Date: | 2023-03-24
Project: | Programming Language C++
Reference: | ISO/IEC IS 14882:2020
Reply to: | Jens Maurer
| jens.maurer@gmx.net

## C++ Standard Core Language Active Issues, Revision 111

This document contains the C++ core language issues on which the Committee (INCITS PL22.16 + WG21) has not yet acted, that is, issues with status "Ready," "Tentatively Ready," "Review," "Drafting," and "Open." (See Issue Status below.)

This document is part of a group of related documents that together describe the issues that have been raised regarding the C++ Standard. The other documents in the group are:

- Closed Issues List, which contains the issues which the Committee has decided are not defects in the International Standard, including a brief rationale explaining the reason for the decision.
- Defect Reports List, which contains the issues that have been categorized by the Committee as Defect Reports, as well as other issues accepted by the Committee, along with their proposed resolutions.
- Table of Contents, which contains a summary listing of all issues in numerical order.
- Index by Section, which contains a summary listing of all issues arranged in the order of the sections of the Standard with which they deal most directly.
- Index by Status, which contains a summary listing of all issues grouped by status.

Section references in this document reflect the section numbering of document WG21 N4944.

The purpose of these documents is to record the disposition of issues that have come before the Core Language Working Group of the ANSI (INCITS PL22.16) and ISO (WG21) C++ Standard Committee.

Some issues represent potential defects in the ISO/IEC IS 14882:2020 document and corrected defects in the earlier 2017, 2014, 2011, 2003, and 1998 documents; others refer to text in the working draft for the next revision of the C++ language and not to any Standard text. Issues are not necessarily formal ISO Defect Reports (DRs). While some issues will eventually be elevated to DR status, others will be disposed of in other ways.

The most current public version of this document can be found at http://www.open-std.org/jtc1/sc22/wg21. Requests for further information about these documents should include the document number, reference ISO/IEC 14882:2020, and be submitted to the InterNational Committee for Information Technology Standards (INCITS), 1250 Eye Street NW, Suite 200, Washington, DC 20005, USA.

Information regarding C++ standardization can be found at http://isocpp.org/std.

### Revision History

Revision 111, 2023-03-24:

Added new issues 2655, 2656, 2657, 2658, 2659, 2660, 2661, 2662, 2663, 2664, 2665, 2666, 2667, 2668, 2669, 2670, 2671, 2672, 2673, 2674, 2675, 2676, 2677, 2678, 2679, 2680, 2681, 2682, 2683, 2684, 2685, 2686, 2687, 2688, 2689, 2690, 2691, 2692, 2693, 2694, 2695, 2696, 2697, 2698, 2699, 2700, 2701, 2702, 2703, 2704, 2705, 2706, 2707, 2708, 2709, and 2710. Reflected the deliberations of the teleconferences up to 2023-03-03. Reflected the decisions of the February, 2023 plenary meeting.

Revision 110, 2022-11-27:

Reflected publication of the C++23 CD N4917. Closed issue 578. Added new issues 2610, 2611, 2612, 2613, 2614, 2615, 2616, 2617, 2618, 2619, 2620, 2621, 2622, 2623, 2624, 2625, 2626, 2627, 2628, 2629, 2630, 2631, 2632, 2633, 2634, 2635, 2636, 2637, 2638, 2641, Created issues for the handling of NB comments on the Committee Draft: 2639, 2640, 2642, 2643, 2644, 2645, 2646, 2647, 2648, 2649, 2650, 2651, 2652, 2653, 2654. Reflected the decisions of the November, 2022 plenary meeting.

Revision 109, 2022-08-07:

Added links for section references. Reset status of issues assigned to people no longer active on the committee. Reflected the deliberations of the teleconferences. Reflected the decisions of the July, 2022 plenary meeting. Added detailed descriptions to issues 2187, 2188, 2189, 2190, 2193, 2195, 2196, 2197, 2200, 2203, 2204, 2211, 2212, 2216, 2221, 2225, 2228, 2230, 2231, 2236, 2245, 2246, 2247, 2248, 2250, 2251, 2258, 2263, 2268, 2269, 2270, 2272, 2274, 2275, 2276, 2279, 2283, 2284, 2286, 2288, 2311, 2314, 2319, and 2320. Closed issue 1326 and 2420 as duplicates. Closed issues 476, 687, 689, 728, 916, 944, 1326, 1393, 1469, 1561, 1742, 1912, 1931, 1972, 2212, and 2341. Dissolved the "concurrency" status in favor of liaison indications, affecting issues 1842 and 2298. Reopened issues 2169 and 2355 after obtaining EWG guidance. Added new issues 2531, 2532, 2533, 2534, 2535, 2536, 2537, 2538, 2539, 2540, 2541, 2542, 2543, 2544, 2545, 2546, 2547, 2548, 2549, 2550, 2551, 2552, 2553, 2554, 2555, 2556, 2557, 2558, 2559, 2560, 2561, 2562, 2563, 2564, 2565, 2566, 2567, 2568, 2569, 2570, 2571, 2572, 2573, 2574, 2575, 2576, 2577, 2578, 2579, 2580, 2581, 2582, 2583, 2584, 2585, 2586, 2587, 2588, 2589, 2591, 2592, 2593, 2594, 2595, 2596, 2597, 2598, 2599, 2600, 2601, 2602, 2603, 2604, 2605, 2606, 2607, 2608, and 2609.

Revision 108, 2022-01-25:

Reflected the deliberations of the January, 2022 teleconferences and the decisions of the February, 2022 plenary meeting. Added new issues 2518, 2519, 2520, 2521, 2522, 2523, 2524, 2525, 2526, 2527, 2528, 2529, and 2530.

Revision 107, 2021-12-09:

Updated the status of issues 2482 and 2493. Reflected the deliberations of the December, 2021 CWG teleconference. Added new issues 2506, 2507, 2508, 2509, 2510, 2511, 2512, 2513, 2514, 2515, 2516, and 2517.

Revision 106, 2021-11-22:

Reflected the deliberations of CWG teleconferences from September through November, 2021, and the decisions of the October, 2021 plenary meeting. Added new issues 2502, 2503, 2504, and 2505.

Revision 105, 2021-06-08:

Reflected the deliberations of CWG teleconferences from March through August, 2021, and the decisions of the June, 2021 plenary meeting. Added new issues 2481, 2482, 2483, 2484, 2485, 2486, 2487, 2488, 2489, 2490, 2491, 2492, 2493, 2494, 2495, 2496, 2497, 2498, 2499, 2500, and 2501.

Revision 104, 2021-02-24:

Reflected the decisions of the February, 2021 meeting. Updated the status of issues 1332 and 2402, which were previously resolved by editorial action and adoption of a paper, respectively. Added new issue 2480.

Revision 103, 2021-02-17:

Updated the status of a number of issues that were resolved or rendered moot by other issue resolutions or papers, including issues 156, 192, 278, 297, 555, 560, 617, 670, 1230, 1285, 1331, 1426, 1451, 1452, 1545, 1640, 1644, 1646, 1713, 1801, 1857, 1859, 1880, 1917, 1943, 1974, 2045, 2080, 2112, 2121, 2215, 2295, 2343, 2364, 2367, 2371, and 2411. Reflected the deliberations of the February, 2021 teleconference. Added new issues 2470, 2471, 2472, 2473, 2474, 2475, 2476, 2477, 2478, and 2479.

Revision 102, 2020-12-15: Changed issue 2331 back to "drafting" status after problems were found with the proposed resolution. Fixed transcription errors in the proposed resolution for issue 2020. Corrected the date for resolutions approved at the February, 2019 meeting (they previously indicated approval at the “November, 2019” meeting). Reflected actions at committee meetings and teleconferences. Added new issues 2403, 2404, 2405, 2406, 2407, 2408, 2409, 2410, 2411, 2412, 2413, 2414, 2415, 2416, 2417, 2418, 2419, 2420, 2421, 2422, 2423, 2424, 2425, 2426, 2427, 2428, 2429, 2430, 2431, 2432, 2433, 2434, 2435, 2436, 2437, 2438, 2439, 2440, 2441, 2442, 2443, 2444, 2445, 2446, 2447, 2448, 2449, 2450, 2451, 2452, 2453, 2454, 2455, 2456, 2457, 2458, 2459, 2460, 2461, 2462, 2463, 2464, 2465, 2466, 2467, 2468, and 2469.

Revision 101, 2019-02-27: Reflected actions at committee meetings and teleconferences. Added new issues 2366, 2367, 2368, 2369, 2370, 2371, 2372, 2373, 2374, 2375, 2376, 2377, 2378, 2379, 2380, 2381, 2382, 2383, 2384, 2385, 2386, 2387, 2388, 2389, 2390, 2391, 2392, 2393, 2394, 2395, 2396, 2397, 2398, 2399, 2400, 2401, and 2402,.

Revision 100, 2018-04-11: Reflected actions at the March, 2018 committee meeting and the April, 2018 teleconference. Added new issues 2357, 2358, 2359, 2360, 2361, 2362, 2363, 2364, and 2365.

Revision 99, 2018-02-27: Reflected actions of committee meetings and teleconferences. Added new C++17 status and moved issues incorporated in that IS to have that status. Added new issues 2278, 2279, 2280, 2281, 2282, 2283, 2284, 2285, 2286, 2287, 2288, 2289, 2290, 2291, 2292, 2293, 2294, 2295, 2296, 2297, 2298, 2299, 2300, 2301, 2302, 2303, 2304, 2305, 2306, 2307, 2308, 2309, 2310, 2311, 2312, 2313, 2314, 2315, 2316, 2317, 2318, 2319, 2320, 2321, 2322, 2323, 2324, 2325, 2326, 2327, 2328, 2329, 2330, 2331, 2332, 2333, 2334, 2335, 2336, 2337, 2338, 2339, 2340, 2341, 2342, 2343, 2344, 2345, 2346, 2347, 2348, 2349, 2350, 2351, 2352, 2353, 2354, 2355, and 2356.

Revision 98, 2017-03-20: Reflected the deliberations of the February-March, 2017 Committee meeting.

Revision 97, 2017-02-07: Reflected the deliberations of the 2016 Committee meetings and teleconferences. Added new issues 2123, 2124, 2125, 2126, 2127, 2128, 2129, 2130, 2131, 2132, 2133, 2134, 2135, 2136, 2137, 2138, 2139, 2140, 2141, 2142, 2143, 2144, 2145, 2146, 2147, 2148, 2149, 2150, 2151, 2152, 2153, 2154, 2155, 2156, 2157, 2158, 2159, 2160, 2161, 2162, 2163, 2164, 2165, 2166, 2167, 2168, 2169, 2170, 2171, 2172, 2173, 2174, 2175, 2176, 2177, 2178, 2179, 2180, 2181, 2182, 2183, 2184, 2185, 2186, 2187, 2188, 2189, 2190, 2191, 2192, 2193, 2194, 2195, 2196, 2197, 2198, 2199, 2200, 2201, 2202, 2203, 2204, 2205, 2206, 2207, 2208, 2209, 2210, 2211, 2212, 2213, 2214, 2215, 2216, 2217, 2218, 2219, 2220, 2221, 2222, 2223, 2224, 2225, 2226, 2227, 2228, 2229, 2230, 2231, 2232, 2233, 2234, 2235, 2236, 2237, 2238, 2239, 2240, 2241, 2242, 2243, 2244, 2245, 2246, 2247, 2248, 2249, 2250, 2251, 2252, 2253, 2254, 2255, 2256, 2257, 2258, 2259, 2260, 2261, 2262, 2263, 2264, 2265, 2266, 2267, 2268, 2269, 2270, 2271, 2272, 2273, 2274, 2275, 2276, and 2277. (Note that a number of the new issues have only titles at this point; full descriptions will be added in the next revision of the list.)

Revision 96, 2016-02-15: Changed the status of issue 2047 from "ready" to "tentatively ready" to reflect revisions to accommodate changes in the underlying wording after the October, 2015 meeting. Reflected the results of drafting review teleconferences held 2016-01-11 and 2016-02-08.

Revision 95, 2015-11-10: Reflected the deliberations of the October, 2015 Committee meeting. Moved issue 1893 back to "drafting" status; it was incorrectly inadvertently accepted as a DR at the November, 2014 meeting.XS Issue 1992 was moved back to drafting to allow application to additional text. Added new issues 2123, 2124, 2125, 2126, 2127, 2128, 2129, 2130, 2131, 2132, 2133, 2134, 2135, 2136, 2137, 2138, 2139, 2140, 2141, 2142, 2143, 2144, 2145, 2146, 2147, 2148, 2149, 2150, 2151, 2152, 2153, 2154, 2155, 2156, 2157, 2158, 2159, 2160, 2161, 2162, 2163, 2164, 2165, 2166, 2167, 2168, 2169, 2170, 2171, 2172, 2173, 2174, 2175, 2176, 2177, 2178, 2179, 2180, 2181, 2182, 2183, 2184, 2185, and 2186.

Revision 94, 2015-05-25: Reflected deliberations of the May, 2015 meeting. Moved issues 1734 and 1928 to "extension" status, reflecting EWG's request to develop a position on trivial special functions before CWG addresses them. Added new issues 2036, 2037, 2038, 2039, 2040, 2041, 2042, 2043, 2044, 2045, 2046, 2047, 2048, 2049, 2050, 2051, 2052, 2053, 2054, 2055, 2056, 2057, 2058, 2059, 2060, 2061, 2062, 2063, 2064, 2065, 2066, 2067, 2068, 2069, 2070, 2071, 2072, 2073, 2074, 2075, 2076, 2077, 2078, 2079, 2080, 2081, 2082, 2083, 2084, 2085, 2086, 2087, 2088, 2089, 2090, 2091, 2092, 2093, 2094, 2095, 2096, 2097, 2098, 2099, 2100, 2101, 2102, 2103, 2104, 2105, 2106, 2107, 2108, 2109, 2110, 2111, 2112, 2113, 2114, 2115, 2116, 2117, 2118, 2119, 2120, 2121, and 2122.

Revision 93, 2015-04-13: Incorporated EWG decisions on issues 92, 203, 622, 822, 1077, 1259, 1272, 1564, 1577, 1582, 1586, 1643, 1754, 1798, 1826, and 1833. Moved issue 1657 to "accepted" status, reflecting adoption of paper N4266 at the Urbana meeting. Moved issue 1683 to "DRWP" status, reflecting approval of N4140 as the working paper at the November, 2014 meeting. Incorporated resolutions from the drafting review teleconference held 2015-04-06.

Revision 92, 2014-11-24: Changed all issues approved by the Committee since the April, 2013 Committee Draft to "C++14" status, to reflect the adoption of ISO/IEC 14882:2014. Moved issues 1299, 1651, 1893, and 1817 to "drafting" status, 1584 to "open" status, and 314, 343, 1710, 1794, and 1812 to "review" status in light of concerns that were raised with their proposed resolutions. Moved issue 1048 to "CD3" status to reflect the fact that it had been addressed by paper N3638, adopted at the April, 2013 (Bristol) meeting. Closed issues 550 and 1855 as duplicates. Reflected the actions of the November, 2014 (Urbana) meeting. Added new issues 1948, 1949, 1950, 1951, 1952, 1953, 1954, 1955, 1956, 1957, 1958, 1959, 1960, 1961, 1962, 1963, 1964, 1965, 1966, 1967, 1968, 1969, 1970, 1971, 1972, 1973, 1974, 1975, 1976, 1977, 1978, 1979, 1980, 1981, 1982, 1983, 1984, 1985, 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021, 2022, 2023, 2024, 2025, 2026, 2027, 2028, 2029, 2030, 2031, 2032, 2033, 2034, and 2035.

Revision 91, 2014-10-13: Incorporated deliberations of drafting review teleconferences held 2014-07-14 and 2014-10-06. Added new issue 1947.

Revision 90, 2014-07-07: Issue 1715 was returned to "drafting" status in light of an alternative suggestion for its resolution. Issue 1927 was closed as a duplicate of issue 1695. Reflected the deliberations of CWG at the 2014-06 (Rapperswil) meeting. Added new issues 1932, 1933, 1934, 1935, 1936, 1937, 1938, 1939, 1940, 1941, 1942, 1943, 1944, 1945, and 1946.

Revision 89, 2014-05-27: Issues 1351 1356 1465, 1590, 1639 1708, and 1810 were returned to "review" status for further discussion. Restored issue 1397 to "ready"; it had incorrectly been moved back to "drafting" because of a misunderstood comment. Added new issues 1866, 1867, 1868, 1869, 1870, 1871, 1872, 1873, 1874, 1875, 1876, 1877, 1878, 1879, 1880, 1881, 1882, 1883, 1884, 1885, 1886, 1887, 1888, 1889, 1890, 1891, 1892, 1893, 1894, 1895, 1896, 1897, 1898, 1899, 1900, 1901, 1902, 1903, 1904, 1905, 1906, 1907, 1908, 1909, 1910, 1911, 1912, 1913, 1914, 1915, 1916, 1917, 1918, 1919, 1920, 1921, 1922, 1923, 1924, 1925, 1926, 1927, 1928, 1929, 1930, and 1931.

- Revision 88, 2014-03-03: Changed the "FDIS" status to "C++11", reflecting having voted out a new DIS. Changed issues in "WP" and "DRWP" status to "CD3", reflecting the fact that their resolutions were contained in the Committee Draft approved for balloting at the April, 2013 meeting. Changed the status of issue 344 to "CD3" when it was noticed that it was resolved by the resolution of issue 1435. Changed the status of issue 1491 to "CD3" when it was noticed that it was resolved by the resolution of issue 1402. Reflected the deliberations of the February, 2014 (Issaquah) meeting. Added new issues 1837, 1838, 1839, 1840, 1841, 1842, 1843, 1844, 1845, 1846, 1847, 1848, 1849, 1850, 1851, 1852, 1853, 1854, 1855, 1856, 1857, 1858, 1859, 1860, 1861, 1862, 1863, 1864, and 1865.
- Revision 87, 2014-01-20: Incorporated the results of the drafting review teleconferences held 2013-11-25 and 2014-01-13. Changed status of issue 1469 to "extension", reflecting the consensus of the 2012-08-13 drafting review teleconference. Moved issue 1480 to "WP" status because it was rendered moot by the adoption of N3652 at the April, 2013 meeting. Added new issues 1796, 1797, 1798, 1799, 1800, 1801, 1802, 1803, 1804, 1805, 1806, 1807, 1808, 1809, 1810, 1811, 1812, 1813, 1814, 1815, 1816, 1817, 1818, 1819, 1820, 1821, 1822, 1823, 1824, 1825, 1826, 1827, 1828, 1829, 1830, 1831, 1832, 1833, 1834, 1835, and 1836.
- Revision 86, 2013-10-14: Reflected the deliberations of the September, 2013 (Chicago) meeting. Added new issues 1743, 1744, 1745, 1746, 1747, 1748, 1749, 1750, 1751, 1752, 1753, 1754, 1755, 1756, 1757, 1758, 1759, 1760, 1761, 1762, 1763, 1764, 1765, 1766, 1767, 1768, 1769, 1770, 1771, 1772, 1773, 1774, 1775, 1776, 1777, 1778, 1779, 1780, 1781, 1782, 1783, 1784, 1785, 1786, 1787, 1788, 1789, 1790, 1791, 1792, 1793, 1794, and 1795.
- Revision 85, 2013-09-03: Incorporated the results of the drafting review teleconferences held 2013-06-24 and 2013-08-26. Changed the status of issue 583 to "DR" because it was (silently) addressed by N3624. Changed the status of issue 1531 to "DR" because it had been inadvertently overlooked in recording the motions of the April, 2013 meeting. Changed the status of issue 1597 to "accepted" because it is moot after the adoption of N3652. Returned issue 1609 to "open" status in light of additional discussion. Added new information to the description of issue 1335. Added new issues 1657, 1658, 1659, 1660, 1661, 1662, 1663, 1664, 1665, 1666, 1667, 1668, 1669, 1670, 1671, 1672, 1673, 1674, 1675, 1676, 1677, 1678, 1679, 1680, 1681, 1682, 1683, 1684, 1685, 1686, 1687, 1688, 1689, 1690, 1691, 1692, 1693, 1694, 1695, 1696, 1697, 1698, 1699, 1700, 1701, 1702, 1703, 1704, 1705, 1706, 1707, 1708, 1709, 1710, 1711, 1712, 1713, 1714, 1715, 1716, 1717, 1718, 1719, 1720, 1721, 1722, 1723, 1724, 1725, 1726, 1727, 1728, 1729, 1730, 1731, 1732, 1733, 1734, 1735, 1736, 1737, 1738, 1739, 1740, 1741, and 1742.
- Revision 84, 2013-05-03: Incorporated the results of the drafting review teleconference held 2013-03-25 and the deliberations of the April, 2013 (Bristol) meeting. Changed issue 1568 to "dup" status. Changed the status of issue 1492 to "drafting" status, although it was approved as a DR at the meeting, because its resolution is to be given by another resolution that was not moved. Similarly, issues 1356 and 1465 were incorrectly moved as defect reports and have been returned to "review" status awaiting the adoption of issue 1351. Also, issue 1531 was moved as a DR but was lacking the proposed resolution in the document in the pre-meeting mailing, so it has been returned to "ready" status to be moved at the next meeting. Changed the resolutions of issues 1374 and 1543 to correct clerical errors transcribing the original proposed resolutions. Began the practice of marking proposed resolutions as "[SUPERSEDED]" when a newer one is added. Added new issues 1642, 1643, 1644, 1645, 1646, 1647, 1648, 1649, 1650, 1651, 1652, 1653, 1654, 1655, and 1656.
- Revision 83, 2013-03-18: Incorporated the results of the drafting review teleconference held 2013-02-04, including moving issue 1566 to "NAD" status. Moved issues 1417 and 1460 back to "review" status in light of new questions. Updated the “more information” link to refer to `isocpp.org`. Added new issues 1606, 1607, 1608, 1609, 1610, 1611, 1612, 1613, 1614, 1615, 1616, 1617, 1618, 1619, 1620, 1621, 1622, 1623, 1624, 1625, 1626, 1627, 1628, 1629, 1630, 1631, 1632, 1633, 1634, 1635, 1636, 1637, 1638, 1639, 1640, and 1641.
- Revision 82, 2013-01-14: Incorporated the results of the drafting review teleconference held 2012-12-10. Moved issues 903, 1213, 1358, 1492, 1508, and 1552 to "review" status to allow further consideration in light of subsequent discussion. Added new issues 1569, 1570, 1571, 1572, 1573, 1574, 1575, 1576, 1577, 1578, 1579, 1580, 1581, 1582, 1583, 1584, 1585, 1586, 1587, 1588, 1589, 1590, 1591, 1592, 1593, 1594, 1595, 1596, 1597, 1598, 1599, 1600, 1601, 1602, 1603, 1604, and 1605.
- Revision 81, 2012-11-03: Reflected the deliberations of the October, 2012 (Portland) meeting. Added new issues 1560, 1561, 1562, 1563, 1564, 1565, 1566, 1567, and 1568.
- Revision 80, 2012-09-24: Added remaining descriptions omitted from issues added in revision 77. Corrected status of issues 915, 1313, 1399, and 1437. Changed the status of issues 616, 903, 1318, 1358, and 1402 in light of additional discussion. Incorporated the results of the drafting review teleconference held 2012-08-13. Moved issue 321 to "dup" status. Moved issue 591 to "open" status after the discussion in "dup" issue 1526. Added additional examples to issue 203. Added new issues 1467, 1468, 1469, 1470, 1471, 1472, 1473, 1474, 1475, 1476, 1477, 1478, 1479, 1480, 1481, 1482, 1483, 1484, 1485, 1486, 1487, 1488, 1489, 1490, 1491, 1492, 1493, 1494, 1495, 1496, 1497, 1498, 1499, 1500, 1501, 1502, 1503, 1504, 1505, 1506, 1507, 1508, 1509, 1510, 1511, 1512, 1513, 1514, 1515, 1516, 1517, 1518, 1519, 1520, 1521, 1522, 1523, 1524, 1525, 1526, 1527, 1528, 1529, 1530, 1531, 1532, 1533, 1534, 1535, 1536, 1537, 1538, 1539, 1540, 1541, 1542, 1543, 1544, 1545, 1546, 1547, 1548, 1549, 1550, 1551, 1552, 1553, 1554, 1555, 1556, 1557, 1558, and 1559.
- Revision 79, 2012-02-27: Reflected deliberations at the February, 2012 (Kona) meeting. Added descriptions omitted from some issues added in revision 77. Moved issue 1427 to "NAD" status. Moved issue 1377 to "dup" status (with respect to issue 1279). Moved issue 1422 to "dup" status (with respect to issue 912). Added a new status, "concurrency", to track issues that will be considered and resolved by the Concurrency Working Group. Added new issues 1444, 1445, 1446, 1447, 1448, 1449, 1450, 1451, 1452, 1453, 1454, 1455, 1456, 1457, 1458, 1459, 1460, 1461, 1462, 1463, 1464, 1465, and 1466.
- Revision 78, 2012-01-17: Reflected the results of the December 5, 2011 teleconference. Changed the status of issues 496, 675, 739, 1268, 1269, 1287, and 1312 from "ready" to "review" in response to further discussion of the proposed resolutions. Added new issues 1370, 1371, 1372, 1373, 1374, 1375, 1376, 1377, 1378, 1379, 1380, 1381, 1382, 1383, 1384, 1385, 1386, 1387, 1388, 1389, 1390, 1391, 1392, 1393, 1394, 1395, 1396, 1397, 1398, 1399, 1400, 1401, 1402, 1403, 1404, 1405, 1406, 1407, 1408, 1409, 1410, 1411, 1412, 1413, 1414, 1415, 1416, 1417, 1418, 1419, 1420, 1421, 1422, 1423, 1424, 1425, 1426, 1427, 1428, 1429, 1430, 1431, 1432, 1433, 1434, 1435, 1436, 1437, 1438, 1439, 1440, 1441, 1442, and 1443.
- Revision 77, 2011-09-06: Reflected the deliberations of the August, 2011 (Bloomington) meeting. Added new issues 1247, 1248, 1249, 1250, 1251, 1252, 1253, 1254, 1255, 1256, 1257, 1258, 1259, 1260, 1261, 1262, 1263, 1264, 1265, 1266, 1267, 1268, 1269, 1270, 1271, 1272, 1273, 1274, 1275, 1276, 1277, 1278, 1279, 1280, 1281, 1282, 1283, 1284, 1285, 1286, 1287, 1288, 1289, 1290, 1291, 1292, 1293, 1294, 1295, 1296, 1297, 1298, 1299, 1300, 1301, 1302, 1303, 1304, 1305, 1306, 1307, 1308, 1309, 1310, 1311, 1312, 1313, 1314, 1315, 1316, 1317, 1318, 1319, 1320, 1321, 1322, 1323, 1324, 1325, 1326, 1327, 1328, 1329, 1330, 1331, 1332, 1333, 1334, 1335, 1336, 1337, 1338, 1339, 1340, 1341, 1342, 1343, 1344, 1345, 1346, 1347, 1348, 1349, 1350, 1351, 1352, 1353, 1354, 1355, 1356, 1357, 1358, 1359, 1360, 1361, 1362, 1363, 1364, 1365, 1366, 1367, 1368, and 1369. (Note that many of the preceding issues are missing descriptions. These will be provided in the next revision of the issues list.)
- Revision 76, 2011-04-10: Reflected the results of the March 7, 2011 teleconference and the March, 2011 (Madrid) meeting, including adding a new status, "FDIS," for issues that are resolved in the draft advanced at that meeting for FDIS balloting. In respect for ISO rules governing work during the balloting process, no new issues have been included in this version of the list.
- Revision 75, 2011-02-28: Reflected results of two between-meetings teleconferences, including moving a number of issues to "tentatively ready" status. Added drafting for a number of issues and moved them to "review" status. Reopened issue 109 in light of recent discussion. Moved issue 1028 back to "open" status after further analysis. Moved issues 1109 and 1127 to "DR" status, as they had been overlooked in the results of the November, 2010 (Batavia) meeting. Added new issues 1227, 1228, 1229, 1230, 1231, 1232, 1233, 1234, 1235, 1236, 1237, 1238, 1239, 1240, 1241, 1242, 1243, 1244, 1245, and 1246.
- Revision 74, 2010-11-29: Reflected deliberations at the November, 2010 (Batavia) meeting. Added new issues 1211, 1212, 1213, 1214, 1215, 1216, 1217, 1218, 1219, 1220, 1221, 1222, 1223, 1224, 1225, and 1226.
- Revision 73, 2010-10-18: Moved issues 994 and 1058 back to "open" status for potential reconsideration. Added analysis to issue 1038. Moved issues 1125 and 1127 back to "drafting" status after further discussion revealed problems with the proposed resolution approved at the August, 2010 meeting. Moved issue 1167 to "review" status to allow consideration of extending the resolution to cover a new but related question. Reflected results of two between-meetings teleconferences, including moving a number of issues to "tentatively ready" status. Added new issues 1174, 1175, 1176, 1177, 1178, 1179, 1180, 1181, 1182, 1183, 1184, 1185, 1186, 1187, 1188, 1189, 1190, 1191, 1192, 1193, 1194, 1195, 1196, 1197, 1198, 1199, 1200, 1201, 1202, 1203, 1204, 1205, 1206, 1207, 1208, 1209, and 1210,
- Revision 72, 2010-08-23: Reflected deliberations at the August, 2010 (Rapperswil) meeting. Added new issues 1091, 1092, 1093, 1094, 1095, 1096, 1097, 1098, 1099, 1100, 1101, 1102, 1103, 1104, 1105, 1106, 1107, 1108, 1109, 1110, 1111, 1112, 1113, 1114, 1115, 1116, 1117, 1118, 1119, 1120, 1121, 1122, 1123, 1124, 1125, 1126, 1127, 1128, 1129, 1130, 1131, 1132, 1133, 1134, 1135, 1136, 1137, 1138, 1139, 1140, 1141, 1142, 1143, 1144, 1145, 1146, 1147, 1148, 1149, 1150, 1151, 1152, 1153, 1154, 1155, 1156, 1157, 1158, 1159, 1160, 1161, 1162, 1163, 1164, 1165, 1166, 1167, 1168, 1169, 1170, 1171, 1172, and 1173.
- Revision 71, 2010-08-07: Added new issues 1069, 1070, 1071, 1072, 1073, 1074, 1075, 1076, 1077, 1078, 1079, 1080, 1081, 1082, 1083, 1084, 1085, 1086, 1087, 1088, 1089, and 1090.
- Revision 70, 2010-08-06: Reflected results of between-meetings teleconference, including moving a number of issues to "tentatively ready" status, updating proposed resolutions, and moving issues 341, 355, 472, and 529 to "drafting" status for more work. Changed issue 979 to "CD2" status, which was overlooked in the results of the March, 2010 meeting. Closed issues 1041, 1019, and 507 as duplicates of issues 924, 314, and 260, respectively.
- Revision 69, 2010-03-29: Reflected deliberations at the March, 2010 (Pittsburgh, PA) meeting. Placed all issues with "WP" status and newly-approved "DR" issues into "CD2" status, to reflect advancing the Final Committee Draft for balloting. Added new issues 1001, 1002, 1003, 1004, 1005, 1006, 1007, 1008, 1009, 1010, 1011, 1012, 1013, 1014, 1015, 1016, 1017, 1018, 1019, 1020, 1021, 1022, 1023, 1024, 1025, 1026, 1027, 1028, 1029, 1030, 1031, 1032, 1033, 1034, 1035, 1036, 1037, 1038, 1039, 1040, 1041, 1042, 1043, 1044, 1045, 1046, 1047, 1048, 1049, 1050, 1051, 1052, 1053, 1054, 1055, 1056, 1057, 1058, 1059, 1060, 1061, 1062, 1063, 1064, 1065, 1066, 1067, and 1068.
- Revision 68, 2010-02-16: Reflected results of between-meeting teleconferences, including moving many issues to "tentatively ready" status and incorporating draft proposed resolutions for many others. Added new issues 999 and 1000.
- Revision 67, 2009-11-08: Reflected deliberations at the October, 2009 (Santa Cruz, CA) meeting. Added new issues 972, 973, 974, 975, 976, 977, 978, 979, 980, 981, 982, 983, 984, 985, 986, 987, 988, 989, 990, 991, 992, 993, 994, 995, 996, 997, and 998.
- Revision 66, 2009-09-29: Incorporated a new status, "tentatively ready", for issues whose resolutions were reviewed and approved by teleconference between meetings. Changed issue 378 to "CD1" status, as it was resolved by the resolution of issue 276. Added new issues 949, 950, 951, 952, 953, 954, 955, 956, 957, 958, 959, 960, 961, 962, 963, 964, 965, 966, 967, 968, 969, 970, and 971.
- Revision 65, 2009-08-03: Reflected deliberations from the Frankfurt, Germany (July, 2009) meeting. Added a new status, "concepts," for issues related to the concepts proposal that was removed from the working document and moved all concepts-related issues to that status, effectively closing them. Changed the presentation of proposed resolutions to use background colors in addition to font effects to identify changes (i.e., inserted and deleted text). Added new issues 922, 923, 924, 925, 926, 927, 928, 929, 930, 931, 932, 933, 934, 935, 936, 937, 938, 939, 940, 941, 942, 943, 944, 945, 946, 947, and 948.
- Revision 64, 2009-06-19: Fixed incorrect cross-references in issues 867 and 868. Added proposed resolutions for issues 527, 587, 589, 604, 719, 732, and 770 and moved them to "review" status. Added discussion to issues 721, 760, 802, and 872. Added new issues 889, 890, 891, 892, 893, 894, 895, 896, 897, 898, 899, 900, 901, 902, 903, 904, 905, 906, 907, 908, 909, 910, 911, 912, 913, 914, 915, 916, 917, 918, 919, 920, and 921.
- Revision 63, 2009-05-06: Closed issue 839 as a duplicate of issue 803. Added notes to the discussion of issues 325, 626, 667 690, 739, and 841. Moved issue 662 back to "open" status with additional discussion. Added new issues 843, 844, 845, 846, 847, 848, 849, 850, 851, 852, 853, 854, 855, 856, 857, 858, 859, 860, 861, 862, 863, 864, 865, 866, 867, 868, 869, 870, 871, 872, 873, 874, 875, 876, 877, 878, 879, 880, 881, 882, 883, 884, 885, 886, 887, and 888.
- Revision 62, 2009-03-23: Reflected deliberations from the Summit, NJ (March, 2009) meeting. Marked issues directly related to National Body comments on the 2008-10 Committee Draft with a tag identifying the relevant comment; the tag is both printed in the issue body and reflected in the "Index by Status" document. Added new issues 767, 768, 769, 770, 771, 772, 773, 774, 775, 776, 777, 778, 779, 780, 781, 782, 783, 784, 785, 786, 787, 788, 789, 790, 791, 792, 793, 794, 795, 796, 797, 798, 799, 800, 801, 802, 803, 804, 805, 806, 807, 808, 809, 810, 811, 812, 813, 814, 815, 816, 817, 818, 819, 820, 821, 822, 823, 824, 825, 826, 827, 828, 829, 830, 831, 832, 833, 834, 835, 836, 837, 838, 839, 840, 841, and 842.
- Revision 61, 2009-02-08: Provided a reference to a paper containing a proposed resolution for issues 695 and 699 and moved them to "review" status. Added new issues 749, 750, 751, 752, 753, 754, 755, 756, 757, 758, 759, 760, 761, 762, 763, 764, 765, and 766.
- Revision 60, 2008-12-09: Revised the resolution of issue 653 and moved to "review" status. Added new issues 724, 725, 726, 727, 728, 729, 730, 731, 732, 733, 734, 735, 736, 737, 738, 739, 740, 741, 742, 743, 744, 745, 746, 747, and 748.
- Revision 59, 2008-10-05: Reflected deliberations from the San Francisco (September, 2008) meeting. Placed all issues with "WP" status and newly-approved "DR" issues into "CD1" status, to reflect advancing the Committee Draft for balloting. Added new proposed resolutions to issues 696, 704, and 705 and moved them to "review" status. Changed issue 265 to "dup" status in favor of issue 353, which was approved in 2003. Added new issues 710, 711, 712, 713, 714, 715, 716, 717, 718, 719, 720, 721, 722, and 723.
- Revision 58, 2008-08-25: Fixed some incorrect section references. Fixed the title of issue 692. Changed the status of issue 603 to "WP", as the paper that resolved it was approved in April, 2007. Moved issue 657 back to "review" status and added proposed wording. Added discussion to issues 693 and 697. Added or revised resolutions for issues 606, 614, 652, 685, and 694 and moved them to "review" status. Added new issues 704, 705, 706, 707, 708, and 709.
- Revision 57, 2008-07-27: Updated the status of issues 222, 309, and 632 to reflect actions at the June, 2008 meeting that were inadvertently omitted in the preceding revision. Added proposed wording for issue 683 and moved it to "review" status. Added new issues 697, 698, 699, 700, 701, 702, and 703.
- Revision 56, 2008-06-29: Reflected deliberations from the Sophia Antipolis (June, 2008) meeting. Added discussion to issues 507 and 527. Added proposed resolution for issue 653. Added new issues 695 and 696.
- Revision 55, 2008-05-18: Added proposed resolutions for issues 220, 257, 292, 341, 539, 554, 570, 571, 576, 597, 621, 624, 626, 633, 634, 639, and 642, and moved them to "review" status. Moved issue 155 to "dup" status in favor of issue 632, which was changed to "review" status in light of its connection with the initializer-list proposal. Moved issues 530 and 551 to "WP" status because they were resolved by the constexpr proposal. Moved issue 646 to "review" status because it appears to be "NAD". Changed issue 240 to "review" status (from "ready") in light of input from WG14 regarding a similar issue. Added new issues 685, 686, 687, 688, 689, 690, 691, 692, 693, and 694.
- Revision 54, 2008-03-17: Reflected deliberations from the Bellevue (February, 2008) meeting. Restructured references inside “definitions” sections (Clause 3 [intro.defs], _N4567_.17.3 [definitions]) because the individual definitions are not sections. Returned issue 646 to "open" status (it was previously erroneously in "drafting" status). Moved issues 347, 512, and 236 to "review" status with a recommendation to close them as "NAD." Changed issues 220 and 256 back to "open" status and annotated them to indicate that they have been accepted by EWG and referred to CWG for action for C++0x. Changed issues 6 and 150 back to "open" status and annotated them to indicate that they have been accepted by EWG and referred to CWG for action, but not for C++0x. Closed issues 107, 168, 229, 294, and 359 as "NAD" to reflect the recommendation of EWG. Added new issues 673, 674, 675, 676, 677, 678, 679, 680, 681, 682, 683, and 684.
- Revision 53, 2008-02-03: Updated the proposed resolutions for issues 288 and 342. Updated the status of issues 199 and 430 to reflect actions at the Oxford (April, 2007) meeting. Added proposed resolutions and changed the status to "review" for issues 28, 111, 118, 141, 240, 276, 309, 355, 373, 378, 462, 485, 535, 574, 601, 608, 641, 645, and 651. Added new issues 667, 668, 669, 670, 671, and 672.
- Revision 52, 2007-12-09: Updated the status of issues 568 and 620 to reflect actions at the Toronto (July, 2007) meeting. Fixed a typographical error in the example for issue 606. Added new issues 654, 655, 656, 657, 658, 659, 660, 661, 662, 663, 664, 665, and 666.
- Revision 51, 2007-10-09: Reflected deliberations from the Kona (October, 2007) meeting. Added new issues 652 and 653.
- Revision 50, 2007-09-09: Updated section reference numbers to use the numbering of the most recent working draft and added text identifying the document number of that draft at the beginning of each issues list document. Updated issue 475 with discussion regarding the point at which `std::uncaught_exception()` becomes `false`. Added new issues 642, 643, 644, 645, 646, 647, 648, 649, 650, and 651.
- Revision 49, 2007-08-05: Reflected deliberations from the Toronto (July, 2007) meeting. Added additional discussion to issues 219 and 339. Added new issues 637, 638, 639, 640, and 641.
- Revision 48, 2007-06-24: Added new issues 632, 633, 634, 635, and 636.
- Revision 47, 2007-05-06: Reflected deliberations from the Oxford (April, 2007) meeting. Added new issues 626, 627, 628, 629, 630, and 631.
- Revision 46, 2007-03-11: Added proposed wording to issue 495 and moved it to “review” status. Added new issues 612, 613, 614, 615, 616, 617, 618, 619, 620, 621, 622, 623, 624, and 625.
- Revision 45, 2007-01-14: Changed the status of issue 78 from TC1 to WP. Added new issues 603, 604, 605, 606, 607, 608, 609, 610, and 611.
- Revision 44, 2006-11-05: Reflected deliberations from the Portland (October, 2006) meeting. Added proposed wording for issues 288, 342, and 572, and moved them to “review” status. Added new issues 596, 597, 598, 599, 600, 601, and 602.
- Revision 43, 2006-09-09: Updated issues 218, 357, and 537 with additional discussion and proposed resolutions and moved them to “review” status; added issues 589, 590, 591, 592, 593, 594, and 595.
- Revision 42, 2006-06-23: Updated issues 408 and 561 with additional discussion; added a reference to a paper on the topic to issue 567; and added issues 578, 579, 580, 581, 582, 583, 584, 585, 586, 587, and 588.
- Revision 41, 2006-04-22: Reflected deliberations from the Berlin (April, 2006) meeting. Added issues 562, 563, 564, 565, 566, 567, 568, 569, 570, 571, 572, 573, 574, 575, 576, and 577.
- Revision 40, 2006-02-24: Updated issue 540 to refer to paper J16/06-0022 = WG21 N1952 for its resolution. Updated issue 453 to note the need for additional drafting. Reopened issue 504 and broadened its scope to include object declarations as well. Updated issue 150 (in “extension” status) with additional commentary. Added issues 552, 553, 554, 555, 556, 557, 558, 559, 560, and 561.
- Revision 39, 2005-12-16: Updated issue 488 with additional discussion. Added issues 538, 539, 540, 541, 542, 543, 544, 545, 546, 547, 548, 549, 550, and 551.
- Revision 38, 2005-10-22: Reflected deliberations from the Mont Tremblant (October, 2005) meeting. Added isues 530, 531, 532, 533, 534, 535, 536, and 537.
- Revision 37, 2005-08-27: Added issues 523, 524, 525, 526, 527, 528, and 529.
- Revision 36, 2005-06-27: Reopened issue 484 for additional discussion. Added issues 517, 518, 519, 520, 521, and 522.
- Revision 35, 2005-05-01: Reflected deliberations from the Lillehammer (April, 2005) meeting. Updated issues 189 and 459 with additional discussion. Added new issues 504, 505, 506, 507, 508, 509, 510, 511, 512, 513, 514, 515, and 516.
- Revision 34: 2005-03-06: Closed issue 471 as NAD; updated issues 58, 232, 339, 407, and 494 with additional discussion; and added new issues 498, 499, 500, 501, 502, and 503.
- Revision 33: 2005-01-14: Updated issue 36 with additional discussion. Added new issues 485, 486, 487, 488, 489, 490, 491, 492, 493, 494, 495, 496, and 497.
- Revision 32: 2004-11-07: Reflected deliberations from the Redmond (October, 2004) meeting. Added new issues 475, 476, 477, 478, 479, 480, 481, 482, 483, and 484.
- Revision 31: 2004-09-10: Updated issue 268 with comments from WG14; added comments and changed the status of issue 451 back to “open”; added discussion to issues 334, 341, 385, 399, and 430; and added new issues 470, 471, 472, 473, and 474.
- Revision 30: 2004-04-09: Reflected deliberations from the Sydney (March, 2004) meeting. Added issues 461-469, updated issues 39, 86, 257, 291, 391, 389, 435, 436, 437, 439, 441, 442, 446, 450, 453, and 458.
- Revision 29: 2004-02-13: Added issues 441-460, updated issues 39, 291, 306, 319, 389, 394, 413, and 417.
- Revision 28: 2003-11-15: Reflected deliberations from the Kona (October, 2003) meeting. Added issues 435-438.
- Revision 27: 2003-09-19: Added new issues 412-434, updated issues 54, 301, 372, 382, 391, and 399.
- Revision 26: 2003-04-25: Reflected deliberations from the Oxford (April, 2003) meeting. Added new issues 402-411.
- Revision 25: 2003-03-03: Added new issues 390-401, updated issue 214.
- Revision 24: 2002-11-08: Reflected deliberations from the Santa Cruz (October, 2002) meeting. Added new issues 379-389.
- Revision 23: 2002-09-10: Added new issues 355-378, updated issue 298 and issue 214.
- Revision 22: 2002-05-10: Reflected deliberations from the Curacao (April, 2002) meeting. Added issues 342-354.
- Revision 21: 2002-03-11: Added new issues 314-341, updated issues 132, 214, 244, 245, 254, 255, 283.
- Revision 20: 2001-11-09: Reflected deliberations from the Redmond (October, 2001) meeting. Added issue 313.
- Revision 19: 2001-09-12: Added new issues 289-308, updated issues 222, 261, 270.
- Revision 18: 2001-05-19: Reflected deliberations from the Copenhagen (April, 2001) meeting. Added new issues 282-288.
- Revision 17: 2001-04-29: Added new issues 276-81.
- Revision 16: 2001-03-27: Updated issue 138 to discuss the interaction of using-declarations and "forward declarations." Noted a problem with the proposed resolution of issue 139. Added some new discussion to issue 115. Added proposed resolution for issue 160. Updated address of C++ FAQ. Added new issues 265-275.
- Revision 15: 2000-11-18: Reflected deliberations from the Toronto (October, 2000) meeting; moved the discussion of empty and fully-initialized const objects from issue 78 into new issue 253; added new issues 254-264.
- Revision 14: 2000-10-21: Added issues 246-252; added an extra question to issue 221 and changed its status back to "review."
- Revision 13: 2000-09-16: Added issues 229-245; changed status of issue 106 to "review" because of problem detected in proposal; added wording for issues 87 and 216 and moved to "review" status; updated discussion of issues 5, 78, 198, 203, and 222.
- Revision 12: 2000-05-21: Reflected deliberations from the Tokyo (April, 2000) meeting; added new issues 222-228.
- Revision 11, 2000-04-13: Added proposed wording and moved issues 62, 73, 89, 94, 106, 121, 134, 142, and 145 to "review" status. Moved issue 13 from "extension" to "open" status because of recent additional discussion. Added new issues 217-221.
- Revision 10, 2000-03-21: Split the issues list and indices into multiple documents. Added further discussion to issues 84 and 87. Added proposed wording and moved issues 1, 69, 85, 98, 105, 113, 132, and 178 to "review" status. Added new issues 207-216.
- Revision 9, 2000-02-23: Incorporated decisions from the October, 1999 meeting of the Committee; added issues 174 through 206.
- Revision 8, 1999-10-13: Minor editorial changes to issues 90 and 24; updated issue 89 to include a related question; added issues 169, 170, 171, 172, and 173.
- Revision 7, 1999-09-14: Removed unneeded change to 13.9.4 [temp.expl.spec] paragraph 9 from issue 24; changed issue 85 to refer to 6.5.6 [basic.lookup.elab]; added issues 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, and 168.
- Revision 6, 1999-05-31: Moved issue 72 to "dup" status; added proposed wording and moved issue 90 to "review" status; updated issue 98 with additional question; added issues 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, and 121.
- Revision 5, 1999-05-24: Reordered issues by status; added revision history; first public version.

### Issue status

Issues progress through various statuses as the Core Language Working Group and, ultimately, the full PL22.16 and WG21 committees deliberate and act. For ease of reference, issues are grouped in these documents by their status. Issues have one of the following statuses:

Open: The issue is new or the working group has not yet formed an opinion on the issue. If a Suggested Resolution is given, it reflects the opinion of the issue's submitter, not necessarily that of the working group or the Committee as a whole.

Drafting: Informal consensus has been reached in the working group and is described in rough terms in a Tentative Resolution, although precise wording for the change is not yet available.

Review: Exact wording of a Proposed Resolution is now available for an issue on which the working group previously reached informal consensus.

Ready: The working group has reached consensus that a change in the working draft is required, the Proposed Resolution is correct, and the issue is ready to forward to the full Committee for ratification.

Tentatively Ready: Like "ready" except that the resolution was produced and approved by a subset of the working group membership between meetings. Persons not participating in these between-meeting activities are encouraged to review such resolutions carefully and to alert the working group with any problems that may be found.

DR: The full Committee has approved the item as a proposed defect report. The Proposed Resolution in an issue with this status reflects the best judgment of the Committee at this time regarding the action that will be taken to remedy the defect; however, the current wording of the Standard remains in effect until such time as a Technical Corrigendum or a revision of the Standard is issued by ISO.

accepted: Like a DR except that the issue concerns the wording of the current Working Paper rather than that of the current International Standard.

TC1: A DR issue included in Technical Corrigendum 1. TC1 is a revision of the Standard issued in 2003.

CD1: A DR issue not resolved in TC1 but included in Committee Draft 1. CD1 was advanced for balloting at the September, 2008 WG21 meeting.

CD2: A DR issue not resolved in CD1 but included in the Final Committee Draft advanced for balloting at the March, 2010 WG21 meeting.

C++11: A DR issue not resolved in CD2 but included in ISO/IEC 14882:2011.

CD3: A DR/DRWP or Accepted/WP issue not resolved in C++11 but included in the Committee Draft advanceed for balloting at the April, 2013 WG21 meeting.

C++14: A DR/DRWP or Accepted/WP issue not resolved in CD3 but included in ISO/IEC 14882:2014.

CD4: A DR/DRWP or Accepted/WP issue not resolved in C++14 but included in the Committee Draft advanced for balloting at the June, 2016 WG21 meeting.

C++17: a DR/DRWP or Accepted/WP issue not resolved in CD4 but included in ISO/IEC 14882:2017.

CD5: A DR/DRWP or Accepted/WP issue not resolved in C++17 but included in the Committee Draft advanced for balloting at the July, 2019 WG21 meeting.

C++20: a DR/DRWP or Accepted/WP issue not resolved in CD5 but included in ISO/IEC 14882:2020.

CD6: A DR/DRWP or Accepted/WP issue not resolved in C++20 but included in the Committee Draft advanced for balloting at the July, 2022 WG21 meeting.

DRWP: A DR issue whose resolution is reflected in the current Working Paper. The Working Paper is a draft for a future version of the Standard.

WP: An accepted issue whose resolution is reflected in the current Working Paper.

Dup: The issue is identical to or a subset of another issue, identified in a Rationale statement.

NAD: The working group has reached consensus that the issue is not a defect in the Standard. A Rationale statement describes the working group's reasoning.

Extension: The working group has reached consensus that the issue is not a defect in the Standard but is a request for an extension to the language. The working group expresses no opinion on the merits of an issue with this status; however, the issue will be maintained on the list for possible future consideration as an extension proposal.

Concepts: The issue relates to the “Concepts” proposal that was removed from the working paper at the Frankfurt (July, 2009) meeting and hence is no longer under consideration.

### Overview

Section | Issue | Status | Liaison | Title
3 intro.defs | 783 | open |  | Definition of “argument”
3 intro.defs | 2632 | open |  | 'user-declared' is not defined
4.1 intro.compliance | 949 | open |  | Requirements for freestanding implementations
4.1.1 intro.compliance.general | 2700 | review | WG14 | `#error` disallows existing implementation practice
5.2 lex.phases | 1698 | open |  | Files ending in `\`
5.2 lex.phases | 2573 | open | SG12 | Undefined behavior when splicing results in a universal-character-name
5.4 lex.pptoken | 369 | drafting |  | Are `new`/`delete` identifiers or preprocessing-op-or-punc?
5.4 lex.pptoken | 1655 | drafting |  | Line endings in raw string literals
5.4 lex.pptoken | 2574 | open | SG12 | Undefined behavior when lexing unmatched quotes
5.6 lex.token | 1901 | drafting |  | punctuator referenced but not defined
5.12 lex.operators | 189 | drafting |  | Definition of operator and punctuator
5.13 lex.literal | 1924 | review | editor | Definition of “literal” and kinds of literals
5.13.2 lex.icon | 2698 | review | WG14, EWG | Using extended integer types with `z` suffix
5.13.8 lex.ext | 1266 | open |  | user-defined-integer-literal overflow
5.13.8 lex.ext | 1723 | drafting |  | Multicharacter user-defined character literals
5.13.8 lex.ext | 1735 | drafting |  | Out-of-range literals in user-defined-literals
6.1 basic.pre | 1529 | drafting |  | Nomenclature for variable vs reference non-static data member
6.3 basic.def.odr | 1209 | open |  | Is a potentially-evaluated expression in a template definition a “use?”
6.3 basic.def.odr | 1897 | drafting |  | ODR vs alternative tokens
6.4.1 basic.scope.scope | 2488 | open |  | Overloading virtual functions and functions with trailing requires-clauses
6.5.1 basic.lookup.general | 2480 | drafting |  | Lookup for enumerators in modules
6.5.2 class.member.lookup | 380 | open |  | Definition of "ambiguous base class" missing
6.5.2 class.member.lookup | 2567 | open |  | Operator lookup ambiguity
6.5.5.1 basic.lookup.qual.general | 1089 | drafting |  | Template parameters in member selections
6.6 basic.link | 2670 | open |  | Programs and translation units
6.6 basic.link | 2706 | open |  | Repeated structured binding declarations
6.7.1 intro.memory | 1953 | open |  | Data races and common initial sequence
6.7.2 intro.object | 2324 | drafting |  | Size of base class subobject
6.7.2 intro.object | 2325 | drafting |  | `std::launder` and reuse of character buffers
6.7.2 intro.object | 2334 | open |  | Creation of objects by `typeid`
6.7.2 intro.object | 2469 | drafting |  | Implicit object creation vs constant expressions
6.7.3 basic.life | 419 | open |  | Can cast to virtual base class be done on partially-constructed object?
6.7.3 basic.life | 1027 | drafting |  | Type consistency and reallocation of scalar types
6.7.3 basic.life | 1530 | drafting |  | Member access in out-of-lifetime objects
6.7.3 basic.life | 1853 | drafting |  | Defining “allocated storage”
6.7.3 basic.life | 2258 | open |  | Storage deallocation during period of destruction
6.7.3 basic.life | 2514 | review | SG12 | Modifying const subobjects
6.7.3 basic.life | 2551 | open |  | "Refers to allocated storage" has no meaning
6.7.3 basic.life | 2676 | open |  | Replacing a complete object having base subobjects
6.7.3 basic.life | 2677 | review |  | Replacing union subobjects
6.7.4 basic.indet | 1997 | drafting |  | Placement new and previous initialization
6.7.5 basic.stc | 365 | open |  | Storage duration and temporaries
6.7.5 basic.stc | 1634 | drafting |  | Temporary storage duration
6.7.5 basic.stc | 2533 | drafting |  | Storage duration of implicitly created objects
6.7.5.5.2 basic.stc.dynamic.allocation | 1676 | drafting |  | `auto` return type for allocation and deallocation functions
6.7.5.5.2 basic.stc.dynamic.allocation | 1682 | open |  | Overly-restrictive rules on function templates as allocation functions
6.7.5.5.2 basic.stc.dynamic.allocation | 2073 | drafting |  | Allocating memory for exception objects
6.7.5.5.3 basic.stc.dynamic.deallocation | 523 | open |  | Can a one-past-the-end pointer be invalidated by deleting an adjacent object?
6.7.5.5.3 basic.stc.dynamic.deallocation | 2042 | drafting |  | Exceptions and deallocation functions
6.7.6 basic.align | 1211 | drafting |  | Misaligned lvalues
6.7.7 class.temporary | 2434 | open |  | Mandatory copy elision vs non-class objects
6.7.7 class.temporary | 2666 | open |  | Lifetime extension through `static_cast`
6.8 basic.types | 350 | open | WG14 | `signed char` underlying representation for objects
6.8 basic.types | 1701 | drafting |  | Array vs sequence in object representation
6.8.1 basic.types.general | 2519 | tentatively ready |  | Object representation of a bit-field
6.8.2 basic.fundamental | 146 | open |  | Floating-point zero
6.8.2 basic.fundamental | 251 | open |  | How many signed integer types are there?
6.8.2 basic.fundamental | 2689 | open |  | Are cv-qualified `std::nullptr_t` fundamental types?
6.8.4 basic.compound | 2544 | open |  | Address of past-the-end of a potentially-overlapping subobject
6.9.1 intro.execution | 698 | open |  | The definition of “sequenced before” is too narrow
6.9.2 intro.multithread | 1842 | open | SG1 | Unevaluated operands and “carries a dependency”
6.9.2.2 intro.races | 2297 | open |  | Unclear specification of atomic operations
6.9.2.2 intro.races | 2298 | open | SG1 | Actions and expression evaluation
6.9.2.2 intro.races | 2587 | open |  | Visible side effects and initial value of an object
6.9.3.2 basic.start.static | 371 | open |  | Interleaving of constructor calls
6.9.3.2 basic.start.static | 1294 | open |  | Side effects in dynamic/static initialization
6.9.3.2 basic.start.static | 1659 | open |  | Initialization order of thread_local template static data members
6.9.3.2 basic.start.static | 1986 | drafting |  | odr-use and delayed initialization
6.9.3.2 basic.start.static | 2148 | drafting |  | Thread storage duration and order of initialization
6.9.3.3 basic.start.dynamic | 640 | open |  | Accessing destroyed local objects of static storage duration
6.9.3.3 basic.start.dynamic | 2444 | drafting |  | Constant expressions in initialization odr-use
6.9.3.3 basic.start.dynamic | 2684 | open |  | thread_local dynamic initialization
7.3.6 conv.qual | 2438 | open |  | Problems in the specification of qualification conversions
7.3.7 conv.prom | 2485 | tentatively ready |  | Bit-fields in integral promotions
7.3.13 conv.mem | 170 | tentatively ready |  | Pointer-to-member conversions
7.5.4 expr.prim.id | 2503 | drafting |  | Unclear relationship among name, qualified name, and unqualified name
7.5.4.3 expr.prim.id.qual | 2549 | open |  | Implicitly moving the operand of a throw-expression in unevaluated contexts
7.5.4.4 expr.prim.id.dtor | 2473 | drafting |  | Parentheses in pseudo-destructor calls
7.5.5.2 expr.prim.lambda.closure | 1973 | tentatively ready |  | Which parameter-declaration-clause in a lambda-expression?
7.5.5.2 expr.prim.lambda.closure | 2542 | drafting |  | Is a closure type a structural type?
7.5.5.2 expr.prim.lambda.closure | 2561 | open |  | Conversion to function pointer for lambda with explicit object parameter
7.5.5.3 expr.prim.lambda.capture | 2086 | drafting |  | Reference odr-use vs implicit capture
7.5.7.1 expr.prim.req.general | 2560 | open |  | Parameter type determination in a requirement-parameter-list
7.5.7.1 expr.prim.req.general | 2565 | open |  | Invalid types in the parameter-declaration-clause of a requires-expression
7.6 expr.compound | 1642 | open |  | Missing requirements for prvalue operands
7.6.1.3 expr.call | 2284 | open |  | Sequencing of braced-init-list arguments
7.6.1.3 expr.call | 2515 | open |  | Result of a function call
7.6.1.3 expr.call | 2660 | open |  | Confusing term "this parameter"
7.6.1.3 expr.call | 2688 | open |  | Calling explicit object member functions
7.6.1.4 expr.type.conv | 914 | open | EWG | Value-initialization of array types
7.6.1.4 expr.type.conv | 1521 | drafting |  | `T{`expr`}` with reference types
7.6.1.4 expr.type.conv | 2283 | drafting |  | Missing complete type requirements
7.6.1.5 expr.ref | 2557 | drafting |  | Class member access referring to an unrelated class
7.6.1.5 expr.ref | 2705 | open |  | Accessing ambiguous subobjects
7.6.1.6 expr.post.incr | 742 | open |  | Postfix increment/decrement with long bit-field operands
7.6.1.7 expr.dynamic.cast | 1965 | drafting |  | Explicit casts to reference types
7.6.1.8 expr.typeid | 282 | open |  | Namespace for `extended_type_info`
7.6.1.8 expr.typeid | 528 | open |  | Why are incomplete class types not allowed with `typeid`?
7.6.1.8 expr.typeid | 1954 | open |  | `typeid` null dereference check in subexpressions
7.6.1.9 expr.static.cast | 2048 | open |  | C-style casts that cast away constness vs `static_cast`
7.6.1.9 expr.static.cast | 2243 | drafting |  | Incorrect use of implicit conversion sequence
7.6.2.2 expr.unary.op | 232 | drafting |  | Is indirection through a null pointer undefined behavior?
7.6.2.4 expr.await | 2668 | open |  | `co_await` in a lambda-expression
7.6.2.5 expr.sizeof | 2609 | open |  | Padding in class types
7.6.2.8 expr.new | 267 | open |  | Alignment requirement for new-expressions
7.6.2.8 expr.new | 473 | open |  | Block-scope declarations of allocator functions
7.6.2.8 expr.new | 901 | drafting |  | Deleted `operator delete`
7.6.2.8 expr.new | 1628 | open |  | Deallocation function templates
7.6.2.8 expr.new | 2102 | drafting |  | Constructor checking in new-expression
7.6.2.8 expr.new | 2281 | drafting |  | Consistency of aligned `operator delete` replacement
7.6.2.8 expr.new | 2532 | open |  | Kind of pointer value returned by `new T[0]`
7.6.2.8 expr.new | 2566 | open |  | Matching deallocation for uncaught exception
7.6.2.8 expr.new | 2592 | open |  | Missing definition for placement allocation/deallocation function
7.6.2.8 expr.new | 2623 | drafting |  | Invoking destroying `operator delete` for constructor failure
7.6.2.9 expr.delete | 196 | open |  | Arguments to deallocation functions
7.6.4 expr.mptr.oper | 2593 | open |  | Insufficient base class restriction for pointer-to-member expression
7.6.6 expr.add | 2013 | drafting |  | Pointer subtraction in large array
7.6.6 expr.add | 2182 | drafting |  | Pointer arithmetic in array-like containers
7.6.6 expr.add | 2548 | open |  | Array prvalues and additive operators
7.6.9 expr.rel | 2696 | open | SG22 | Relational comparisons of pointers to `void`
7.6.16 expr.cond | 2023 | drafting |  | Composite reference result type of conditional operator
7.6.16 expr.cond | 2316 | drafting |  | Simplifying class conversions in conditional expressions
7.6.18 expr.throw | 2699 | ready |  | Inconsistency of throw-expression specification
7.6.19 expr.ass | 1542 | drafting |  | Compound assignment of braced-init-list
7.7 expr.const | 1255 | open |  | Definition problems with `constexpr` functions
7.7 expr.const | 1256 | open |  | Unevaluated operands are not necessarily constant expressions
7.7 expr.const | 1626 | open |  | `constexpr` member functions in brace-or-equal-initializers
7.7 expr.const | 2166 | drafting |  | Unclear meaning of “undefined `constexpr` function”
7.7 expr.const | 2186 | drafting |  | Unclear point that “preceding initialization” must precede
7.7 expr.const | 2192 | open |  | Constant expressions and order-of-eval undefined behavior
7.7 expr.const | 2301 | open |  | Value-initialization and constexpr constructor evaluation
7.7 expr.const | 2456 | open |  | Viable user-defined conversions in converted constant expressions
7.7 expr.const | 2536 | open | EWG | Partially initialized variables during constant initialization
7.7 expr.const | 2545 | open |  | Transparently replacing objects in constant expressions
7.7 expr.const | 2552 | open |  | Constant evaluation of non-defining variable declarations
7.7 expr.const | 2559 | open |  | Defaulted consteval functions
7.7 expr.const | 2633 | open |  | typeid of constexpr-unknown dynamic type
7.7 expr.const | 2656 | open |  | Converting consteval lambda to function pointer in non-immediate context
7.7 expr.const | 2702 | open |  | Constant destruction of reference members
7.7 expr.const | 2710 | open |  | Loops in constant expressions
8.6.5 stmt.ranged | 1680 | drafting |  | Including `<initializer_list>` for range-based `for`
8.7 stmt.jump | 2115 | drafting |  | Order of implicit destruction vs release of automatic storage
8.7.4 stmt.return | 2495 | open |  | Glvalue result of a function call
8.7.5 stmt.return.coroutine | 2556 | open |  | Unusable `promise::return_void`
8.8 stmt.dcl | 2123 | open |  | Omitted constant initialization of local static variables
8.9 stmt.ambig | 1223 | drafting |  | Syntactic disambiguation and trailing-return-types
9.1 dcl.pre | 157 | open |  | Omitted typedef declarator
9.2.2 dcl.stc | 498 | open |  | Storage class specifiers in definitions of class members
9.2.2 dcl.stc | 2232 | open |  | `thread_local` anonymous unions
9.2.6 dcl.constexpr | 2117 | drafting |  | Explicit specializations and `constexpr` function templates
9.2.6 dcl.constexpr | 2531 | review |  | Static data members redeclared as constexpr
9.2.9.2 dcl.type.cv | 2195 | open |  | Unsolicited reading of trailing volatile members
9.2.9.4 dcl.type.elab | 144 | open |  | Position of `friend` specifier
9.2.9.4 dcl.type.elab | 2634 | open |  | Avoid circularity in specification of scope for friend class declarations
9.2.9.6 dcl.spec.auto | 1348 | drafting |  | Use of `auto` in a trailing-return-type
9.2.9.6 dcl.spec.auto | 1670 | drafting |  | `auto` as conversion-type-id
9.2.9.6 dcl.spec.auto | 1868 | drafting |  | Meaning of “placeholder type”
9.2.9.6 dcl.spec.auto | 2412 | review |  | SFINAE vs undeduced placeholder type
9.2.9.6.1 dcl.spec.auto.general | 2476 | tentatively ready |  | placeholder-type-specifiers and function declarators
9.3.2 dcl.name | 1488 | drafting |  | abstract-pack-declarators in type-ids
9.3.3 dcl.ambig.res | 2228 | open | EWG | Ambiguity resolution for cast to function type
9.3.4.1 dcl.meaning.general | 2671 | open |  | friend named by a template-id
9.3.4.3 dcl.ref | 453 | drafting |  | References may only bind to “valid” objects
9.3.4.3 dcl.ref | 504 | open |  | Should use of a variable in its own initializer require a diagnostic?
9.3.4.3 dcl.ref | 2550 | open |  | Type "reference to cv `void`" outside of a declarator
9.3.4.6 dcl.fct | 1001 | drafting |  | Parameter type adjustment in dependent parameter types
9.3.4.6 dcl.fct | 1668 | drafting |  | Parameter type determination still not clear enough
9.3.4.6 dcl.fct | 1790 | open | WG14 | Ellipsis following function parameter pack
9.3.4.6 dcl.fct | 2537 | drafting |  | Overbroad grammar for parameter-declaration
9.3.4.6 dcl.fct | 2553 | open |  | Restrictions on explicit object member functions
9.3.4.7 dcl.fct.default | 325 | drafting |  | When are default arguments parsed?
9.3.4.7 dcl.fct.default | 361 | open |  | Forward reference to default argument
9.3.4.7 dcl.fct.default | 1580 | drafting |  | Default arguments in explicit instantiations
9.3.4.7 dcl.fct.default | 1609 | open |  | Default arguments and function parameter packs
9.3.4.7 dcl.fct.default | 2683 | ready |  | Default arguments for member functions of templated nested classes
9.3.4.7 dcl.fct.default | 2701 | open |  | Default arguments in multiple scopes / inheritance of array bounds in the same scope
9.4 dcl.init | 2327 | drafting |  | Copy elision for direct-initialization with a conversion function
9.4.1 dcl.init.general | 2708 | open |  | Parenthesized initialization of arrays
9.4.1 dcl.init.general | 2709 | open |  | Parenthesized initialization of reference-to-aggregate
9.4.2 dcl.init.aggr | 2128 | drafting |  | Imprecise rule for reference member initializer
9.4.2 dcl.init.aggr | 2149 | drafting |  | Brace elision and array length deduction
9.4.3 dcl.init.string | 1304 | drafting |  | Omitted array bound with string initialization
9.4.4 dcl.init.ref | 233 | open |  | References vs pointers in UDC overload resolution
9.4.4 dcl.init.ref | 1414 | drafting |  | Binding an rvalue reference to a reference-unrelated lvalue
9.4.4 dcl.init.ref | 1827 | drafting |  | Reference binding with ambiguous conversions
9.4.4 dcl.init.ref | 2018 | drafting |  | Qualification conversion vs reference binding
9.4.4 dcl.init.ref | 2657 | open |  | Cv-qualification adjustment when binding reference to temporary
9.4.4 dcl.init.ref | 2704 | open |  | Clarify meaning of "bind directly"
9.4.5 dcl.init.list | 1996 | drafting |  | Reference list-initialization ignores conversion functions
9.4.5 dcl.init.list | 2168 | open |  | Narrowing conversions and +/- infinity
9.4.5 dcl.init.list | 2252 | review | editor | Enumeration list-initialization from the same type
9.4.5 dcl.init.list | 2638 | open |  | Improve the example for initializing by initializer list
9.5.1 dcl.fct.def.general | 1962 | open | EWG | Type of `__func__`
9.5.1 dcl.fct.def.general | 2144 | drafting |  | Function/variable declaration ambiguity
9.5.1 dcl.fct.def.general | 2362 | open | EWG | `__func__` should be `constexpr`
9.5.2 dcl.fct.def.default | 1854 | drafting |  | Disallowing use of implicitly-deleted functions
9.5.2 dcl.fct.def.default | 2547 | open |  | Defaulted comparison operator function for non-classes
9.5.2 dcl.fct.def.default | 2570 | open |  | Clarify constexpr for defaulted functions
9.5.4 dcl.fct.def.coroutine | 2562 | open |  | Exceptions thrown during coroutine startup
9.5.4 dcl.fct.def.coroutine | 2563 | open | EWG | Initialization of coroutine result object
9.6 dcl.struct.bind | 2340 | open |  | Reference collapsing and structured bindings
9.7.1 dcl.enum | 1485 | drafting |  | Out-of-class definition of member unscoped opaque enumeration
9.7.1 dcl.enum | 2131 | drafting |  | Ambiguity with opaque-enum-declaration
9.8.2.2 namespace.unnamed | 2505 | drafting |  | Nested unnamed namespace of inline unnamed namespace
9.9 namespace.udecl | 813 | open |  | `typename` in a using-declaration with a non-dependent name
9.9 namespace.udecl | 2555 | open |  | Ineffective redeclaration prevention for using-declarators
9.9 namespace.udecl | 2663 | tentatively ready |  | Example for member redeclarations with using-declarations
9.11 dcl.link | 1817 | drafting |  | Linkage specifications and nested scopes
9.12.1 dcl.attr.grammar | 1706 | drafting |  | `alignas` pack expansion syntax
9.12.2 dcl.align | 1617 | open |  | `alignas` and non-defining declarations
9.12.2 dcl.align | 2223 | drafting |  | Multiple `alignas` specifiers
10.1 module.unit | 2541 | open |  | Linkage specifications, module purview, and module attachment
10.2 module.interface | 2607 | drafting |  | Visibility of enumerator names
11.1 class.pre | 2637 | open |  | Injected-class-name as a simple-template-id
11.2 class.prop | 511 | open |  | POD-structs with template assignment operators
11.2 class.prop | 2463 | open | EWG | Trivial copyability and unions with non-trivial members
11.4 class.mem | 1890 | drafting |  | Member type depending on definition of member function
11.4.1 class.mem.general | 2188 | open |  | empty-declaration grammar ambiguity
11.4.1 class.mem.general | 2661 | open |  | Missing disambiguation rule for pure-specifier vs. brace-or-equal-initializer
11.4.4 special | 2595 | open |  | "More constrained" for eligible special member functions
11.4.5 class.ctor | 1353 | tentatively ready |  | Array and variant members and deleted special member functions
11.4.5 class.ctor | 1623 | drafting |  | Deleted default union constructor and member initializers
11.4.5 class.ctor | 1808 | drafting |  | Constructor templates vs default constructors
11.4.5.3 class.copy.ctor | 1092 | drafting |  | Cycles in overload resolution during instantiation
11.4.5.3 class.copy.ctor | 1548 | drafting |  | Copy/move construction and conversion functions
11.4.5.3 class.copy.ctor | 1594 | drafting |  | Lazy declaration of special members vs overload errors
11.4.5.3 class.copy.ctor | 2203 | drafting |  | Defaulted copy/move constructors and UDCs
11.4.5.3 class.copy.ctor | 2264 | drafting |  | Memberwise copying with indeterminate value
11.4.6 class.copy.assign | 1499 | drafting |  | Missing case for deleted move assignment operator
11.4.6 class.copy.assign | 2329 | drafting |  | Virtual base classes and generated assignment operators
11.4.7 class.dtor | 1977 | drafting |  | Contradictory results of failed destructor lookup
11.4.7 class.dtor | 2158 | drafting |  | Polymorphic behavior during destruction
11.4.8.3 class.conv.fct | 2513 | open |  | Ambiguity with requires-clause and operator-function-id
11.4.9.3 class.static.data | 1283 | drafting |  | Static data members of classes with typedef name for linkage purposes
11.4.9.3 class.static.data | 1721 | drafting |  | Diagnosing ODR violations for static data members
11.4.9.3 class.static.data | 2335 | drafting |  | Deduced return types vs member types
11.5 class.union | 57 | open |  | Empty unions
11.5 class.union | 1404 | drafting |  | Object reallocation in unions
11.5 class.union | 1702 | drafting |  | Rephrasing the definition of “anonymous union”
11.5.1 class.union.general | 2591 | open |  | Implicit change of active union member for anonymous union in union
11.5.1 class.union.general | 2675 | open |  | `start_lifetime_as`, placement-new, and active union members
11.7.3 class.virtual | 2554 | open |  | Overriding virtual functions, also with explicit object parameters
11.8.3 class.access.base | 2246 | drafting |  | Access of indirect virtual base class constructors
11.8.4 class.friend | 718 | open | EWG | Non-class, non-function friend declarations
11.8.4 class.friend | 2588 | drafting | EWG | friend declarations and module linkage
11.8.5 class.protected | 472 | drafting |  | Casting across protected inheritance
11.8.5 class.protected | 1883 | drafting |  | Protected access to constructors in mem-initializers
11.8.5 class.protected | 2187 | drafting |  | Protected members and access via qualified-id
11.8.5 class.protected | 2244 | open |  | Base class access in aggregate initialization
11.9.3 class.base.init | 1915 | open | EWG | Potentially-invoked destructors in non-throwing constructors
11.9.3 class.base.init | 2056 | drafting |  | Member function calls in partially-initialized class objects
11.9.3 class.base.init | 2403 | drafting |  | Temporary materialization and base/member initialization
11.9.3 class.base.init | 2669 | open |  | Lifetime extension for aggregate initialization
11.9.4 class.inhctor.init | 2504 | review |  | Inheriting constructors from virtual base classes
11.9.5 class.cdtor | 1517 | drafting |  | Unclear/missing description of behavior during construction/destruction
11.9.6 class.copy.elision | 6 | open |  | Should the optimization that allows a class object to alias another object also allow the case of a parameter in an inline function to alias its argument?
11.9.6 class.copy.elision | 1049 | open |  | Copy elision through reference parameters of inline functions
11.10.1 class.compare.default | 2568 | open |  | Access checking during synthesis of defaulted comparison operator
11.10.3 class.spaceship | 2703 | review |  | Three-way comparison requiring strong ordering for floating-point types, take 2
11.10.4 class.compare.secondary | 2546 | open |  | Defaulted secondary comparison operators defined as deleted
12.2.2.2.1 over.match.call.general | 2687 | review |  | Calling an explicit object member function via an address-of-overload-set
12.2.2.2.2 over.call.func | 1278 | drafting |  | Incorrect treatment of contrived object
12.2.2.2.3 over.call.object | 2189 | open |  | Surrogate call template
12.2.2.2.3 over.call.object | 2564 | open |  | Conversion to function pointer with an explicit object parameter
12.2.2.3 over.match.oper | 545 | open |  | User-defined conversions and built-in operator overload resolution
12.2.2.3 over.match.oper | 1919 | open |  | Overload resolution for `!` with explicit conversion operator
12.2.2.3 over.match.oper | 2089 | drafting |  | Restricting selection of builtin overloaded operators
12.2.2.7 over.match.ref | 2028 | drafting |  | Converting constructors in rvalue reference initialization
12.2.2.7 over.match.ref | 2108 | drafting |  | Conversions to non-class prvalues in reference initialization
12.2.2.8 over.match.list | 2194 | drafting |  | Impossible case in list initialization
12.2.2.8 over.match.list | 2311 | open |  | Missed case for guaranteed copy elision
12.2.2.9 over.match.class.deduct | 2425 | open |  | Confusing wording for deduction from a type
12.2.2.9 over.match.class.deduct | 2467 | drafting |  | CTAD for alias templates and the deducible check
12.2.2.9 over.match.class.deduct | 2471 | drafting |  | Nested class template argument deduction
12.2.2.9 over.match.class.deduct | 2628 | open |  | Implicit deduction guides should propagate constraints
12.2.2.9 over.match.class.deduct | 2680 | open |  | Class template argument deduction for aggregates with designated initializers
12.2.4.2 over.best.ics | 2319 | drafting |  | Nested brace initialization from same type
12.2.4.2.1 over.best.ics.general | 2525 | drafting |  | Incorrect definition of implicit conversion sequence
12.2.4.2.1 over.best.ics.general | 2679 | open |  | Implicit conversion sequence with a null pointer constant
12.2.4.2.5 over.ics.ref | 2077 | drafting |  | Overload resolution and invalid rvalue-reference initialization
12.2.4.2.6 over.ics.list | 1536 | drafting |  | Overload resolution with temporary from initializer list
12.2.4.2.6 over.ics.list | 2169 | open |  | Narrowing conversions and overload resolution
12.2.4.2.6 over.ics.list | 2492 | review |  | Comparing user-defined conversion sequences in list-initialization
12.2.4.3 over.ics.rank | 1459 | open |  | Reference-binding tiebreakers in overload resolution
12.2.4.3 over.ics.rank | 1789 | open |  | Array reference vs array decay in overload resolution
12.2.4.3 over.ics.rank | 2110 | drafting |  | Overload resolution for base class conversion and reference/non-reference
12.2.4.3 over.ics.rank | 2337 | open |  | Incorrect implication of logic ladder for conversion sequence tiebreakers
12.3 over.over | 1038 | review |  | Overload resolution of `&x.static_func`
12.3 over.over | 2572 | open |  | Address of overloaded function with no target
12.4 over.oper | 1989 | drafting |  | Insufficient restrictions on parameters of postfix operators
12.4.3 over.binary | 1549 | open |  | Overloaded comma operator with `void` operand
12.5 over.built | 260 | open |  | User-defined conversions and built-in `operator=`
12.5 over.built | 954 | open |  | Overload resolution of conversion operator templates with built-in types
12.6 over.literal | 1620 | open |  | User-defined literals and extended integer types
13 temp | 205 | drafting |  | Templates and static data members
13.1 temp.pre | 1463 | drafting | EWG | `extern "C"` alias templates
13.2 temp.param | 1444 | drafting |  | Type adjustments of non-type template parameters
13.2 temp.param | 1635 | drafting |  | How similar are template default arguments to function default arguments?
13.2 temp.param | 2395 | drafting |  | Parameters following a pack expansion
13.2 temp.param | 2617 | open |  | Default template arguments for template members of non-template classes
13.3 temp.names | 579 | open |  | What is a “nested” `>` or `>>`?
13.3 temp.names | 2450 | drafting |  | braced-init-list as a template-argument
13.4 temp.arg | 440 | open |  | Allow implicit pointer-to-member conversion on nontype template argument
13.4 temp.arg | 2105 | open |  | When do the arguments for a parameter pack end?
13.4.3 temp.arg.nontype | 2043 | drafting |  | Generalized template arguments and array-to-pointer decay
13.4.3 temp.arg.nontype | 2049 | drafting |  | List initializer in non-type template default argument
13.4.3 temp.arg.nontype | 2401 | drafting |  | Array decay vs prohibition of subobject non-type arguments
13.4.3 temp.arg.nontype | 2459 | drafting |  | Template parameter initialization
13.4.4 temp.arg.template | 2057 | drafting |  | Template template arguments with default arguments
13.4.4 temp.arg.template | 2398 | drafting |  | Template template parameter matching and deduction
13.5.2 temp.constr.constr | 2686 | open |  | Pack expansion into a non-pack parameter of a concept
13.5.2.3 temp.constr.atomic | 2589 | open |  | Context of access checks during constraint satisfaction checking
13.6 temp.type | 2037 | drafting |  | Alias templates and template declaration matching
13.7 temp.decls | 1730 | drafting |  | Can a variable template have an unnamed type?
13.7.2.3 temp.deduct.guide | 2697 | open | EWG | Deduction guides using abbreviated function syntax
13.7.2.3 temp.deduct.guide | 2707 | open |  | Deduction guides cannot have a trailing requires-clause
13.7.5 temp.friend | 1918 | open |  | `friend` templates with dependent scopes
13.7.5 temp.friend | 1945 | open |  | Friend declarations naming members of class templates in non-templates
13.7.5 temp.friend | 2118 | open |  | Stateful metaprogramming via friend injection
13.7.6 temp.spec.partial | 708 | open |  | Partial specialization of member templates of class templates
13.7.6 temp.spec.partial | 1647 | drafting |  | Type agreement of non-type template arguments in partial specializations
13.7.6 temp.spec.partial | 2127 | drafting |  | Partial specialization and nullptr
13.7.6 temp.spec.partial | 2173 | open |  | Partial specialization with non-deduced contexts
13.7.6.1 temp.spec.partial.general | 2179 | drafting |  | Required diagnostic for partial specialization after first use
13.7.6.2 temp.spec.partial.match | 549 | drafting |  | Non-deducible parameters in partial specializations
13.7.6.4 temp.spec.partial.member | 1755 | drafting |  | Out-of-class partial specializations of member templates
13.7.7.2 temp.over.link | 310 | open |  | Can function templates differing only in parameter cv-qualifiers be overloaded?
13.7.7.2 temp.over.link | 2584 | open |  | Equivalent types in function template declarations
13.7.7.3 temp.func.order | 402 | open |  | More on partial ordering of function templates
13.7.7.3 temp.func.order | 1157 | open |  | Partial ordering of function templates is still underspecified
13.7.7.3 temp.func.order | 2160 | open |  | Issues with partial ordering
13.7.8 temp.alias | 1286 | drafting |  | Equivalence of alias templates
13.7.8 temp.alias | 1430 | open |  | Pack expansion into fixed alias template parameter list
13.7.8 temp.alias | 1554 | drafting |  | Access and alias templates
13.7.8 temp.alias | 1979 | drafting |  | Alias template specialization in template member definition
13.7.8 temp.alias | 1980 | drafting |  | Equivalent but not functionally-equivalent redeclarations
13.7.8 temp.alias | 2236 | drafting |  | When is an alias template specialization dependent?
13.8 temp.res | 1257 | open |  | Instantiation via non-dependent references in uninstantiated templates
13.8 temp.res | 2067 | open |  | Generated variadic templates requiring empty pack
13.8.1 temp.res.general | 2462 | drafting |  | Problems with the omission of the `typename` keyword
13.8.1 temp.res.general | 2468 | drafting |  | Omission of the `typename` keyword in a member template parameter list
13.8.2 temp.local | 186 | open |  | Name hiding and template template-parameters
13.8.2 temp.local | 459 | open |  | Hiding of template parameters by base class members
13.8.3.2 temp.dep.type | 1390 | drafting |  | Dependency of alias template specializations
13.8.3.2 temp.dep.type | 1524 | drafting |  | Incompletely-defined class template base
13.8.3.2 temp.dep.type | 1619 | open |  | Definition of current instantiation
13.8.3.2 temp.dep.type | 2074 | drafting |  | Type-dependence of local class of function template
13.8.3.3 temp.dep.expr | 2275 | drafting |  | Type-dependence of function template
13.8.3.3 temp.dep.expr | 2487 | drafting |  | Type dependence of function-style cast to incomplete array type
13.8.3.3 temp.dep.expr | 2600 | review |  | Type dependency of placeholder types
13.8.3.5 temp.dep.temp | 2090 | drafting |  | Dependency via non-dependent base class
13.8.4 temp.dep.res | 2 | drafting |  | How can dependent names be used in member declarations that appear outside of the class template definition?
13.8.4.1 temp.point | 287 | drafting |  | Order dependencies in template instantiation
13.8.4.1 temp.point | 1845 | drafting |  | Point of instantiation of a variable template specialization
13.8.4.1 temp.point | 2245 | drafting |  | Point of instantiation of incomplete class template
13.8.4.1 temp.point | 2250 | open |  | Implicit instantiation, destruction, and TUs
13.8.4.1 temp.point | 2497 | drafting |  | Points of instantiation for constexpr function templates
13.9 temp.spec | 1253 | open |  | Generic non-template members
13.9 temp.spec | 2435 | open |  | Alias template specializations
13.9.2 temp.inst | 1378 | open |  | When is an instantiation required?
13.9.2 temp.inst | 1602 | review |  | Linkage of specialization vs linkage of template arguments
13.9.2 temp.inst | 1856 | open |  | Indirect nested classes of class templates
13.9.2 temp.inst | 2202 | drafting |  | When does default argument instantiation occur?
13.9.2 temp.inst | 2222 | drafting |  | Additional contexts where instantiation is not required
13.9.2 temp.inst | 2263 | drafting |  | Default argument instantiation for `friend`s
13.9.2 temp.inst | 2265 | drafting |  | Delayed pack expansion and member redeclarations
13.9.2 temp.inst | 2596 | drafting |  | Instantiation of constrained non-template friends
13.9.2 temp.inst | 2655 | open |  | Instantiation of default arguments in lambda-expressions
13.9.3 temp.explicit | 293 | open |  | Syntax of explicit instantiation/specialization too permissive
13.9.3 temp.explicit | 1046 | open |  | What is a “use” of a class specialization?
13.9.3 temp.explicit | 1665 | drafting |  | Declaration matching in explicit instantiations
13.9.3 temp.explicit | 2421 | drafting |  | Explicit instantiation of constrained member functions
13.9.3 temp.explicit | 2501 | drafting |  | Explicit instantiation and trailing requires-clauses
13.9.4 temp.expl.spec | 529 | drafting |  | Use of `template<>` with “explicitly-specialized” class templates
13.9.4 temp.expl.spec | 1840 | drafting |  | Non-deleted explicit specialization of deleted function template
13.9.4 temp.expl.spec | 1993 | drafting |  | Use of `template<>` defining member of explicit specialization
13.9.4 temp.expl.spec | 2409 | drafting |  | Explicit specializations of constexpr static data members
13.10.2 temp.arg.explicit | 264 | open |  | Unusable template constructors and conversion functions
13.10.2 temp.arg.explicit | 2055 | drafting |  | Explicitly-specified non-deduced parameter packs
13.10.3 temp.deduct | 697 | open |  | Deduction rules apply to more than functions
13.10.3 temp.deduct | 1172 | drafting |  | “instantiation-dependent” constructs
13.10.3 temp.deduct | 1322 | drafting |  | Function parameter type decay in templates
13.10.3 temp.deduct | 1582 | drafting |  | Template default arguments and deduction failure
13.10.3 temp.deduct | 1844 | open |  | Defining “immediate context”
13.10.3 temp.deduct | 2054 | review |  | Missing description of class SFINAE
13.10.3 temp.deduct | 2296 | open | EWG | Are default argument instantiation failures in the “immediate context”?
13.10.3.1 temp.deduct.general | 2498 | open |  | Partial specialization failure and the immediate context
13.10.3.1 temp.deduct.general | 2672 | open |  | Lambda body SFINAE is still required, contrary to intent and note
13.10.3.2 temp.deduct.call | 503 | open |  | Cv-qualified function types in template argument deduction
13.10.3.2 temp.deduct.call | 1513 | drafting |  | `initializer_list` deduction failure
13.10.3.2 temp.deduct.call | 1584 | drafting |  | Deducing function types from cv-qualified types
13.10.3.2 temp.deduct.call | 1939 | open |  | Argument conversions to nondeduced parameter types revisited
13.10.3.3 temp.deduct.funcaddr | 1486 | drafting |  | Base-derived conversion in member pointer deduction
13.10.3.5 temp.deduct.partial | 1221 | open |  | Partial ordering and reference collapsing
13.10.3.5 temp.deduct.partial | 1610 | drafting |  | Cv-qualification in deduction of reference to array
13.10.3.6 temp.deduct.type | 1763 | open |  | Length mismatch in template type deduction
13.10.3.6 temp.deduct.type | 2328 | drafting |  | Unclear presentation style of template argument deduction rules
14.4 except.handle | 2172 | drafting |  | Multiple exceptions with one exception object
14.4 except.handle | 2219 | drafting |  | Dynamically-unreachable handlers
14.5 except.spec | 2417 | open |  | Explicit instantiation and exception specifications
15 cpp | 2002 | open | WG14 | White space within preprocessing directives
15.2 cpp.cond | 925 | open |  | Type of character literals in preprocessor expressions
15.2 cpp.cond | 1436 | open |  | Interaction of constant expression changes with preprocessor expressions
15.2 cpp.cond | 2190 | open |  | Insufficient specification of `__has_include`
15.2 cpp.cond | 2575 | open | SG12 | Undefined behavior when macro-replacing "defined" operator
15.3 cpp.include | 2576 | open | SG12 | Undefined behavior with macro-expanded `#include` directives
15.6 cpp.replace | 1718 | drafting | WG14 | Macro invocation spanning end-of-file
15.6 cpp.replace | 2003 | drafting |  | Zero-argument macros incorrectly specified
15.6.1 cpp.replace.general | 2577 | open | SG12 | Undefined behavior for preprocessing directives in macro arguments
15.6.3 cpp.stringize | 1625 | open | WG14 | Adding spaces between tokens in stringizing
15.6.3 cpp.stringize | 1709 | drafting |  | Stringizing raw string literals containing newline
15.6.3 cpp.stringize | 2578 | open | SG12 | Undefined behavior when creating an invalid string literal via stringizing
15.6.4 cpp.concat | 2522 | open | WG14 | Removing placemarker tokens and retention of whitespace
15.6.4 cpp.concat | 2579 | open | SG12 | Undefined behavior when token pasting does not create a preprocessing token
15.6.5 cpp.rescan | 268 | open | WG14 | Macro name suppression in rescanned replacement text
15.7 cpp.line | 2580 | open | SG12 | Undefined behavior with `#line`
15.7 cpp.line | 2693 | open | WG14 | Escape sequences for the string-literal of `#line`
15.9 cpp.pragma | 1889 | drafting |  | Unclear effect of `#pragma` on conformance
15.11 cpp.predefined | 2581 | open | SG12 | Undefined behavior for predefined macros
15.12 cpp.pragma.op | 897 | open |  | `_Pragma` and extended string-literals
15.12 cpp.pragma.op | 2694 | open | WG14 | string-literals of the `_Pragma` operator
17.13.3 csetjmp.syn | 2361 | open |  | Unclear description of `longjmp` undefined behavior
Annex B implimits | 2181 | drafting |  | Normative requirements in an informative Annex
Annex C diff | 1944 | open |  | New C incompatibilities
C.5 diff.cpp03 | 1279 | drafting |  | Additional differences between C++ 2003 and C++ 2011
C.6 diff.iso | 1248 | open |  | Updating Annex C to C99

### Issues with "Ready" Status

#### 2699. Inconsistency of throw-expression specification

Section: 7.6.18 [expr.throw] Status: ready Submitter: Krystian Stasiowski Date: 2020-04-06

Subclause 7.6.18 [expr.throw] paragraph 2 and 3 specify:

> Evaluating a throw-expression with an operand throws an exception (14.2 [except.throw]); the type of the exception object is determined by removing any top-level cv-qualifier s from the static type of the operand and adjusting the type from “array of T” or function type T to “pointer to T”.

> A throw-expression with no operand rethrows the currently handled exception (14.4 [except.handle]). The exception is reactivated with the existing exception object; no new exception object is created. The exception is no longer considered to be caught.

This means that throwing a value of type `const char[3]` would throw a `char*` rather than `const char*`, which is not intended.

Proposed resolution (approved by CWG 2023-03-03):

Change in 7.6.18 [expr.throw] paragraph 2 through 4 as follows:

> Evaluating a A throw-expression with an operand throws an exception (14.2 [except.throw]); the. The array-to-pointer (7.3.3 [conv.array]) and function-to-pointer (7.3.4 [conv.func]) standard conversions are performed on the operand. The type of the exception object is determined by removing any top-level cv-qualifiers from the static type of the (possibly converted) operand and adjusting the type from “array of T” or function type T to “pointer to T”.

> A throw-expression with no operand rethrows the currently handled exception (14.4 [except.handle]). If no exception is presently being handled, the function `std:: terminate` is invoked (14.6.2 [except.terminate]). The Otherwise, the exception is reactivated with the existing exception object; no new exception object is created. The exception is no longer considered to be caught.

> If no exception is presently being handled, evaluating a throw-expression with no operand calls std:: terminate() (14.6.2 [except.terminate]).

#### 2683. Default arguments for member functions of templated nested classes

Section: 9.3.4.7 [dcl.fct.default] Status: ready Submitter: Matthew House Date: 2023-01-11

Subclause 9.3.4.7 [dcl.fct.default] paragraph 6 specifies:

> Except for member functions of class templates, the default arguments in a member function definition that appears outside of the class definition are added to the set of default arguments provided by the member function declaration in the class definition; the program is ill-formed if a default constructor (11.4.5.2 [class.default.ctor]), copy or move constructor (11.4.5.3 [class.copy.ctor]), or copy or move assignment operator (11.4.6 [class.copy.assign]) is so declared. Default arguments for a member function of a class template shall be specified on the initial declaration of the member function within the class template.

That rule appears to allow adding default arguments for member functions of classes that are nested within class templates, for example:

      template<class> struct A { struct B { void c(int); }; };
      template<class T> void A<T>::B::c(int = 0) {}

MSVC accepts; gcc and clang reject.

Proposed resolution (approved by CWG 2023-03-03):

Change in 9.3.4.7 [dcl.fct.default] paragraph 6 as follows:

> Except for member functions of class templates templated classes, the default arguments in a member function definition that appears outside of the class definition are added to the set of default arguments provided by the member function declaration in the class definition; the program is ill-formed if a default constructor (11.4.5.2 [class.default.ctor]), copy or move constructor (11.4.5.3 [class.copy.ctor]), or copy or move assignment operator (11.4.6 [class.copy.assign]) is so declared. Default arguments for a member function of a templated class template shall be specified on the initial declaration of the member function within the templated class template.

### Issues with "Tentatively Ready" Status

#### 2519. Object representation of a bit-field

Section: 6.8.1 [basic.types.general] Status: tentatively ready Submitter: Jiang An Date: 2022-01-20

6.7.2 [intro.object] clearly implies that bit-fields are objects; paragraphs 8-9 contain phrases like “unless an object is a bit-field...” and “a non-bit-field subobject”. However, the definition of “object representation” in 6.8.1 [basic.types.general] paragraph 4 is,

> The object representation of an object of type `T` is the sequence of N `unsigned char` objects taken up by the object of type `T`, where N equals `sizeof(T)`.

and thus fails to address bit-fields, which are not necessarily composed of a sequence of complete bytes.

The C Standard (6.2.6.1 paragraph 4) says,

> Values stored in bit-fields consist of m bits, where m is the size specified for the bit-field. The object representation is the set of m bits the bit-field comprises in the addressable storage unit holding it.

Presumably similar wording could be adopted for C++.

Proposed resolution (approved by CWG 2023-01-06) [SUPERSEDED]:

Change in 6.8.1 [basic.types.general] paragraph 4 as follows:

> The object representation of an object of a type `T` is the sequence of N `unsigned char` objects taken up by the a non-bit-field complete object of type `T`, where N equals `sizeof(T)`. The value representation of an object of a type `T` is the set of bits in the object representation of T that participate in representing a value of type `T`. The object and value representation of a non-bit-field complete object of type `T` are the bytes and bits, respectively, of the object corresponding to the object and value representation of its type. The object representation of a bit-field object is the sequence of N bits taken up by the object, where N is the width of the bit-field (11.4.10 [class.bit]). The value representation of a bit-field object is the set of bits in the object representation that participate in representing its value. Bits in the object representation of a type or object that are not part of the value representation are padding bits. For trivially copyable types, the value representation is a set of bits in the object representation that determines a value, which is one discrete element of an implementation-defined set of values. [ Footnote: ... ]

CWG 2023-02-06

Additional drafting is needed to constrain the definition to complete object types.

Proposed resolution (approved for C++26 by CWG 2023-02-06):

Change in 6.8.1 [basic.types.general] paragraph 4 as follows:

> The object representation of an object of a complete object type `T` is the sequence of N `unsigned char` objects taken up by the a non-bit-field complete object of type `T`, where N equals `sizeof(T)`. The value representation of an object of a type `T` is the set of bits in the object representation of T that participate in representing a value of type `T`. The object and value representation of a non-bit-field complete object of type `T` are the bytes and bits, respectively, of the object corresponding to the object and value representation of its type. The object representation of a bit-field object is the sequence of N bits taken up by the object, where N is the width of the bit-field (11.4.10 [class.bit]). The value representation of a bit-field object is the set of bits in the object representation that participate in representing its value. Bits in the object representation of a type or object that are not part of the value representation are padding bits. For trivially copyable types, the value representation is a set of bits in the object representation that determines a value, which is one discrete element of an implementation-defined set of values. [ Footnote: ... ]

#### 2485. Bit-fields in integral promotions

Section: 7.3.7 [conv.prom] Status: tentatively ready Submitter: Richard Smith Date: 2021-04-01

According to 7.3.7 [conv.prom] paragraph 5,

> A prvalue for an integral bit-field (11.4.10 [class.bit]) can be converted to a prvalue of type `int` if `int` can represent all the values of the bit-field; otherwise, it can be converted to `unsigned int` if `unsigned int` can represent all the values of the bit-field. If the bit-field is larger yet, no integral promotion applies to it. If the bit-field has an enumerated type, it is treated as any other value of that type for promotion purposes.

This description has several problems. First, the “bit-field” semantic property only makes sense for glvalue expressions, so it's unclear why these rules are described as applying to a prvalue. Perhaps this should be rephrased as something like “An expression that was a bit-field glvalue prior to the application of the lvalue-to-rvalue conversion”?

Second, suppose that `char32_t` is wider than `int`. Per paragraph 2, a `char32_t` prvalue promotes to `unsigned long` (because `unsigned long` is necessarily at least 32 bits wide). But per paragraph 5, a `char32_t : 32` bitfield does not promote. This seems inconsistent.

Finally, it is not clear that the usual integral promotions are not applied to bit-fields. This should be made explicit.

Proposed resolution (approved by CWG 2023-02-07):

1. Insert a paragraph before 7.3.7 [conv.prom] paragraph 1 as follows:

   > For the purposes of 7.3.7 [conv.prom], a converted bit-field is a prvalue that is the result of an lvalue-to-rvalue conversion (7.3.2 [conv.lval]) applied to a bit-field (11.4.10 [class.bit]).

2. Change in 7.3.7 [conv.prom] paragraph 1 as follows:

   > A prvalue of that is not a converted bit-field and has an integer type other than bool, char8_t, char16_t, char32_t, or wchar_t whose integer conversion rank (6.8.6 [conv.rank]) is less than the rank of int can be converted to a prvalue of type int if int can represent all the values of the source type; otherwise, the source prvalue can be converted to a prvalue of type unsigned int.

3. Change in 7.3.7 [conv.prom] paragraph 4 as follows:

   > A prvalue of an unscoped enumeration type whose underlying type is fixed (9.7.1 [dcl.enum]) can be converted to a prvalue of its underlying type. Moreover, if integral promotion can be applied to its underlying type, a prvalue of an unscoped enumeration type whose underlying type is fixed can also be converted to a prvalue of the promoted underlying type. [ Note: A converted bit-field of enumeration type is treated as any other value of that type for promotion purposes. -- end note ]

4. Change in 7.3.7 [conv.prom] paragraph 5 as follows:

   > A prvalue for an integral bit-field (11.4.10 [class.bit]) converted bit-field of integral type can be converted to a prvalue of type int if int can represent all the values of the bit-field; otherwise, it can be converted to unsigned int if unsigned int can represent all the values of the bit-field. If the bit-field is larger yet, no integral promotion applies to it. If the bit-field has enumeration type, it is treated as any other value of that type for promotion purposes.

5. Move 7.3.7 [conv.prom] paragraph 2 after paragraph 5 and change as follows:

   > A prvalue of type char8_t, char16_t, char32_t, or wchar_t (6.8.2 [basic.fundamental]) (including a converted bit-field that was not already promoted to int or unsigned int according to the rules above) can be converted to a prvalue of the first of the following types that can represent all the values of its underlying type: int, unsigned int, long int, unsigned long int, long long int, or unsigned long long int. If none of the types in that list can represent all the values of its underlying type, a prvalue of type char8_t, char16_t, char32_t, or wchar_t can be converted to a prvalue of , or its underlying type.

#### 170. Pointer-to-member conversions

Section: 7.3.13 [conv.mem] Status: tentatively ready Submitter: Mike Stump Date: 16 Sep 1999

The descriptions of explicit (7.6.1.9 [expr.static.cast] paragraph 9) and implicit (7.3.13 [conv.mem] paragraph 2) pointer-to-member conversions differ in two significant ways:

1. In a `static_cast`, a conversion in which the class in the target pointer-to-member type is a base of the class in which the member is declared is permitted and required to work correctly, as long as the resulting pointer-to-member is eventually dereferenced with an object whose dynamic type contains the member. That is, the class of the target pointer-to-member type is not required to contain the member referred to by the value being converted. The specification of implicit pointer-to-member conversion is silent on this question.

   (This situation cannot arise in an implicit pointer-to-member conversion where the source value is something like `&X::f`, since you can only implicitly convert from pointer-to-base-member to pointer-to-derived-member. However, if the source value is the result of an explicit "up-cast," the target type of the conversion might still not contain the member referred to by the source value.)

2. The target type in a `static_cast` is allowed to be more cv-qualified than the source type; in an implicit conversion, however, the cv-qualifications of the two types are required to be identical.

The first difference seems like an oversight. It is not clear whether the latter difference is intentional or not.

(See also issue 794.)

CWG 2022-11-09

The second concern is NAD; implicit conversions allow chaining a pointer-to-member conversion with a qualification conversion.

Proposed resolution:

1. Change in 7.3.13 [conv.mem] paragraph 2 as follows:

   > A prvalue of type “pointer to member of B of type cv T”, where B is a class type, can be converted to a prvalue of type “pointer to member of D of type cv T”, where D is a complete class derived (11.7 [class.derived]) from B. If B is an inaccessible (11.8 [class.access]), ambiguous (6.5.2 [class.member.lookup]), or virtual (11.7.2 [class.mi]) base class of D, or a base class of a virtual base class of D, a program that necessitates this conversion is ill-formed. If class D does not contain the original member and is not a base class of the class containing the original member, the behavior is undefined. Otherwise, The the result of the conversion refers to the same member as the pointer to member before the conversion took place, but it refers to the base class member as if it were a member of the derived class. The result refers to the member in D's instance of B. Since the result has type “pointer to member of D of type cv T”, indirection through it with a D object is valid. The result is the same as if indirecting through the pointer to member of B with the B subobject of D. The null member pointer value is converted to the null member pointer value of the destination type. [ Footnote: ... ]

2. Change in 7.6.1.9 [expr.static.cast] paragraph 13 as follows:

   > ... If class B contains the original member, or is a base or derived class of the class containing the original member, the resulting pointer to member points to the original member. ...

#### 1973. Which parameter-declaration-clause in a lambda-expression?

Section: 7.5.5.2 [expr.prim.lambda.closure] Status: tentatively ready Submitter: Dinka Ranns Date: 2014-07-16

According to 7.5.5.2 [expr.prim.lambda.closure] paragraph 3,

> The closure type for a lambda-expression has a public inline function call operator (for a non-generic lambda) or function call operator template (for a generic lambda) (12.4.4 [over.call]) whose parameters and return type are described by the lambda-expression's parameter-declaration-clause and trailing-return-type respectively, and whose template-parameter-list consists of the specified template-parameter-list, if any.

This is insufficiently precise because the trailing-return-type might itself contain a parameter-declaration-clause.

Suggested resolution [SUPERSEDED]:

Change in 7.5.5.1 [expr.prim.lambda.general] paragraph 5 as follows:

> If a lambda-declarator does not include a start with a parenthesized parameter-declaration-clause, it is as if `()` were inserted at the start of the lambda-declarator. A lambda-expression's parameter-declaration-clause is the (possibly empty) parameter-declaration-clause of the lambda-expression's lambda-declarator. If the lambda-declarator does not include a trailing-return-type, the lambda return type is auto, which is deduced from return statements as described in 9.2.9.6 [dcl.spec.auto].

Proposed resolution (approved by CWG 2023-02-06):

1. Change in 7.5.5.2 [expr.prim.lambda.closure] paragraph 3 as follows:

   > The closure type for a lambda-expression has a public inline function call operator (for a non-generic lambda) or function call operator template (for a generic lambda) (12.4.4 [over.call]) whose parameters and return type are described by those of the lambda-expression's parameter-declaration-clause and trailing-return-type respectively, and whose template-parameter-list consists of the specified template-parameter-list, if any.

2. Change in 7.5.5.1 [expr.prim.lambda.general] paragraph 5 as follows:

   > If a lambda-declarator does not include a parameter-declaration-clause, it is as if `()` were inserted at the start of the lambda-declarator. A lambda-expression's parameter-declaration-clause is the parameter-declaration-clause of the lambda-expression's lambda-declarator, if any, or empty otherwise. If the lambda-declarator does not include a trailing-return-type, the lambda return type is auto, which is deduced from return statements as described in 9.2.9.6 [dcl.spec.auto].

#### 2476. placeholder-type-specifiers and function declarators

Section: 9.2.9.6.1 [dcl.spec.auto.general] Status: tentatively ready Submitter: Davis Herring Date: 2021-01-29

According to 9.2.9.6.1 [dcl.spec.auto.general] paragraph 3,

> The placeholder type can appear with a function declarator in the decl-specifier-seq, type-specifier-seq, conversion-function-id, or trailing-return-type, in any context where such a declarator is valid. If the function declarator includes a trailing-return-type (9.3.4.6 [dcl.fct]), that trailing-return-type specifies the declared return type of the function. Otherwise, the function declarator shall declare a function.

This wording disallows a declaration like

       int f();
       auto (*fp)()=f;

The requirement to declare a function was introduced by the resolution of issue 1892.

Proposed resolution (April, 2021):

Change 9.2.9.6.1 [dcl.spec.auto.general] paragraph 3 as follows:

> The placeholder type can appear with a function declarator in the decl-specifier-seq, type-specifier-seq, conversion-function-id, or trailing-return-type, in any context where such a declarator is valid if the function declarator includes a trailing-return-type `T` (9.3.4.6 [dcl.fct]) or declares a function. If the function declarator includes a trailing-return-type (9.3.4.6 [dcl.fct]), that trailing-return-type specifies In the former case, `T` is the declared return type of the function. Otherwise, the function declarator shall declare a function. If the declared return type of the a function contains a placeholder type, the return type of the function is deduced from non-discarded `return` statements, if any, in the body of the function (8.5.2 [stmt.if]).

Additional notes (May, 2021):

It was observed that the proposed resolution above does not address the example in the issue, since `fp` neither has a trailing-return-type nor declares a function. Presumably another case in which a function declarator with a placeholder return type should be permitted is in the declaration of a variable in which the type is deduced from its initializer.

It was also noted in passing that the deduction in the example is only partial: the parameter-type-list is specified by the declarator and only the return type is deduced from the initializer. Although this example is supported by current implementations, there is implementation divergence in the support of another case in which only part of the variable's type is deduced:

        auto (&ar)[2] = L"a";  // Array bound declared, element type deduced

This issue is related to issue 1892, which prohibited cases like

        std::vector<auto(*)()> v;

The ultimate outcome of the two issues should be:

        int f();
        auto (*fp1)() = f;       // OK
        auto (*fp2)()->int = f;  // OK
        auto (*fp3)()->auto = f; // OK

        template<typename T> struct C { };
        C<auto(*)()> c1;         // Not OK
        C<auto(*)()->int> c2;    // OK
        C<auto(*)()->auto> c3;   // Not OK

Proposed resolution (January, 2023):

1. Change in 9.2.9.6.1 [dcl.spec.auto.general] paragraph 1 as follows:

   > A placeholder-type-specifier designates a placeholder type that will be replaced later, typically by deduction from an initializer.

2. Change and split 9.2.9.6.1 [dcl.spec.auto.general] paragraph 3 as follows:

   > A placeholder type can appear with a function declarator in the decl-specifier-seq, type-specifier-seq, conversion-function-id, or trailing-return-type, in any context where such for a function declarator is valid that includes a trailing-return-type (9.3.4.6 [dcl.fct]). If the function declarator includes a trailing-return-type (9.3.4.6 [dcl.fct]), that trailing-return-type specifies the declared return type of the function.

   > Otherwise, the A placeholder type can appear in the decl-specifier-seq or type-specifier-seq in the declared return type of a function declarator shall declare that declares a function. If the declared return type of the function contains a placeholder type, ; the return type of the function is deduced from non-discarded `return` statements, if any, in the body of the function (8.5.2 [stmt.if]).

3. Change in 9.2.9.6.1 [dcl.spec.auto.general] paragraph 4 as follows:

   > The type of a variable declared using a placeholder type is deduced from its initializer. This use is allowed in an initializing declaration (9.4 [dcl.init]) of a variable. The placeholder type shall appear as one of the decl-specifiers in the decl-specifier-seq and or as one of the type-specifiers in a trailing-return-type that specifies the type that replaces such a decl-specifier; the decl-specifier-seq shall be followed by one or more declarators, each of which shall be followed by a non-empty initializer. [ Example:

   >       ...
   >       auto f() -> int;                // OK, f returns int
   >       auto (*fp)() -> auto = f;       // OK
   >       ...

   > -- end example ]

4. Change in 9.3.4.6 [dcl.fct] paragraph 1 as follows:

   > In a declaration T D where D has the form

   >     D1 ( parameter-declaration-clause ) cv-qualifier-seqopt
   >       ref-qualifieropt noexcept-specifieropt attribute-specifier-seqopt trailing-return-typeopt

   > and the type of the contained declarator-id in the declaration `T D1` is "derived-declarator-type-list `T`",:
   > - If the trailing-return-type is present, `T` shall be the single type-specifier `auto`, and the declared return type of the function type is the type specified by the trailing-return-type.
   > - Otherwise, the declared return type of the function type is `T`.
   > theThe type of the declarator-id in `D` is "derived-declarator-type-list noexceptopt function of parameter-type-list cv-qualifier-seqopt ref-qualifieropt returning T U", where
   > - the parameter-type-list is derived from the parameter-declaration-clause as described below,
   > - `U` is the declared return type, and
   > - the optional noexcept is present if and only if the exception specification (14.5 [except.spec]) is non-throwing.
   > The optional attribute-specifier-seq appertains to the function type.

5. Remove 9.3.4.6 [dcl.fct] paragraph 2:

   > In a declaration T D where D has the form

   >     D1 ( parameter-declaration-clause ) cv-qualifier-seqopt
   >       ref-qualifieropt noexcept-specifieropt attribute-specifier-seqopt trailing-return-type

   > and the type ... The optional attribute-specifier-seq appertains to the function type.

6. Change in 11.4.8.3 [class.conv.fct] paragraph 1 as follows:

   > A declaration whose declarator-id has an unqualified-id that is a conversion-function-id declares a conversion function; its declarator shall be a function declarator (9.3.4.6 [dcl.fct]) of the form

   >     ptr-declarator noptr-declarator ( parameter-declaration-clause ) cv-qualifier-seqopt
   >           ref-qualifier-seqopt noexcept-specifieropt attribute-specifier-seqopt parameters-and-qualifiers

   > where the ptr-declarator noptr-declarator consists solely of an id-expression, an optional attribute-specifier-seq, and optional surrounding parentheses, and the id-expression has one of the following forms: ...

7. Change in 11.4.8.3 [class.conv.fct] paragraph 2 as follows:

   > A conversion function shall have no non-object parameters and shall be a non-static member function of a class or class template X; it specifies a conversion from X to the type specified by its declared return type is the conversion-type-id, interpreted as a type-id (9.3.2 [dcl.name]) and it specifies a conversion from `X` to the type specified by the conversion-type-id, interpreted as a type-id (9.3.2 [dcl.name]). A decl-specifier in the decl-specifier-seq of a conversion function (if any) shall not be a defining-type-specifier .

8. Remove 11.4.8.3 [class.conv.fct] paragraph 3:

   > The type of the conversion function is “noexceptopt function taking no parameter cv-qualifier-seq opt ref-qualifier opt returning conversion-type-id”.

#### 2663. Example for member redeclarations with using-declarations

Section: 9.9 [namespace.udecl] Status: tentatively ready Submitter: Shafik Yaghmour Date: 2022-11-28

Issue 36 was resolved by P1787R6, but no example was added.

Proposed resolution:

1. Add an example to 9.9 [namespace.udecl] paragraph 8 as follows:

   > [ Example:

   >     struct C {
   >       int i;
   >     };

   >     struct D1 : C { };
   >     struct D2 : C { };

   >     struct D3 : D1, D2 {
   >       using D1::i;   // OK; equivalent to using C::i
   >       using D1::i;   // error: duplicate
   >       using D2::i;   // error: duplicate, also names C::i
   >     };

   > -- end example ]

2. Change the example in 9.9 [namespace.udecl] paragraph 10 as follows:

   >       using B::x;
   >       using A::x;              // OK, hides struct B::x
   >       using A::x;              // OK, does not conflict with previous using A::x
   >       x = 99;                  // assigns to A::x
   >       struct x x1;             // x1 has class type B::x
   >     }

CWG 2023-01-06

There is implementation divergence in handling this example.

CWG 2023-02-07

P1787R6 clarified that the example added to 9.9 [namespace.udecl] paragraph 10 is accepted, even in the non-function case.

#### 1353. Array and variant members and deleted special member functions

Section: 11.4.5 [class.ctor] Status: tentatively ready Submitter: Sean Hunt Date: 2011-08-16

The specification of when a defaulted special member function is to be defined as deleted sometimes overlooks variant and array members.

Proposed resolution (approved by CWG 2023-02-07):

1. Change in 11.4.5.2 [class.default.ctor] paragraph 2 as follows:

   > A defaulted default constructor for class X is defined as deleted if:
   > - X is a union that has a variant member with a non-trivial default constructor and no variant member of X has a default member initializer,
   > - X is a non-union class that has a variant member M with a non-trivial default constructor and no variant member of the anonymous union containing M has a default member initializer,
   > - any non-static data member with no default member initializer (11.4 [class.mem]) is of reference type,
   > - any non-variant non-static data member of const-qualified type (or possibly multi-dimensional array thereof) with no brace-or-equal-initializer is not const-default-constructible (9.4 [dcl.init]),
   > - X is a union and all of its variant members are of const-qualified type (or possibly multi-dimensional array thereof),
   > - X is a non-union class and all members of any anonymous union member are of const-qualified type (or possibly multi-dimensional array thereof),
   > - any potentially constructed subobject, except for a non-static data member with a brace-or-equal-initializer or a variant member of a union where another non-static data member has a brace-or-equal-initializer, has class type M (or possibly multi-dimensional array thereof) and either M has no default constructor or overload resolution (12.2 [over.match]) as applied to find M's corresponding constructor results in an ambiguity or in a function that is deleted or inaccessible from the defaulted default constructor either does not result in a usable candidate (12.2.1 [over.match.general]) or, in the case of a variant member, selects a non-trivial function, or
   > - any potentially constructed subobject has a type with class type M (or possibly multi-dimensional array thereof) and M has a destructor that is deleted or inaccessible from the defaulted default constructor.

2. Change in 11.4.5.3 [class.copy.ctor] paragraph 10 as follows:

   > ... A defaulted copy/move constructor for a class X is defined as deleted (9.5.3 [dcl.fct.def.delete]) if X has:
   > - a potentially constructed subobject of type M (or possibly multi-dimensional array thereof) that cannot be copied/moved because for which overload resolution (12.2 [over.match]), as applied to find M's corresponding constructor, results in an ambiguity or a function that is deleted or inaccessible from the defaulted constructor either does not result in a usable candidate (12.2.1 [over.match.general]) or, in the case of a variant member, selects a non-trivial function,
   > - a variant member whose corresponding constructor as selected by overload resolution is non-trivial,
   > - any potentially constructed subobject of a type with class type M (or possibly multi-dimensional array thereof) where M has a destructor that is deleted or inaccessible from the defaulted constructor, or,
   > - for the copy constructor, a non-static data member of rvalue reference type.

3. Change in 11.4.6 [class.copy.assign] paragraph 7 as follows:

   > A defaulted copy/move assignment operator for class X is defined as deleted if X has:
   > - a variant member with a non-trivial corresponding assignment operator and X is a union-like class, or
   > - a non-static data member of const non-class type (or possibly multi-dimensional array thereof), or
   > - a non-static data member of reference type, or
   > - a direct non-static data member of class type M (or possibly multi-dimensional array thereof) or a direct base class M that cannot be copied/moved because overload resolution (12.2 [over.match]), as applied to find M's corresponding assignment operator, results in an ambiguity or a function that is deleted or inaccessible from the defaulted assignment operator either does not result in a usable candidate (12.2.1 [over.match.general]) or, in the case of a variant member, selects a non-trivial function.

4. Change in 11.4.7 [class.dtor] paragraph 7 as follows:

   > A defaulted destructor for a class X is defined as deleted if:
   > - X is a union-like class that has a variant member with a non-trivial destructor,
   > - any potentially constructed subobject has class type M (or possibly multi-dimensional array thereof) and M has a deleted destructor that is deleted or a destructor that is inaccessible from the defaulted destructor or, in the case of a variant member, is non-trivial,
   > - or, for a virtual destructor, lookup of the non-array deallocation function results in an ambiguity or in a function that is deleted or inaccessible from the defaulted destructor.

### Issues with "Review" Status

#### 2700. `#error` disallows existing implementation practice

Section: 4.1.1 [intro.compliance.general] Status: review Submitter: Richard Smith Date: 2023-02-13 Liaison: WG14

The resolution for issue 2518 disallows existing implementation practice, as detailed below:

- The requirement to issue a diagnostic message for each appearance of `#warning` conflicts with existing implementations having a diagnostic limit. Furthermore, an implementation with an on-the-fly incremental preprocessor is no longer allowed to abort the compilation when an error deemed fatal happens (e.g. a `#include` file that does not exist).
- The wording does not allow for implementation-specific mechanisms for creating a preprocessing translation unit, such as vendor-specific forms of `#if`-like directives and `#pragma once`.

Suggested resolution:

1. Change in 4.1.1 [intro.compliance.general] paragraph 2 as follows:

   > Furthermore, a conforming implementation
   > - shall not accept a preprocessing translation unit containing a #error preprocessing directive (15.8 [cpp.error]), and
   > - shall issue at least one diagnostic message for each #warning or #error preprocessing directive not following a #error preprocessing directive in a preprocessing translation unit, and
   > - shall not accept a translation unit with a `static_assert`-declaration that fails (9.1 [dcl.pre]).

2. Change in 5.1 [lex.separate] paragraph 1 as follows:

   > The text of the program is kept in units called source files in this document. A source file together with all the headers (16.4.2.3 [headers]) and source files included (15.3 [cpp.include]) via the preprocessing directive #include, less any source lines skipped by any of the conditional inclusion (15.2 [cpp.cond]) preprocessing directives or by the implementation-defined behavior of any conditionally-supported-directives (15.1 [cpp.pre]), is called a preprocessing translation unit.

CWG 2023-03-03

Permit that `#warning` can be ignored if another diagnostic is produced.

Proposed resolution:

1. Change in 4.1.1 [intro.compliance.general] bullet 2.3 as follows:

   > - ...
   > - Otherwise, if a program contains
   >   - a violation of any diagnosable rule or,
   >   - a preprocessing translation unit with a `#warning` preprocessing directive (15.8 [cpp.error]), or
   >   - an occurrence of a construct described in this document as “conditionally-supported” when the implementation does not support that construct,
   >   a conforming implementation shall issue at least one diagnostic message.

2. Change in 4.1.1 [intro.compliance.general] paragraph 2 as follows:

   > Furthermore, a conforming implementation shall not accept
   > - a preprocessing translation unit containing a #error preprocessing directive (15.8 [cpp.error]), or
   > - shall issue at least one diagnostic message for each #warning or #error preprocessing directive not following a #error preprocessing directive in a preprocessing translation unit, and
   > - shall not accept a translation unit with a `static_assert`-declaration that fails (9.1 [dcl.pre]).

3. Change in 5.1 [lex.separate] paragraph 1 as follows:

   > The text of the program is kept in units called source files in this document. A source file together with all the headers (16.4.2.3 [headers]) and source files included (15.3 [cpp.include]) via the preprocessing directive #include, less any source lines skipped by any of the conditional inclusion (15.2 [cpp.cond]) preprocessing directives, as modified by the implementation-defined behavior of any conditionally-supported-directives (15.1 [cpp.pre]) and pragmas (15.9 [cpp.pragma]), if any, is called a preprocessing translation unit.

4. Change in 15.8 [cpp.error] as follows:

   > A preprocessing directive of either of the following forms the form

   >     # error pp-tokensopt new-line

   > renders the program ill-formed. A preprocessing directive of the form

   >     # warning pp-tokensopt new-line

   > causes requires the implementation to produce a at least one diagnostic message for the preprocessing translation unit (4.1.1 [intro.compliance.general]) that.

   > Recommended practice: Any diagnostic message caused by either of these directives should include the specified sequence of preprocessing tokens; the `#error` directive renders the program ill-formed.

#### 1924. Definition of “literal” and kinds of literals

Section: 5.13 [lex.literal] Status: review Submitter: Saeed Amrollah Boyouki Date: 2014-05-12 Liaison: editor

The term “literal” is used without definition except the implicit connection with the syntactic nonterminal literal. The relationships of English terms to syntactic nonterminals (such as “integer literal” and integer-literal) should be examined throughout 5.13 [lex.literal] and its subsections.

Notes from the November, 2016 meeting:

This issue will be handled editorially. It is being placed in "review" status until that point.

#### 2698. Using extended integer types with `z` suffix

Section: 5.13.2 [lex.icon] Status: review Submitter: Mike Miller Date: 2023-02-17 Liaison: WG14, EWG

Subclause 5.13.2 [lex.icon] paragraph 4 specifies:

> If an integer-literal cannot be represented by any type in its list and an extended integer type (6.8.2 [basic.fundamental]) can represent its value, it may have that extended integer type. If all of the types in the list for the integer-literal are signed, the extended integer type shall be signed. If all of the types in the list for the integer-literal are unsigned, the extended integer type shall be unsigned. If the list contains both signed and unsigned types, the extended integer type may be signed or unsigned. A program is ill-formed if one of its translation units contains an integer-literal that cannot be represented by any of the allowed types.

This implies that an integer-literal with a `z` suffix can be of extended integer type, if the literal is larger than what is representable in `std::size_t`.

According to the author of the paper P0330R8 (Literal Suffix for (signed) size_t) introducing the feature, this is unintentional; `z` should only yield `std::size_t` and its corresponding signed integer type.

See also the corresponding WG14 paper N2998 Literal Suffixes for size_t.

Proposed resolution (reviewed by CWG 2023-03-03):

Change in 5.13.2 [lex.icon] paragraph 4 as follows:

> If Except for integer-literals containing a size-suffix, if the value of an integer-literal cannot be represented by any type in its list and an extended integer type (6.8.2 [basic.fundamental]) can represent its value, it may have that extended integer type. If all of the types in the list for the integer-literal are signed, the extended integer type shall be is signed. If all of the types in the list for the integer-literal are unsigned, the extended integer type shall be is unsigned. If the list contains both signed and unsigned types, the extended integer type may be signed or unsigned. A program is ill-formed if one of its translation units contains If an integer-literal that cannot be represented by any of the allowed types, the program is ill-formed. [ Note: An integer-literal with a `z` or `Z` suffix is ill-formed if it cannot be represented by `std::size_t`. -- end note ]

Additional notes (February, 2023)

Alerted the chair of SG22 (C/C++ Liaison).

Forwarded to EWG at the request of the EWG chair via cplusplus/papers#1467.

#### 2514. Modifying const subobjects

Section: 6.7.3 [basic.life] Status: review Submitter: Jiang An Date: 2021-11-07 Liaison: SG12

The change in C++20 for RU007 allows transparently replacing a const subobject whose complete object is not const, and the new object can be non-const. However, if the reuse of the object has not happened, modifying such subobjects is still undefined behavior.

This restriction causes problems in the implementation of `std::map` and `std::unordered_map`; see this bug report. Here, the `key_type` objects in map containers are const, and implementations generally can't replace these objects after construction.

Perhaps these restrictions can be relaxed to assist in this case: if

- the complete object of a `key_type` subobject in a container is not const (as the `mapped_type` subobject is not const), or

- the complete object has dynamic storage duration

a const subobject could be modified.

(Is it meaningful to allow a new-expression like `new const int(42)` to create cv-qualified objects? Perhaps such objects should be unqualified, while maintaining the cv-qualification in the type of the expression?)

Notes from the November, 2022 meeting

The advice of SG12 is solicited; see cplusplus/papers#1395.

#### 2677. Replacing union subobjects

Section: 6.7.3 [basic.life] Status: review Submitter: Richard Smith Date: 2022-12-06

The resolution to NB comment US 041 (C++20 CD) does not seem to have fully addressed the original issue, allowing:

      union U { int i, j; };
      U u;
      new (&u) U{.i = 5};
      int k = u.j;  // OK! New U::i transparently replaces existing u.j!

The suggestion is to allow a newly-created complete object to transparently replace any object of the same type in the same storage, except for a potentially-overlapping subobject or a const complete object, and to allow a newly-created subobject to transparently replace only a corresponding subobject of an existing object.

Suggested resolution [SUPERSEDED]:

Change in 6.7.3 [basic.life] paragraph 8 as follows:

> ... An object o1 is transparently replaceable by an object o2 if:
> - o2 is a complete object and:
>   - the storage that o2 occupies exactly overlays the storage that o1 occupied, and
>   - o1 and o2 are of the same type (ignoring the top-level cv-qualifiers), and
>   - o1 is not a const, complete object, and
>   - neither o1 nor o2 is not a potentially-overlapping subobject (6.7.2 [intro.object]), and or
> - either o1 and o2 are both complete objects, or o1 and o2 are corresponding direct subobjects of objects p1 and p2, respectively, and p1 is transparently replaceable by p2.

Additional notes (February, 2023)

The suggested resolution above does not handle the additional example in issue 2676.

Suggested resolution:

1. Change in 6.7.2 [intro.object] paragraph 2 as follows:

   > Objects can contain other objects, called subobjects. A subobject can be a member subobject (11.4 [class.mem]), a base class subobject (11.7 [class.derived]), or an array element. An object that is not a subobject of any other object is called a complete object. If an object is created in storage associated with a member subobject or array element subobject e (which may or may not be within its lifetime), the created object is a subobject of e's containing object if:
   > - the lifetime of e's containing object has begun and not ended, and
   > - the storage for the new object exactly overlays the storage location associated with e, and
   > - e is not a potentially-overlapping subobject, and
   > - the new object is of the same type as e (ignoring cv-qualification).
   > In this case, e and the created object are corresponding direct subobjects.

2. Change in 6.7.3 [basic.life] paragraph 8 as follows:

   > ... An object o1 is transparently replaceable by an object o2 if either
   > - o1 and o2 are complete objects for which:
   >   - o1 is not const,
   >   - the storage that o2 occupies exactly overlays the storage that o1 occupied, and
   >   - o1 and o2 are of the same type (ignoring the top-level cv-qualifiers), and or
   >   - o1 is not a const, complete object, and
   >   - neither o1 nor o2 is a potentially-overlapping subobject (6.7.2 [intro.object]), and
   > - either o1 and o2 are both complete objects, or o1 and o2 are corresponding direct subobjects of objects p1 and p2, respectively, and p1 is transparently replaceable by p2 (6.7.2 [intro.object]) for which:
   >   - the complete object of o1 is not const or
   >   - o1 is a mutable member subobject or a subobject thereof.

#### 2531. Static data members redeclared as constexpr

Section: 9.2.6 [dcl.constexpr] Status: review Submitter: Davis Herring Date: 2022-02-16

C++17 made constexpr static data members implicitly inline (9.2.6 [dcl.constexpr] paragraph 1):

> A function or static data member declared with the `constexpr` or `consteval` specifier is implicitly an inline function or variable (9.2.8 [dcl.inline]).

However, that makes the following well-formed C++14 program ill-formed, no diagnostic required, per 9.2.8 [dcl.inline] paragraph 5:

> If a function or variable with external or module linkage is declared inline in one definition domain, an inline declaration of it shall be reachable from the end of every definition domain in which it is declared; no diagnostic is required.

      // x.hh
      struct X {
        static const int x;
      };

      // TU 1
      #include "x.hh"
      constexpr int X::x{};

      // TU 2
      #include "x.hh"
      int main() { return !&X::x; }

Proposed resolution (approved by CWG 2023-02-07):

Change 9.2.6 [dcl.constexpr] paragraph 1 as follows:

> A function or static data member declared with the `constexpr` or `consteval` specifier on its first declaration is implicitly an inline function or variable (9.2.8 [dcl.inline]).

Drafting note: Functions must be declared `constexpr` on every declaration if on any, so this isn't a change for them.

#### 2412. SFINAE vs undeduced placeholder type

Section: 9.2.9.6 [dcl.spec.auto] Status: review Submitter: Mike Miller Date: 2019-05-03

The status of the following example is not clear:

      template <typename T> auto foo(T);  // Not defined

      template <typename T> struct FooCallable {
        template<class U>
        static constexpr bool check_foo_callable(...) { return false; }

        template<class U, class = decltype(foo(U{})) >
        static constexpr bool check_foo_callable(int) { return true; }

        static constexpr bool value = check_foo_callable<T>(0);
      };
      static_assert(FooCallable<int>::value == false, "");

The `static_assert` causes the evaluation of the default template argument `decltype(foo<int>(int{}))`. However, `foo` is not defined, leaving it with an undeduced placeholder return type. This situation could conceivably be handled in two different ways. According to 9.2.9.6 [dcl.spec.auto] paragraph 9,

> If the name of an entity with an undeduced placeholder type appears in an expression, the program is ill-formed.

This would thus appear to be an invalid expression resulting from substitution in the immediate context of the declaration and thus a substitution failure.

The other alternative would be to treat the presence of an undeduced placeholder type for a function template as satisfying the requirements of 13.9.2 [temp.inst] paragraph 4,

> Unless a function template specialization has been explicitly instantiated or explicitly specialized, the function template specialization is implicitly instantiated when the specialization is referenced in a context that requires a function definition to exist or if the existence of the definition affects the semantics of the program.

and attempt to instantiate `foo<int>`. That instantiation fails because the definition is not provided, which would then be an error outside the immediate context of the declaration and thus a hard error instead of substitution failure.

CWG 2022-11-10

There is no implementation divergence on the handling of this example.

Possible resolution:

Change in 9.2.9.6.1 [dcl.spec.auto.general] paragraph 11 as follows:

> If a variable or function with an undeduced placeholder type is named by an expression (6.3 [basic.def.odr]), the program is ill-formed. Once a non-discarded return statement has been seen in a function, however, the return type deduced from that statement can be used in the rest of the function, including in other return statements. [ Example: ...

>       template <typename T> auto f(T);     // not defined

>       template <typename T> struct F {
>         template<class U>
>         static constexpr bool g(...) { return false; }

>         template<class U, class = decltype(f(U{})) >
>         static constexpr bool g(int) { return true; }

>         static constexpr bool value = g<T>(0);
>       };
>       static_assert(F<int>::value == false, "");

> -- end example ]

#### 2252. Enumeration list-initialization from the same type

Section: 9.4.5 [dcl.init.list] Status: review Submitter: Richard Smith Date: 2016-03-22 Liaison: editor

According to 9.4.5 [dcl.init.list] bullet 3.8,

> Otherwise, if `T` is an enumeration with a fixed underlying type (9.7.1 [dcl.enum]), the initializer-list has a single element `v`, and the initialization is direct-list-initialization, the object is initialized with the value `T(v)` (7.6.1.4 [expr.type.conv]); if a narrowing conversion is required to convert `v` to the underlying type of `T` , the program is ill-formed.

This could be read as requiring that there be a conversion from `v` to the underlying type of `T`, leaving the status of an example like the following unclear:

      enum class E {};
      struct X { operator E(); };
      E{X()}; // ok?

Notes from the March, 2018 meeting:

CWG disagreed that the existing wording requires such a conversion, only that if such a conversion is possble, it must not narrow. A formulation along the lines of “if that initialization involves a narrowing conversion to the underlying type of `T`...” was suggested to clarify the intent. This will be handled editorially, and the issue will be left in "review" status until the change has been verified.

#### 2504. Inheriting constructors from virtual base classes

Section: 11.9.4 [class.inhctor.init] Status: review Submitter: Hubert Tong Date: 2021-11-03

According to 11.9.4 [class.inhctor.init] paragraph 1,

> When a constructor for type `B` is invoked to initialize an object of a different type `D` (that is, when the constructor was inherited (9.9 [namespace.udecl])), initialization proceeds as if a defaulted default constructor were used to initialize the `D` object and each base class subobject from which the constructor was inherited, except that the `B` subobject is initialized by the invocation of the inherited constructor. The complete initialization is considered to be a single function call; in particular, the initialization of the inherited constructor's parameters is sequenced before the initialization of any part of the `D`object.

First, this assumes that the base class constructor will be invoked from the derived class constructor, which will not be true if the base is virtual and initialized by a more-derived constructor.

If the call to the virtual base constructor is omitted, the last sentence is unclear whether the initialization of the base class constructor's parameters by the inheriting constructor occurs or not. There is implementation divergence in the initialization of `V`'s parameter in the following example:

      struct NonTriv {
        NonTriv(int);
        ~NonTriv();
      };
      struct V { V() = default; V(NonTriv); };
      struct Q { Q(); };
      struct A : virtual V, Q {
        using V::V;
        A() : A(42) { }
      };
      struct B : A { };
      void foo() { B b; }

CWG telecon 2022-09-23:

Inheriting constructors from a virtual base class ought to be ill-formed. Inform EWG accordingly.

Possible resolution [SUPERSEDED]:

1. Change in 9.9 [namespace.udecl] paragraph 3 as follows:

   > ... If a using-declarator names a constructor, its nested-name-specifier shall name a direct non-virtual base class of the current class. If the immediate (class) scope is associated with a class template, it shall derive from the specified base class or have at least one dependent base class.

2. Change the example in 11.9.4 [class.inhctor.init] paragraph 1 as follows:

       D2 f(1.0);  // error: B1 has a deleted no default constructor

       struct W { W(int); };
       struct X : virtual W { using W::W; X() = delete; };
       struct Y : X { using X::X; };
       struct Z : Y, virtual W { using Y::Y; };
       Z z(0);  // OK, initialization of Y does not invoke default constructor of X

3. Change the example in 11.9.4 [class.inhctor.init] paragraph 2 as follows:

       struct V1 : virtual B { using B::B; };
       struct V2 : virtual B { using B::B; };

       struct D2 : V1, V2 {
         using V1::V1;
         using V2::V2;
       };
       D1 d1(0);  // error: ambiguous
       D2 d2(0);  // OK, initializes virtual B base class, which initializes the A base class
                  // then initializes the V1 and V2 base classes as if by a defaulted default constructor

CWG telecon 2022-10-07:

Given that there are examples that discuss inheriting constructors from virtual base classes and given the existing normative wording, making it clear that `NonTriv` is not constructed, CWG felt that the implementation divergence is best addressed by amending the examples.

Possible resolution:

Add another example before 11.9.4 [class.inhctor.init] paragraph 2 as follows:

> [ Example:

>     struct NonTriv {
>       NonTriv(int);
>       ~NonTriv();
>     };
>     struct V { V() = default; V(NonTriv); };
>     struct Q { Q(); };
>     struct A : virtual V, Q {
>       using V::V;
>       A() : A(42) { }    // #1, A(42) is equivalent to V(42)
>     };
>     struct B : A { };
>     void foo() { B b; }

> In this example, the `V` subobject of `b` is constructed using the defaulted default constructor. The mem-initializer naming the constructor inherited from `V` at #1 is not evaluated and thus no object of type `NonTriv` is constructed. -- end example ]

> If the constructor was inherited from multiple base class subobjects of type B, the program is ill-formed.

Additional notes (October, 2022)

Possible resolution:

1. Change in 11.9.4 [class.inhctor.init] paragraph 1 as follows:

   > When a constructor for type `B` is invoked to initialize an object of a different type `D` (that is, when the constructor was inherited (9.9 [namespace.udecl])), initialization proceeds as if a defaulted default constructor were used to initialize the `D` object and each base class subobject from which the constructor was inherited, if the base class subobject were to be initialized as part of the `D` object (11.9.3 [class.base.init]), except that the `B` subobject is initialized by the invocation of the inherited constructor. The invocation of the inherited constructor, including the evaluation of any arguments, is omitted if the `B` subobject is not to be initialized as part of the `D` object. The complete initialization is considered to be a single function call; in particular, unless omitted, the initialization of the inherited constructor's parameters is sequenced before the initialization of any part of the `D`object.

2. Add another example before 11.9.4 [class.inhctor.init] paragraph 2 as follows:

   > [ Example:

   >     struct V { V() = default; V(int); };
   >     struct Q { Q(); };
   >     struct A : virtual V, Q {
   >       using V::V;
   >       A() = delete;
   >     };
   >     int bar() { return 42; }
   >     struct B : A {
   >       B() : A(bar()) {}  // ok
   >     };
   >     struct C : B {};
   >     void foo() { C c; } // bar is not invoked, because the V subobject is not initialized as part of B

   > -- end example ]

CWG telecon 2022-10-21:

This is an ABI break for implementations when transitioning to the C++17 model for inheriting constructors.

#### 2703. Three-way comparison requiring strong ordering for floating-point types, take 2

Section: 11.10.3 [class.spaceship] Status: review Submitter: Richard Smith Date: 2023-02-13

The resolution accepted for issue 2539 does not actually address the example in the issue, because overload resolution is never performed for expressions involved only built-in types.

Suggested resolution:

Change in 11.10.3 [class.spaceship] paragraph 1 as follows:

> The synthesized three-way comparison of type R (17.11.2 [cmp.categories]) of glvalues a and b of the same type is defined as follows:
> - If a <=> b is usable (11.10.1 [class.compare.default]) and can be explicitly converted to R using static_cast, `static_cast<R>(a <=> b)`.
> - Otherwise, if `a <=> b` is usable or overload resolution for `a <=> b` is performed and finds at least one viable candidate, the synthesized three-way comparison is not defined.
> - Otherwise, ...

#### 2687. Calling an explicit object member function via an address-of-overload-set

Section: 12.2.2.2.1 [over.match.call.general] Status: review Submitter: Matthew House Date: 2023-01-16

Subclause 12.2.2.2.1 [over.match.call.general] paragraph 2 specifies:

> If the postfix-expression is the address of an overload set, overload resolution is applied using that set as described above. If the function selected by overload resolution is a non-static member function, the program is ill-formed.

However, 7.6.2.2 [expr.unary.op] paragraph 3 states that the address of an explicit object member function is a plain pointer to function, not a pointer to member. The former can be invoked using the regular function call syntax (7.6.1.3 [expr.call]) without the need for a pointer-to-member expression (7.6.4 [expr.mptr.oper]). For example, absent any overloading and given some function `f`, the expression `(&A::f)(A())` could be valid if `f` is a static member function or an explicit object member function. However, that expression cannot possibly be valid if `f` is an implicit object member function.

Proposed resolution (approved by CWG 2023-01-27):

Change in 12.2.2.2.1 [over.match.call.general] paragraph 2 as follows:

> If the postfix-expression is the address of an overload set, overload resolution is applied using that set as described above. If the function selected by overload resolution is a non-static an implicit object member function, the program is ill-formed.

EWG 2023-02-06

Subclause 12.3 [over.over] paragraph 4 says that explicit-object member functions match both kinds of pointer type, and 11.4.3 [class.mfct.non.static] paragraph 2 seems to try to transform `(&A::f)(A())` into `(&(*this).A::f)(A())` if `f` is an explicit-object member function.

Editor's observation: 12.3 [over.over] paragraph 4 was a missed edit applying P0847R7; it was rectified with commit 0c9dd96bb on 2023-01-17.

Additional notes (March, 2023)

The concern about incorrectly applying the `this` transformation was addressed by P2797R0 (Proposed resolution for CWG2692 Static and explicit object member functions with the same parameter-type-lists).

#### 2492. Comparing user-defined conversion sequences in list-initialization

Section: 12.2.4.2.6 [over.ics.list] Status: review Submitter: Jim X Date: 2021-01-11

Consider the following example:

      #include <initializer_list>
      struct A{
        operator short(){
          return 0;
        }
      };
      struct B{
        operator bool(){
          return 0;
        }
      };
      void fun(std::initializer_list<int>){}
      void fun(std::initializer_list<bool>){}
      int main(){
        fun({A{},B{}});
      }

According to 12.2.4.2.6 [over.ics.list] paragraph 6,

> Otherwise, if the parameter type is `std::initializer_list<X>` and all the elements of the initializer list can be implicitly converted to `X`, the implicit conversion sequence is the worst conversion necessary to convert an element of the list to `X`, or if the initializer list has no elements, the identity conversion. This conversion can be a user-defined conversion even in the context of a call to an initializer-list constructor.

In this example, all of the conversions from list elements to the `initializer_list` template argument type are user-defined conversions. According to 12.2.4.3 [over.ics.rank] bullet 3.3,

> User-defined conversion sequence `U1` is a better conversion sequence than another user-defined conversion sequence `U2` if they contain the same user-defined conversion function or constructor or they initialize the same class in an aggregate initialization and in either case the second standard conversion sequence of `U1` is better than the second standard conversion sequence of `U2`.

Since in both cases the two elements of the initializer-list argument involve different user-defined conversion functions, the two user-defined conversion sequences for the elements cannot be distinguished, so the determination of the “worst conversion” for the two candidates does not consider the second standard conversion sequence. This presumably makes it impossible to distinguish the conversion sequences for the two candidates in the function call, making the call ambiguous.

However, there is implementation divergence on the handling of this example, with g++ reporting an ambiguity and clang, MSVC, and EDG calling the `int` overload, presumably on the basis that `short->int` is a promotion while `short->bool` is a conversion.

Notes from the August, 2021 teleconference:

CWG agreed with the reasoning expressed in the analysis, that conversions involving different user-defined conversion functions cannot be compared, and thus the call is ambiguous. The use of the phrase “worst conversion” is insufficiently clear, however, and requires definition.

Proposed resolution, August, 2021:

Change 12.2.4.2.6 [over.ics.list] paragraphs 5 and 6 as follows:

> Otherwise, if the parameter type is `std::initializer_list<X>` and either the initializer list is empty or all the elements of the initializer list can be implicitly converted to `X`, the implicit conversion sequence is the worst conversion worst conversion necessary to convert an element of the list to `X`, or if defined as follows. If the initializer list has no elements, the worst conversion is the identity conversion. Otherwise, the worst conversion is an implicit conversion sequence for a list element that is not better than any other implicit conversion sequence required by list elements, compared as described in 12.2.4.3 [over.ics.rank]. If more than one implicit conversion sequence satisfies this criterion, then if they are user-defined conversion sequences that do not all contain the same user-defined conversion function or constructor, the worst conversion sequence is the ambiguous conversion sequence (12.2.4.2.1 [over.best.ics.general]); otherwise, it is unspecified which of those conversion sequences is chosen as worst. This conversion can be a user-defined conversion even in the context of a call to an initializer-list constructor. [Example 2:

>       void f(std::initializer_list<int>);
>       f( {} );        // OK: f(initializer_list<int>) identity conversion
>       f( {1,2,3} );   // OK: f(initializer_list<int>) identity conversion
>       f( {'a','b'} ); // OK: f(initializer_list<int>) integral promotion
>       f( {1.0} );     // error: narrowing

>       struct A {
>         A(std::initializer_list<double>);            // #1
>         A(std::initializer_list<complex<double>>);   // #2
>         A(std::initializer_list<std::string>);       // #3
>       };
>       A a{ 1.0,2.0 };        // OK, uses #1

>       void g(A);
>       g({ "foo", "bar" });   // OK, uses #3

>       typedef int IA[3];
>       void h(const IA&);
>       h({ 1, 2, 3 });        // OK: identity conversion

>       void x(std::initializer_list<int>);
>       void x(std::initializer_list<bool>);
>       struct S1 { operator short(); };
>       struct S2 { operator bool(); };
>       void y() {
>         x({S1{}, S2{}});   // error: ambiguous. The ICSes for each list element are indistinguishable because
>                            // they do not contain the same conversion function, so the worst conversion is
>                            // the ambiguous conversion sequence.
>       }

> —end example]

> Otherwise, if the parameter type is “array of `N X` ” or “array of unknown bound of `X`”, if there exists an implicit conversion sequence from each element of the initializer list (and from `{}` in the former case if `N` exceeds the number of elements in the initializer list) to `X`, the implicit conversion sequence is the worst such implicit conversion sequence conversion necessary to convert an element of the list (including, if there are too few list elements, `{}`) to `X`, determined as described above for a `std::initializer_list<X>` with a non-empty initializer list.

Editorial issue required: define "worst conversion sequence"

#### 1038. Overload resolution of `&x.static_func`

Section: 12.3 [over.over] Status: review Submitter: Mike Miller Date: 2010-03-02

The Standard is not clear whether the following example is well-formed or not:

        struct S {
            static void f(int);
            static void f(double);
        };
        S s;
        void (*pf)(int) = &s.f;

According to 7.6.1.5 [expr.ref] bullet 4.3, you do function overload resolution to determine whether `x.f` is a static or non-static member function. 7.6.2.2 [expr.unary.op] paragraph 6 says that you can only take the address of an overloaded function in a context that determines the overload to be chosen, and the initialization of a function pointer is such a context (12.3 [over.over] paragraph 1) . The problem is that 12.3 [over.over] is phrased in terms of “an overloaded function name,” and this is a member access expression, not a name.

There is variability among implementations as to whether this example is accepted; some accept it as written, some only if the `&` is omitted, and some reject it in both forms.

Additional note (October, 2010):

A related question concerns an example like

        struct S {
            static void g(int*) {}
            static void g(long) {}
        } s;

        void foo() {
            (&s.g)(0L);
        }

Because the address occurs in a call context and not in one of the contexts mentioned in 12.3 [over.over] paragraph 1, the call expression in `foo` is presumably ill-formed. Contrast this with the similar example

        void g1(int*) {}
        void g1(long) {}

        void foo1() {
            (&g1)(0L);
        }

This call presumably is well-formed because 12.2.2.2 [over.match.call] applies to “the address of a set of overloaded functions.” (This was clearer in the wording prior to the resolution of issue 704: “...in this context using `&F` behaves the same as using the name `F` by itself.”) It's not clear that there's any reason to treat these two cases differently.

This question also bears on the original question of this issue, since the original wording of 12.2.2.2 [over.match.call] also described the case of an ordinary member function call like `s.g(0L)` as involving the “name” of the function, even though the postfix-expression is a member access expression and not a “name.” Perhaps the reference to “name” in 12.3 [over.over] should be similarly understood as applying to member access expressions?

Additional notes (February, 2023)

This appears to be resolved, in part by P1787R6 (accepted November, 2020).

#### 2600. Type dependency of placeholder types

Section: 13.8.3.3 [temp.dep.expr] Status: review Submitter: Hubert Tong Date: 2022-06-18

Subclause 13.8.3.2 [temp.dep.type] paragraph 7 has a list of types considered to be dependent. This list covers placeholder types only insofar as it has an entry about `decltype(expression)`. Subclause 13.8.3.3 [temp.dep.expr] paragraph 3 has a list of expression forms not considered dependent unless specific types named by the expressions are dependent. This list includes forms where placeholder types are allowed. For example, the wording does not say that the new-expression at #1 (below) is dependent, but it ought to be:

      template <typename T> struct A { A(bool, T); };

      void g(...);

      template <typename T>
      auto f(T t) { return g(new A(t, 0)); }  // #1

      int g(A<int> *);
      int h() { return f<void *>(nullptr); }

Some implementation even treats an obviously non-dependent case as dependent:

      template <typename T, typename U> struct A { A(T, U); };

      void g(...); // #1

      template <typename T>
      auto f() { return g(new A(0, 0)); } // #1 or #2?

      int g(A<int, int> *); // #2
      void h() { return f<void *>(); }

A similar example that is non-dependent:

      template <typename T, typename U = T> struct A { A(T, U); };

      void g(...);

      template <typename T>
      auto f() { return g(new A(0, 0)); }

      int g(A<int> *);
      void h() { return f<void *>(); }

And another non-dependent one:

      template <typename T, typename U = T> struct A { A(T); };

      void g(...);

      template <typename T>
      auto f() { return g(new A(0)); }

      int g(A<int> *);
      void h() { return f<void *>(); }

And here is an example that is dependent:

      template<class T>
      struct S {
       template<class U = T> struct A { A(int); };

       auto f() { return new A(0); } // dependent return type
      };

Proposed resolution (November, 2022) [SUPERSEDED]:

1. Change in 7.6.2.8 [expr.new] paragraph 2 as follows:

   > If a placeholder type (9.2.9.6 [dcl.spec.auto]) or a placeholder for a deduced class type (9.2.9.7 [dcl.type.class.deduct]) appears in the type-specifier-seq of a new-type-id or type-id of a new-expression, the allocated type is deduced as follows: Let init be the new-initializer , if any, and T be the new-type-id or type-id of the new-expression, then the allocated type is the type deduced for the variable x in the invented declaration (9.2.9.6 [dcl.spec.auto]):

   >     T x init ;

2. Insert new paragraphs before 13.8.3.2 [temp.dep.type] paragraph 7 and change the latter as follows:

   > An initializer is dependent if any constituent expression (6.9.1 [intro.execution]) of the initializer is type-dependent. A placeholder type (9.2.9.6.1 [dcl.spec.auto.general]) is dependent if it designates a type deduced from a dependent initializer.

   > A placeholder for a deduced class type (9.2.9.7 [dcl.type.class.deduct]) is dependent if

   > - it has a dependent initializer or
   > - any default template-argument of the primary class template named by the placeholder is dependent when considered in the scope enclosing the primary class template.

   > A type is dependent if it is

   > - ...
   > - a function type whose exception specification is value-dependent,
   > - denoted by a dependent placeholder type,
   > - denoted by a dependent placeholder for a deduced class type,
   > - ...

Proposed resolution (February, 2023):

1. Change in 7.6.2.8 [expr.new] paragraph 2 as follows:

   > If a placeholder type (9.2.9.6 [dcl.spec.auto]) or a placeholder for a deduced class type (9.2.9.7 [dcl.type.class.deduct]) appears in the type-specifier-seq of a new-type-id or type-id of a new-expression, the allocated type is deduced as follows: Let init be the new-initializer , if any, and T be the new-type-id or type-id of the new-expression, then the allocated type is the type deduced for the variable x in the invented declaration (9.2.9.6 [dcl.spec.auto]):

   >     T x init ;

2. Insert new paragraphs before 13.8.3.2 [temp.dep.type] paragraph 7 and change the latter as follows:

   > An initializer is dependent if any constituent expression (6.9.1 [intro.execution]) of the initializer is type-dependent. A placeholder type (9.2.9.6.1 [dcl.spec.auto.general]) is dependent if it designates a type deduced from a dependent initializer.

   > A placeholder for a deduced class type (9.2.9.7 [dcl.type.class.deduct]) is dependent if

   > - it has a dependent initializer, or
   > - it refers to an alias template that is a member of the current instantiation whose definition is dependent.

   > [ Example:

   >       template<class T, class V>
   >       struct S { S(T); };

   >       template<class U>
   >       struct A {
   >         template<class T> using X = S<T, U>;
   >         template<class T> using Y = S<T, int>;
   >         void f() {
   >           new X(1);    // dependent
   >           new Y(1);    // not dependent
   >         }
   >       };

   > -- end example ]

   > A type is dependent if it is

   > - ...
   > - a function type whose exception specification is value-dependent,
   > - denoted by a dependent placeholder type,
   > - denoted by a dependent placeholder for a deduced class type,
   > - ...

#### 1602. Linkage of specialization vs linkage of template arguments

Section: 13.9.2 [temp.inst] Status: review Submitter: Richard Smith Date: 2013-01-09

The Standard does not appear to specify the linkage of a template specialization. 13.9.2 [temp.inst] paragraph 11 does say,

> Implicitly instantiated class and function template specializations are placed in the namespace where the template is defined.

which could be read as implying that the specialization has the same linkage as the template itself. Implementation practice seems to be that the weakst linkage of the template and the arguments is used for the specialization.

Additional notes (February, 2023)

Template specializations do not have linkage.

#### 2054. Missing description of class SFINAE

Section: 13.10.3 [temp.deduct] Status: review Submitter: Ville Voutilainen Date: 2014-12-07

Presumably something like the following should be well-formed, where a deduction failure in a partial specialization is handled as a SFINAE case as it is with function templates and not a hard error:

      template <class T, class U> struct X   {
        typedef char member;
      };

      template<class T> struct X<T,
       typename enable_if<(sizeof(T)>sizeof(
         float)), float>::type>
      {
        typedef long long member;
      };

      int main() {
        cout << sizeof(X<double, float>::member);
      }

However, this does not appear to be described anywhere in the Standard.

Additional notes (January, 2023)

The section on SFINAE (13.10.3.1 [temp.deduct.general] paragraph 8) is not specific to function templates, and 13.7.6.2 [temp.spec.partial.match] paragraph 2 hands off the "matching" determination for partial specializations to 13.10.3 [temp.deduct] in general. However, the definition of deduction substitution loci in 13.10.3.1 [temp.deduct.general] paragraph 7 does not account for the template argument list of a partial specialization.

Possible resolution:

Change in 13.10.3.1 [temp.deduct.general] paragraph 7 as follows:

> The deduction substitution loci are
> - the function type outside of the noexcept-specifier,
> - the explicit-specifier, and
> - the template parameter declarations., and
> - the template argument list of a partial specialization (13.7.6.1 [temp.spec.partial.general]).
> The substitution occurs in all types and expressions that are used in the deduction substitution loci. ...

### Issues with "Drafting" Status

#### 369. Are `new`/`delete` identifiers or preprocessing-op-or-punc?

Section: 5.4 [lex.pptoken] Status: drafting Submitter: Martin v. Loewis Date: 30 July 2002

5.4 [lex.pptoken] paragraph 2 specifies that there are 5 categories of tokens in phases 3 to 6. With 5.12 [lex.operators] paragraph 1, it is unclear whether `new` is an identifier or a preprocessing-op-or-punc; likewise for `delete`. This is relevant to answer the question whether

    #define delete foo

is a well-formed control-line, since that requires an identifier after the `define` token.

(See also issue 189.)

#### 1655. Line endings in raw string literals

Section: 5.4 [lex.pptoken] Status: drafting Submitter: Mike Miller Date: 2013-04-26

According to 5.4 [lex.pptoken] paragraph 3,

> If the input stream has been parsed into preprocessing tokens up to a given character:

> - If the next character begins a sequence of characters that could be the prefix and initial double quote of a raw string literal, such as `R"`, the next preprocessing token shall be a raw string literal. Between the initial and final double quote characters of the raw string, any transformations performed in phases 1 and 2 (trigraphs, universal-character-names, and line splicing) are reverted; this reversion shall apply before any d-char, r-char, or delimiting parenthesis is identified.

However, phase 1 is defined as:

> Physical source file characters are mapped, in an implementation-defined manner, to the basic source character set (introducing new-line characters for end-of-line indicators) if necessary. The set of physical source file characters accepted is implementation-defined. Trigraph sequences (_N4140_.2.4 [lex.trigraph]) are replaced by corresponding single-character internal representations. Any source file character not in the basic source character set (5.3 [lex.charset]) is replaced by the universal-character-name that designates that character.

The reversion described in 5.4 [lex.pptoken] paragraph 3 specifically does not mention the replacement of physical end-of-line indicators with new-line characters. Is it intended that, for example, a CRLF in the source of a raw string literal is to be represented as a newline character or as the original characters?

#### 1901. punctuator referenced but not defined

Section: 5.6 [lex.token] Status: drafting Submitter: Richard Smith Date: 2014-03-25

The syntactic nonterminal punctuator appears in the grammar for token in 5.6 [lex.token], but it is nowhere defined. It should be merged with operator and given an appropriate list of tokens as a definition for the merged term.

Proposed resolution (October, 2017):

1. Change 5.5 [lex.digraph] paragraph 2 as follows

> In all respects of the language except in an attribute-token (9.12.1 [dcl.attr.grammar]), each alternative token behaves the same, respectively, as its primary token, except for its spelling.18 The set of alternative tokens...

2. Change the grammar in 5.6 [lex.token] as follows:

token:

identifier
keyword
literal-token
operator
punctuator

literal-token:

integer-literal
floating-literal
character-literal
string-literal
user-defined-literal

symbol: one of

    {    }    [    ]    (    )    ;    :    ...  ?    ::   .    .*
    +    -    *    /    %    ^    &    |    ~    !    =    <    >
    +=   -=   *=   /=   %=   ^=   &=   |=   <<   >>   <<=  >>=
    ==   !=   <=   >=   &&   ||   ++   --   ,    ->*  ->

3. Change 5.6 [lex.token] paragraph 1 as follows:

> There are five four kinds of tokens: identifiers, keywords, literals,19 operators, and other separators and symbols. Blanks, horizontal and vertical tabs, newlines, formfeeds, and comments (collectively, “white space”), as described below, are ignored except as they serve to separate tokens. [Note: Some white space is required to separate otherwise adjacent identifiers, keywords, numeric literals, and alternative tokens containing alphabetic characters. —end note] Each preprocessing-token resulting from translation phase 6 is converted into the corresponding token as follows:

> - If the preprocessing-token is an identifier or is one of the preprocessing-op-or-punc tokens `new` or `delete`, the resulting token is a keyword if it is listed in Table 5, and otherwise is an identifier.

> - Otherwise, if the preprocessing-token is a pp-number with the lexical form of an integer-literal or floating-literal, or is a character-literal or string-literal, the resulting token is of the corresponding form.

> - Otherwise, if the preprocessing-token is a pp-number with the lexical form of a user-defined-integer-literal or user-defined-floating-literal or is a user-defined-character-literal or user-defined-string-literal, the resulting token is a user-defined-literal.

> - Otherwise, if the preprocessing-token is a preprocessing-op-or-punc, and there is a corresponding symbol (after converting alternative token representations to their primary tokens), the resulting token is that symbol.

> - Otherwise, the program is ill-formed.

> [Note: Within an attribute-token (9.12.1 [dcl.attr.grammar]), a token formed from a preprocessing-token that satisfies the syntactic requirements of an identifier is considered to be an identifier with the spelling of the preprocessing-token. —end note]

4. Delete the final sentence of 5.12 [lex.operators] paragraph 1.

> Each preprocessing-op-or-punc is converted to a single token in translation phase 7 (5.2 [lex.phases]).

#### 189. Definition of operator and punctuator

Section: 5.12 [lex.operators] Status: drafting Submitter: Mike Miller Date: 20 Dec 1999

The nonterminals operator and punctuator in 5.6 [lex.token] are not defined. There is a definition of the nonterminal operator in 12.4 [over.oper] paragraph 1, but it is apparent that the two nonterminals are not the same: the latter includes keywords and multi-token operators and does not include the nonoverloadable operators mentioned in paragraph 3.

There is a definition of preprocessing-op-or-punc in 5.12 [lex.operators] , with the notation that

> Each preprocessing-op-or-punc is converted to a single token in translation phase 7 (2.1).

However, this list doesn't distinguish between operators and punctuators, it includes digraphs and keywords (can a given token be both a keyword and an operator at the same time?), etc.

Suggested resolution:

1. Change 12.4 [over.oper] to use the term overloadable-operator.
2. Change 5.6 [lex.token] to use the term operator-token instead of operator (since there are operators that are keywords and operators that are composed of more than one token).
3. Change 5.12 [lex.operators] to define the nonterminals operator-token and punctuator.

Additional note (April, 2005):

The resolution for this problem should also address the fact that `sizeof` and `typeid` (and potentially others like `decltype` that may be added in the future) are described in some places as “operators” but are not listed in 12.4 [over.oper] paragraph 3 among the operators that cannot be overloaded.

(See also issue 369.)

#### 1723. Multicharacter user-defined character literals

Section: 5.13.8 [lex.ext] Status: drafting Submitter: Mike Miller Date: 2013-07-31

According to 5.13.3 [lex.ccon] paragraph 1, a multicharacter literal like `'ab'` is conditionally-supported and has type `int`.

According to 5.13.8 [lex.ext] paragraph 6,

> If L is a user-defined-character-literal, let ch be the literal without its ud-suffix. S shall contain a literal operator (12.6 [over.literal]) whose only parameter has the type of ch and the literal L is treated as a call of the form

> `operator ""` X`(`ch`)`

A user-defined-character-literal like `'ab'_foo` would thus require a literal operator

`operator "" _foo(int)`

However, that is not one of the signatures permitted by 12.6 [over.literal] paragraph 3.

Should multicharacter user-defined-character-literals be conditionally-supported? If so, 12.6 [over.literal] paragraph 3 should be adjusted accordingly. If not, a note in 5.13.8 [lex.ext] paragraph 6 saying explicitly that they are not supported would be helpful.

#### 1735. Out-of-range literals in user-defined-literals

Section: 5.13.8 [lex.ext] Status: drafting Submitter: Mike Miller Date: 2013-08-12

The description of the numeric literals occurring as part of user-defined-integer-literals and user-defined-floating-literals in 5.13.8 [lex.ext] says nothing about whether they are required to satisfy the same constraints as literals that are not part of a user-defined-literal. In particular, because it is the spelling, not the value, of the literal that is used for raw literal operators and literal operator templates, there is no particular reason that they should be restricted to the maximum values and precisions that apply to ordinary literals (and one could imagine that this would be a good notation for allowing literals of extended-precision types).

Is this relaxation of limits intended to be required, or is it a quality-of-implementation issue? Should something be said, either normatively or non-normatively, about this question?

#### 1529. Nomenclature for variable vs reference non-static data member

Section: 6.1 [basic.pre] Status: drafting Submitter: Daniel Krügler Date: 2012-07-24

According to 6.1 [basic.pre] paragraph 6,

> A variable is introduced by the declaration of a reference other than a non-static data member or of an object.

In other words, non-static data members of reference type are not variables. This complicates the wording in a number of places, where the text refers to “variable or data member,” presumably to cover the reference case, but that phrasing could lead to the mistaken impression that all data members are not variables. It would be better if either there were a term for the current phrase “variable or data member” or if there were a less-unwieldy term for “non-static data member of reference type” that could be used in place of “data member” in the current phrasing.

#### 1897. ODR vs alternative tokens

Section: 6.3 [basic.def.odr] Status: drafting Submitter: Hubert Tong Date: 2014-03-21

According to 5.5 [lex.digraph] paragraph 2,

> In all respects of the language, each alternative token behaves the same, respectively, as its primary token, except for its spelling.

However, the primary and alternative tokens are different tokens, which runs afoul of the ODR requirement in 6.3 [basic.def.odr] paragraph 6 that the definitions consist of the “same sequence of tokens.” This wording should be amended to allow for use of primary and alternative tokens.

#### 2480. Lookup for enumerators in modules

Section: 6.5.1 [basic.lookup.general] Status: drafting Submitter: Richard Smith Date: 2021-02-12

According to 6.5.1 [basic.lookup.general] paragraphs 2-3,

> ...A declaration X precedes a program point P in a translation unit L if P follows X, X inhabits a class scope and is reachable from P, or else...

> A single search in a scope S for a name N from a program point P finds all declarations that precede P to which any name that is the same as N (6.1 [basic.pre]) is bound in S.

These rules cause problems for finding enumerators when qualified by an exported name of its enumeration type, unlike a member of a class. For example:

      export module A;
      enum class X { x };
      enum Y { y };

      export module B;
      import A;
      export using XB = X;
      export using YB = Y;

      // client code
      import B;
      int main() {
        XB x = XB::x; // should be OK because definition of X is reachable, even
                      // though A is not imported
        YB y = YB::y; // similarly OK
        YB z = ::y;   // error, because y from module A is not visible
      }

It would seem that this problem could be addressed by changing “inhabits a class scope” to “does not inhabit a namespace scope.”

#### 1089. Template parameters in member selections

Section: 6.5.5.1 [basic.lookup.qual.general] Status: drafting Submitter: Daveed Vandevoorde Date: 2010-06-29

In an example like

        template<typename T> void f(T p)->decltype(p.T::x);

The nested-name-specifier `T::` looks like it refers to the template parameter. However, if this is instantiated with a type like

        struct T { int x; };
        struct S: T { };

the reference will be ambiguous, since it is looked up in both the context of the expression, finding the template parameter, and in the class, finding the base class injected-class-name, and this could be a deduction failure. As a result, the same declaration with a different parameter name

        template<typename U> void f(U p)->decltype(p.U::x);

is, in fact, not a redeclaration because the two can be distinguished by SFINAE.

It would be better to add a new lookup rule that says that if a name in a template definition resolves to a template parameter, that name is not subject to further lookup at instantiation time.

Additional note (November, 2020):

Paper P1787R6, adopted at the November, 2020 meeting, partially addresses this issue.

#### 2324. Size of base class subobject

Section: 6.7.2 [intro.object] Status: drafting Submitter: GB Date: 2017-02-27
P0488R0 comment GB 9

According to 6.7.2 [intro.object] paragraph 7,

> Unless it is a bit-field (11.4.10 [class.bit]), a most derived object shall have a nonzero size and shall occupy one or more bytes of storage. Base class subobjects may have zero size.

Base class objects of zero size is a misleading term, as `sizeof` such an object is non-zero. Size should not be a property of an object, rather of a type.

#### 2325. `std::launder` and reuse of character buffers

Section: 6.7.2 [intro.object] Status: drafting Submitter: CA Date: 2017-02-27
P0488R0 comment CA 12

The status of the following code should be explicitly indicated in the Standard to avoid surprise:

      #include <new>
      int bar() {
        alignas(int) unsigned char space[sizeof(int)];
        int *pi = new (static_cast<void *>(space)) int;
        *pi = 42;
        return [=]() mutable {
          return   *std::launder(reinterpret_cast<int *>(space)); }();
       }

In particular, it appears that the call to `std::launder` has undefined behaviour because the captured copy of space is not established to provide storage for an object of type `int` (sub 6.7.2 [intro.object] paragraph 1). Furthermore, the code has undefined behaviour also because it attempts to access the stored value of the `int` object through a glvalue of an array type other than one of the ones allowed by sub 7.2.1 [basic.lval] paragraph 8.

#### 2469. Implicit object creation vs constant expressions

Section: 6.7.2 [intro.object] Status: drafting Submitter: Hubert Tong Date: 2020-12-07

It is not intended that implicit object creation, as described in 6.7.2 [intro.object] paragraph 10, should occur during constant expression evaluation, but there is currently no wording prohibiting it.

Notes from the February, 2021 teleconference:

This issue was occasioned by issue 2464, which is also the subject of LWG issue 3495. CWG reviewed the proposed resolution and agrees with it. The intended approach for this issue is to wait for LWG to resolve that issue, then add a note in the core section pointing out the implications of that requirement for implicit object creation.

#### 1027. Type consistency and reallocation of scalar types

Section: 6.7.3 [basic.life] Status: drafting Submitter: Gabriel Dos Reis Date: 2010-02-03

Is the following well-formed?

        int f() {
            int i = 3;
            new (&i) float(1.2);
            return i;
        }

The wording that is intended to prevent such shenanigans, 6.7.3 [basic.life] paragraphs 7-9, doesn't quite apply here. In particular, paragraph 7 reads,

> If, after the lifetime of an object has ended and before the storage which the object occupied is reused or released, a new object is created at the storage location which the original object occupied, a pointer that pointed to the original object, a reference that referred to the original object, or the name of the original object will automatically refer to the new object and, once the lifetime of the new object has started, can be used to manipulate the new object, if:

> - the storage for the new object exactly overlays the storage location which the original object occupied, and

> - the new object is of the same type as the original object (ignoring the top-level cv-qualifiers), and...

The problem here is that this wording only applies “after the lifetime of an object has ended and before the storage which the object occupied is reused;” for an object of a scalar type, its lifetime only ends when the storage is reused or released (paragraph 1), so it appears that these restrictions cannot apply to such objects.

(See also issues 1116 and 1338.)

Proposed resolution (August, 2010):

This issue is resolved by the resolution of issue 1116.

#### 1530. Member access in out-of-lifetime objects

Section: 6.7.3 [basic.life] Status: drafting Submitter: Howard Hinnant Date: 2012-07-26

According to 6.7.3 [basic.life] paragraphs 5 and 6, a program has undefined behavior if a pointer or glvalue designating an out-of-lifetime object

> is used to access a non-static data member or call a non-static member function of the object

It is not clear what the word “access” means in this context. A reasonable interpretation might be using the pointer or glvalue as the left operand of a class member access expression; alternatively, it might mean to read or write the value of that member, allowing a class member access expression that is used only to form an address or bind a reference.

This needs to be clarified. A relevant consideration is the recent adoption of the resolution of issue 597, which eased the former restriction on simple address manipulations involving out-of-lifetime objects: if base-class offset calculations are now allowed, why not non-static data member offset calculations?

(See also issue 1531 for other uses of the term “access.”)

Additional note (January, 2013):

A related question is the meaning of the phrase “before the constructor begins execution” in 11.9.5 [class.cdtor] paragraph 1 means:

> For an object with a non-trivial constructor, referring to any non-static member or base class of the object before the constructor begins execution results in undefined behavior.

For example:

      struct DerivedMember { ... };

      struct Base {
        Base(DerivedMember const&);
      };

      struct Derived : Base {
        DerivedMember x;
        Derived() : Base(x) {}
      };

      Derived a;

Is the reference to `Derived::x` in the mem-initializer valid?

Additional note (March, 2013):

This clause is phrased in terms of the execution of the constructor. However, it is possible for an aggregate to have a non-trivial default constructor and be initialized without executing a constructor. The wording needs to be updated to allow for non-constructor initialization to avoid appearing to imply undefined behavior for an example like:

      struct X {
        std::string s;
      } x = {};
      std::string t = x.s;  // No constructor called for x: undefined behavior?

#### 1853. Defining “allocated storage”

Section: 6.7.3 [basic.life] Status: drafting Submitter: Jeffrey Yasskin Date: 2014-02-09

The term “allocated storage” is used in several places in the Standard to refer to memory in which an object may be created (dynamic, static, or automatic storage), but it has no formal definition.

See also issue 2551.

#### 1997. Placement new and previous initialization

Section: 6.7.4 [basic.indet] Status: drafting Submitter: Jason Merrill Date: 2014-09-08

Given the following example,

      #include <new>

      int main() {
        unsigned char buf[sizeof(int)] = {};
        int *ip = new (buf) int;
        return *ip; // 0 or undefined?
      }

Should the preceding initializsation of the buffer carry over to the value of `*ip`? According to 6.7.4 [basic.indet] paragraph 1,

> When storage for an object with automatic or dynamic storage duration is obtained, the object has an indeterminate value, and if no initialization is performed for the object, that object retains an indeterminate value until that value is replaced (7.6.19 [expr.ass]).

In this case, no new storage is being obtained for the `int` object created by the new-expression.

#### 1634. Temporary storage duration

Section: 6.7.5 [basic.stc] Status: drafting Submitter: Richard Smith Date: 2013-03-04

According to 6.7.5 [basic.stc] paragraph 2,

> Static, thread, and automatic storage durations are associated with objects introduced by declarations (6.2 [basic.def]) and implicitly created by the implementation (6.7.7 [class.temporary]).

The apparent intent of the reference to 6.7.7 [class.temporary] is that a temporary whose lifetime is extended to be that of a reference with one of those storage durations is considered also to have that storage duration. This interpretation is buttressed by use of the phrase “an object with the same storage duration as the temporary” (twice) in 6.7.7 [class.temporary] paragraph 5.

There are two problems, however: first, the specification of lifetime extension of temporaries (also in 6.7.7 [class.temporary] paragraph 5) does not say anything about storage duration. Also, nothing is said in either of these locations about the storage duration of a temporary whose lifetime is not extended.

The latter point is important because 6.7.3 [basic.life] makes a distinction between the lifetime of an object and the acquisition and release of the storage the object occupies, at least for objects with non-trivial initialization and/or a non-trivial destructor. The assumption is made in 6.7.7 [class.temporary] and elsewhere that the storage in which a temporary is created is no longer available for reuse, as specified in 6.7.3 [basic.life], after the lifetime of the temporary has ended, but this assumption is not explicitly stated. One way to make that assumption explicit would be to define a storage duration for temporaries whose lifetime is not extended.

See also issues 365 and 2256.

#### 2533. Storage duration of implicitly created objects

Section: 6.7.5 [basic.stc] Status: drafting Submitter: Andrey Erokhin Date: 2022-02-17

In subclause 6.7.2 [intro.object] paragraph 10, operations implicitly creating objects are defined:

> Some operations are described as implicitly creating objects within a specified region of storage. For each operation that is specified as implicitly creating objects, that operation implicitly creates and starts the lifetime of zero or more objects of implicit-lifetime types (6.8.1 [basic.types.general]) in its specified region of storage if...

However, the standard does not specify the storage duration that such an implicitly-created object has; this new method of object creation is not mentioned in 6.7.5.1 [basic.stc.general] paragraph 2:

> Static, thread, and automatic storage durations are associated with objects introduced by declarations (6.2 [basic.def]) and implicitly created by the implementation (6.7.7 [class.temporary]). The dynamic storage duration is associated with objects created by a new-expression (7.6.2.8 [expr.new]).

With the exception of `malloc`, the storage duration should probably be that of the object providing storage (if any), similar to the provision for subobjects in 6.7.5.6 [basic.stc.inherit]:

> The storage duration of subobjects and reference members is that of their complete object (6.7.2 [intro.object]).

The storage duration of an object created by a non-allocating form of an allocation function (17.6.3.4 [new.delete.placement]) should be treated similarly.

Possible resolution:

1. Change in 6.7.2 [intro.object] paragraph 13 as follows:

   > Any implicit or explicit invocation of a function named `operator new` or `operator new[]` implicitly creates objects with dynamic storage duration in the returned region of storage and returns a pointer to a suitable created object.

2. No change in 6.7.5.1 [basic.stc.general] paragraph 2:

   > Static, thread, and automatic storage durations are associated with objects introduced by declarations (6.2 [basic.def]) and implicitly created by the implementation (6.7.7 [class.temporary]). The dynamic storage duration is associated with objects created by a new-expression (7.6.2.8 [expr.new]).

3. Change in 6.7.5.6 [basic.stc.inherit] paragraph 1 as follows:

   > The storage duration of subobjects and reference members is that of their complete object. The storage duration of an object nested within another object x is the storage duration of x (6.7.2 [intro.object]).

4. Change in 7.6.2.8 [expr.new] paragraph 9 as follows:

   > An object created by a new-expression that invokes an allocation function with a non-allocating form (see below) has the storage duration of the object that used to occupy the region of storage where the new object is created. Objects Any other object created by a new-expression have has dynamic storage duration (6.7.5.5 [basic.stc.dynamic]). [Note 5: The lifetime of such an object is not necessarily restricted to the scope in which it is created. —end note]

5. Change in 20.2.12 [c.malloc] paragraph 4 as follows:

   > These functions implicitly create objects (6.7.2 [intro.object]) with dynamic storage duration in the returned region of storage and return a pointer to a suitable created object. In the case of calloc and realloc, the objects are created before the storage is zeroed or copied, respectively.

#### 1676. `auto` return type for allocation and deallocation functions

Section: 6.7.5.5.2 [basic.stc.dynamic.allocation] Status: drafting Submitter: Richard Smith Date: 2013-05-04

Do we need explicit language to forbid `auto` as the return type of allocation and deallocation functions?

(See also issue 1669.)

#### 2073. Allocating memory for exception objects

Section: 6.7.5.5.2 [basic.stc.dynamic.allocation] Status: drafting Submitter: Jonathan Wakely Date: 2015-01-20

According to 6.7.5.5.2 [basic.stc.dynamic.allocation] paragraph 4,

> [Note: In particular, a global allocation function is not called to allocate storage for objects with static storage duration (6.7.5.2 [basic.stc.static]), for objects or references with thread storage duration (6.7.5.3 [basic.stc.thread]), for objects of type `std::type_info` (7.6.1.8 [expr.typeid]), or for an exception object (14.2 [except.throw]). —end note]

The restriction against allocating exception objects on the heap was intended to ensure that heap exhaustion could be reported by throwing an exception, i.e., that obtaining storage for `std::bad_alloc` could not fail because the heap was full. However, this implicitly relied on the assumption of a single thread and does not scale to large numbers of threads, so the restriction should be lifted and another mechanism found for guaranteeing the ability to throw `std::bad_alloc`.

Notes from the February, 2016 meeting:

The prohibition of using an allocation function appears only in a note, although there is a normative reference to the rule in 14.2 [except.throw] paragraph 4. CWG was in favor of retaining the prohibition of using a C++ allocation function for the memory of an exception object, with the implicit understanding that use of `malloc` would be permitted. The resolution for this issue should delete the note and move the prohibition to normative text in the relevant sections.

#### 2042. Exceptions and deallocation functions

Section: 6.7.5.5.3 [basic.stc.dynamic.deallocation] Status: drafting Submitter: Richard Smith Date: 2014-11-13

According to 6.7.5.5.3 [basic.stc.dynamic.deallocation] paragraph 3,

> If a deallocation function terminates by throwing an exception, the behavior is undefined.

This seems to be in conflict with the provisions of 14.5 [except.spec]: if a deallocation function throws an exception that is not allowed by its exception-specification, 14.5 [except.spec] paragraph 10 would appear to give the program defined behavior (calling `std::unexpected()` or `std::terminate()`). (Note that 14.5 [except.spec] paragraph 18 explicitly allows an explicit exception-specification for a deallocation function.)

#### 1211. Misaligned lvalues

Section: 6.7.6 [basic.align] Status: drafting Submitter: David Svoboda Date: 2010-10-20

6.7.6 [basic.align] speaks of “alignment requirements,” and 6.7.5.5.2 [basic.stc.dynamic.allocation] requires the result of an allocation function to point to “suitably aligned” storage, but there is no explicit statement of what happens when these requirements are violated (presumably undefined behavior).

#### 1701. Array vs sequence in object representation

Section: 6.8 [basic.types] Status: drafting Submitter: Lawrence Crowl Date: 2013-06-14

According to 6.8 [basic.types] paragraph 4,

> The object representation of an object of type `T` is the sequence of N `unsigned char` objects taken up by the object of type `T`, where N equals `sizeof(T)`.

However, it is not clear that a “sequence” can be indexed, as an array can and as is required for the implementation of `memcpy` and similar code.

Additional note, November, 2014:

An additional point of concern has been raised as to whether it is appropriate to refer to the constituent bytes of an object as being “objects” themselves, along with the interaction of this specification with copying or not copying parts of the object representation that do not participate in the value representation of the object (“padding” bytes).

#### 1986. odr-use and delayed initialization

Section: 6.9.3.2 [basic.start.static] Status: drafting Submitter: Richard Smith Date: 2014-08-21

The current wording of 6.9.3.2 [basic.start.static] allows deferral of static and thread_local initialization until a variable or function in the containing translation unit is odr-used. This requires implementations to avoid optimizing away the relevant odr-uses. We should consider relaxing the rule to allow for such optimizations.

Proposed resolution (November, 2014):

> For a variable `V` with thread or static storage duration, let `X` be the set of all variables with the same storage duration as `V` that are defined in the same translation unit as `V`. If the observable behavior of the abstract machine (6.7.2 [intro.object]) depends on the value of `V` through an evaluation E, and E is not sequenced before the end of the initialization of any variable in `X`, then the end of the initialization of all variables in `X` is sequenced before E.

There is also a problem (submitted by David Majnemer) if the odr-use occurs in a constexpr context that does not require the variable to be constructed. For example,

      struct A { A(); };
      thread_local A a;

      constexpr bool f() { return &a != nullptr; }

It doesn't seem possible to construct `a` before its odr-use in `f`.

There is implementation divergence in the handling of this example.

Notes from the November, 2014 meeting:

CWG determined that the second part of the issue (involving `constexpr`) is not a defect because the address of an object with thread storage duration is not a constant expression.

Additional note, May, 2015:

CWG failed to indicate where and how to apply the wording in the proposed resolution. In addition, further review has raised concern that “sequenced before” may be the wrong relation to use for the static storage duration case because it implies “in the same thread.”

Notes from the October, 2015 meeting:

The suggested wording is intended to replace some existing wording in 6.9.3.2 [basic.start.static] paragraph 2. CWG affirmed that the correct relationship is “happens before” and not “sequenced before.”

#### 2148. Thread storage duration and order of initialization

Section: 6.9.3.2 [basic.start.static] Status: drafting Submitter: Hubert Tong Date: 2015-06-22

The terms “ordered” and “unordered” initialization are only defined in 6.9.3.2 [basic.start.static] paragraph 2 for entities with static storage duration. They should presumably apply to entities with thread storage duration as well.

#### 2444. Constant expressions in initialization odr-use

Section: 6.9.3.3 [basic.start.dynamic] Status: drafting Submitter: Davis Herring Date: 2019-11-06

According to 6.9.3.3 [basic.start.dynamic] paragraph 3,

> A non-initialization odr-use is an odr-use (6.3 [basic.def.odr]) not caused directly or indirectly by the initialization of a non-local static or thread storage duration variable.

Paragraphs 4-6 uses this term to exclude such odr-uses from consideration in determining the point by which a deferred initialization must be performed. A `static_assert` or a template argument expression can odr-use a variable, but it cannot be said to define any time during execution.

Suggestion: Add constant expression evaluation to the definition. Rename the term to “initializing odr-use” (based on effect rather than cause). Add a note saying that no such odr-use can occur before `main` begins.

Notes from the February, 2021 teleconference:

CWG agreed with the direction.

#### 2503. Unclear relationship among name, qualified name, and unqualified name

Section: 7.5.4 [expr.prim.id] Status: drafting Submitter: Jens Maurer Date: 2021-08-04

The phrases “name”, “qualified name” and “unqualified name” are used in various places. It is not clear that all names are either one or the other; there could, in fact, be a third kind of name that is neither.

See also editorial issue 4793.

#### 2473. Parentheses in pseudo-destructor calls

Section: 7.5.4.4 [expr.prim.id.dtor] Status: drafting Submitter: Mike Miller Date: 2020-12-15

According to 7.5.4.4 [expr.prim.id.dtor] paragraph 2,

> If the id-expression names a pseudo-destructor, `T` shall be a scalar type and the id-expression shall appear as the right operand of a class member access (7.6.1.5 [expr.ref]) that forms the postfix-expression of a function call (7.6.1.3 [expr.call]).

This would appear to make the following example ill-formed, because it is the parenthesized expression and not the class member access that is the postfix-expression in the function call:

      typedef int T;
      void f(int* p) {
        (p->~T)();   // Ill-formed?
      }

Presumably this is an oversight.

#### 2542. Is a closure type a structural type?

Section: 7.5.5.2 [expr.prim.lambda.closure] Status: drafting Submitter: Zhihao Yuan Date: 2022-03-01

Consider:

      template <auto V>
      void foo() {}

      void bar() {
        foo<[i = 3] { return i; }>();
      }

It is unclear whether the data members of a closure type are public or private. This makes a difference, since it affects whether a closure type is a structural type or not (13.2 [temp.param] paragraph 7:

> A structural type is one of the following:
> - a scalar type, or
> - an lvalue reference type, or
> - a literal class type with the following properties:
>   - all base classes and non-static data members are public and non-mutable and
>   - the types of all bases classes and non-static data members are structural types or (possibly multi-dimensional) array thereof.

Possible resolution:

Change in 7.5.5.2 [expr.prim.lambda.closure] paragraph 2 as follows:

> ... The closure type is not an aggregate type (9.4.2 [dcl.init.aggr]) and not a structural type (13.2 [temp.param]). ...

#### 2086. Reference odr-use vs implicit capture

Section: 7.5.5.3 [expr.prim.lambda.capture] Status: drafting Submitter: Hubert Tong Date: 2015-02-14

Whether a reference is odr-used or not has less to do with the context where it is named and more to do with its initializer. In particular, 7.5.5 [expr.prim.lambda] bullet 12.2 leads to cases where references that can never be odr-used are implicitly captured:

> A lambda-expression with an associated capture-default that does not explicitly capture `this` or a variable with automatic storage duration (this excludes any id-expression that has been found to refer to an init-capture's associated non-static data member), is said to implicitly capture the entity (i.e., this or a variable) if the compound-statement:

> - odr-uses (6.3 [basic.def.odr]) the entity, or

> - names the entity in a potentially-evaluated expression (6.3 [basic.def.odr]) where the enclosing full-expression depends on a generic lambda parameter declared within the reaching scope of the lambda-expression.

For example, `ref` should not be captured in the following:

      struct A {
        A() = default;
        A(const A &) = delete;
      } globalA;

      constexpr bool bar(int &, const A &a) { return &a == &globalA; }

      int main() {
        A &ref = globalA;
        [=](auto q) { static_assert(bar(q, ref), ""); }(0);
      }

#### 1521. `T{`expr`}` with reference types

Section: 7.6.1.4 [expr.type.conv] Status: drafting Submitter: Steve Adamczyk Date: 2012-07-10

According to 7.6.1.4 [expr.type.conv] paragraph 4,

> Similarly, a simple-type-specifier or typename-specifier followed by a braced-init-list creates a temporary object of the specified type direct-list-initialized (9.4.5 [dcl.init.list]) with the specified braced-init-list, and its value is that temporary object as a prvalue.

This wording does not handle the case where `T` is a reference type: it is not possible to create a temporary object of that type, and presumably the result would be an xvalue, not a prvalue.

#### 2283. Missing complete type requirements

Section: 7.6.1.4 [expr.type.conv] Status: drafting Submitter: Richard Smith Date: 2016-06-27

P0135R1 (Wording for guaranteed copy elision through simplified value categories) removes complete type requirements from 7.6.1.3 [expr.call] (under the assumption that subclause 9.4 [dcl.init] has them; apparently it does not) and from 7.6.1.8 [expr.typeid] paragraph 3. These both appear to be bad changes and should presumably be reverted.

#### 2557. Class member access referring to an unrelated class

Section: 7.6.1.5 [expr.ref] Status: drafting Submitter: Jens Maurer Date: 2022-03-25

Consider:

      struct A {
        static int x;
      };

      struct B {
        using type = A;
      };

      int y = B().type::x;

There seems to be no requirement that the member named in a class member access actually is a member of the class of the object expression. Subclause 7.5.4.1 [expr.prim.id.general] paragraph 3 does not cover static members:

> An id-expression that denotes a non-static data member or non-static member function of a class can only be used:
> - as part of a class member access in which the object expression refers to the member's class or a class derived from that class, or
> - ...

Suggested resolution:

1. Change in 7.6.1.5 [expr.ref] paragraph 4 as follows:

   > Otherwise, the object expression shall be of class type. The class type shall be complete unless the class member access appears in the definition of that class.
   > [Note: The program is ill-formed if the result differs from that when the class is complete (6.5.2 [class.member.lookup]). —end note]
   > [Note: 6.5.5 [basic.lookup.qual] describes how names are looked up after the `.` and `->` operators. —end note] If `E2` is a qualified-id, the terminal name of its nested-name-specifier shall denote the type of `E1` or a base class thereof.

   > [Example:

   >       struct A {
   >         static int x;
   >       };

   >       struct B {
   >         static int x;
   >       };

   >       struct D : B {
   >         using type = A;
   >       };

   >       int y1 = D().B::x;         // OK, B is a base class of D
   >       int y2 = D().type::x;      // error: A is not a base class of D
   >       int y3 = D::type::x;       // OK, evaluates A::x

   > —end example ]

2. Change in 7.6.1.5 [expr.ref] bullet 6.5 as follows:

   - ...
   - If `E2` is a member an enumerator and the type of `E2` is `T`, the expression `E1.E2` is a prvalue. The type of `E1.E2` is `T`.

     [Example:

           enum E { e };
           struct X {
             using E::e;
           };
           int f(X x) {
             return x.e;
           }

     —end example ]

3. Change in 7.5.4.1 [expr.prim.id.general] paragraph 3 as follows:

   > An id-expression that denotes a non-static data member or non-static member function of a class can only be used:
   > - as part of a class member access (7.6.1.5 [expr.ref]) in which the object expression refers to the member's class [ Footnote: ... ] or a class derived from that class, or
   > - to form a pointer to member (7.6.2.2 [expr.unary.op]), or
   > - if that id-expression denotes a non-static data member and it appears in an unevaluated operand.

#### 1965. Explicit casts to reference types

Section: 7.6.1.7 [expr.dynamic.cast] Status: drafting Submitter: Richard Smith Date: 2014-07-07

The specification of `dynamic_cast` in 7.6.1.7 [expr.dynamic.cast] paragraph 2 (and `const_cast` in 7.6.1.11 [expr.const.cast] is the same) says that the operand of a cast to an lvalue reference type must be an lvalue, so that

      struct A { virtual ~A(); }; A &&make_a();

      A &&a = dynamic_cast<A&&>(make_a());   // ok
      const A &b = dynamic_cast<const A&>(make_a()); // ill-formed

The behavior of `static_cast` is an odd hybrid:

      struct B : A { }; B &&make_b();
      A &&c = static_cast<A&&>(make_b()); // ok
      const A &d = static_cast<const A&>(make_b()); // ok
      const B &e = static_cast<const B&>(make_a()); // ill-formed

(Binding a const lvalue reference to an rvalue is permitted by 7.6.1.9 [expr.static.cast] paragraph 4 but not by paragraphs 2 and 3.)

There is implementation divergence on the treatment of these examples.

Also, `const_cast` permits binding an rvalue reference to a class prvalue but not to any other kind of prvalue, which seems like an unnecessary restriction.

Finally, 7.6.1.9 [expr.static.cast] paragraph 3 allows binding an rvalue reference to a class or array prvalue, but not to other kinds of prvalues; those are covered in paragraph 4. This would be less confusing if paragraph 3 only dealt with binding rvalue references to glvalues and left all discussion of prvalues to paragraph 4, which adequately handles the class and array cases as well.

Notes from the May, 2015 meeting:

CWG reaffirmed the status quo for `dynamic_cast` but felt that `const_cast` should be changed to permit binding an rvalue reference to types that have associated memory (class and array types).

#### 2243. Incorrect use of implicit conversion sequence

Section: 7.6.1.9 [expr.static.cast] Status: drafting Submitter: Hubert Tong Date: 2016-03-08

The term “implicit conversion sequence” is now used in some non-call contexts (e.g., 7.6.1.9 [expr.static.cast] paragraph 4, 7.6.16 [expr.cond] paragraph 4, 7.6.10 [expr.eq] paragraph 4) ) and it is not clear that the current definition is suited for these additional uses. In particular, passing an argument in a function call is always copy-initialization, but some of these contexts require consideration of direct-initialization.

Notes from the December, 2016 teleconference:

The problem is that overload resolution relies on copy initalization and thus does not describe direct initialization. See also issue 1781.

#### 232. Is indirection through a null pointer undefined behavior?

Section: 7.6.2.2 [expr.unary.op] Status: drafting Submitter: Mike Miller Date: 5 Jun 2000

At least a couple of places in the IS state that indirection through a null pointer produces undefined behavior: 6.9.1 [intro.execution] paragraph 4 gives "dereferencing the null pointer" as an example of undefined behavior, and 9.3.4.3 [dcl.ref] paragraph 4 (in a note) uses this supposedly undefined behavior as justification for the nonexistence of "null references."

However, 7.6.2.2 [expr.unary.op] paragraph 1, which describes the unary "*" operator, does not say that the behavior is undefined if the operand is a null pointer, as one might expect. Furthermore, at least one passage gives dereferencing a null pointer well-defined behavior: 7.6.1.8 [expr.typeid] paragraph 2 says

> If the lvalue expression is obtained by applying the unary * operator to a pointer and the pointer is a null pointer value (7.3.12 [conv.ptr]), the `typeid` expression throws the `bad_typeid` exception (17.7.5 [bad.typeid]).

This is inconsistent and should be cleaned up.

Bill Gibbons:

At one point we agreed that dereferencing a null pointer was not undefined; only using the resulting value had undefined behavior.

For example:

        char *p = 0;
        char *q = &*p;

Similarly, dereferencing a pointer to the end of an array should be allowed as long as the value is not used:

        char a[10];
        char *b = &a[10];   // equivalent to "char *b = &*(a+10);"

Both cases come up often enough in real code that they should be allowed.

Mike Miller:

I can see the value in this, but it doesn't seem to be well reflected in the wording of the Standard. For instance, presumably `*p` above would have to be an lvalue in order to be the operand of "`&`", but the definition of "lvalue" in 7.2.1 [basic.lval] paragraph 2 says that "an lvalue refers to an object." What's the object in `*p`? If we were to allow this, we would need to augment the definition to include the result of dereferencing null and one-past-the-end-of-array.

Tom Plum:

Just to add one more recollection of the intent: I was very happy when (I thought) we decided that it was only the attempt to actually fetch a value that creates undefined behavior. The words which (I thought) were intended to clarify that are the first three sentences of the lvalue-to-rvalue conversion, 7.3.2 [conv.lval]:

> An lvalue (7.2.1 [basic.lval]) of a non-function, non-array type `T` can be converted to an rvalue. If `T` is an incomplete type, a program that necessitates this conversion is ill-formed. If the object to which the lvalue refers is not an object of type `T` and is not an object of a type derived from `T`, or if the object is uninitialized, a program that necessitates this conversion has undefined behavior.

In other words, it is only the act of "fetching", of lvalue-to-rvalue conversion, that triggers the ill-formed or undefined behavior. Simply forming the lvalue expression, and then for example taking its address, does not trigger either of those errors. I described this approach to WG14 and it may have been incorporated into C 1999.

Mike Miller:

If we admit the possibility of null lvalues, as Tom is suggesting here, that significantly undercuts the rationale for prohibiting "null references" -- what is a reference, after all, but a named lvalue? If it's okay to create a null lvalue, as long as I don't invoke the lvalue-to-rvalue conversion on it, why shouldn't I be able to capture that null lvalue as a reference, with the same restrictions on its use?

I am not arguing in favor of null references. I don't want them in the language. What I am saying is that we need to think carefully about adopting the permissive approach of saying that it's all right to create null lvalues, as long as you don't use them in certain ways. If we do that, it will be very natural for people to question why they can't pass such an lvalue to a function, as long as the function doesn't do anything that is not permitted on a null lvalue.

If we want to allow `&*(p=0)`, maybe we should change the definition of "`&`" to handle dereferenced null specially, just as `typeid` has special handling, rather than changing the definition of lvalue to include dereferenced nulls, and similarly for the array_end+1 case. It's not as general, but I think it might cause us fewer problems in the long run.

Notes from the October 2003 meeting:

See also issue 315, which deals with the call of a static member function through a null pointer.

We agreed that the approach in the standard seems okay: `p = 0; *p;` is not inherently an error. An lvalue-to-rvalue conversion would give it undefined behavior.

Proposed resolution (October, 2004):

(Note: the resolution of issue 453 also resolves part of this issue.)

1. Add the indicated words to 7.2.1 [basic.lval] paragraph 2:

   > An lvalue refers to an object or function or is an empty lvalue (7.6.2.2 [expr.unary.op]).

2. Add the indicated words to 7.6.2.2 [expr.unary.op] paragraph 1:

   > The unary `*` operator performs indirection: the expression to which it is applied shall be a pointer to an object type, or a pointer to a function type and the result is an lvalue referring to the object or function to which the expression points, if any. If the pointer is a null pointer value (7.3.12 [conv.ptr]) or points one past the last element of an array object (7.6.6 [expr.add]), the result is an empty lvalue and does not refer to any object or function. An empty lvalue is not modifiable. If the type of the expression is “pointer to `T`,” the type of the result is “`T`.” [Note: a pointer to an incomplete type (other than cv void) can be dereferenced. The lvalue thus obtained can be used in limited ways (to initialize a reference, for example); this lvalue must not be converted to an rvalue, see 7.3.2 [conv.lval].—end note]

3. Add the indicated words to 7.3.2 [conv.lval] paragraph 1:

   > If the object to which the lvalue refers is not an object of type `T` and is not an object of a type derived from `T`, or if the object is uninitialized, or if the lvalue is an empty lvalue (7.6.2.2 [expr.unary.op]), a program that necessitates this conversion has undefined behavior.

4. Change 6.9.1 [intro.execution] as indicated:

   > Certain other operations are described in this International Standard as undefined (for example, the effect of dereferencing the null pointer division by zero).

Note (March, 2005):

The 10/2004 resolution interacts with the resolution of issue 73. We added wording to 6.8.4 [basic.compound] paragraph 3 to the effect that a pointer containing the address one past the end of an array is considered to “point to” another object of the same type that might be located there. The 10/2004 resolution now says that it would be undefined behavior to use such a pointer to fetch the value of that object. There is at least the appearance of conflict here; it may be all right, but it at needs to be discussed further.

Notes from the April, 2005 meeting:

The CWG agreed that there is no contradiction between this direction and the resolution of issue 73. However, “not modifiable” is a compile-time concept, while in fact this deals with runtime values and thus should produce undefined behavior instead. Also, there are other contexts in which lvalues can occur, such as the left operand of `.` or `.*`, which should also be restricted. Additional drafting is required.

(See also issue 1102.)

#### 901. Deleted `operator delete`

Section: 7.6.2.8 [expr.new] Status: drafting Submitter: John Spicer Date: 20 May, 2009

It is not clear from 7.6.2.8 [expr.new] whether a deleted `operator delete` is referenced by a new-expression in which there is no initialization or in which the initialization cannot throw an exception, rendering the program ill-formed. (The question also arises as to whether such a new-expression constitutes a “use” of the deallocation function in the sense of 6.3 [basic.def.odr].)

Notes from the July, 2009 meeting:

The rationale for defining a deallocation function as deleted would presumably be to prevent such objects from being freed. Treating the new-expression as a use of such a deallocation function would mean that such objects could not be created in the first place. There is already an exemption from freeing an object if “a suitable deallocation function [cannot] be found;” a deleted deallocation function should be treated similarly.

#### 2102. Constructor checking in new-expression

Section: 7.6.2.8 [expr.new] Status: drafting Submitter: Richard Smith Date: 2015-03-16

According to 7.6.2.8 [expr.new] paragraph 19,

> If the new-expression creates an object or an array of objects of class type, access and ambiguity control are done for the allocation function, the deallocation function (11.4.11 [class.free]), and the constructor (11.4.5 [class.ctor]).

The mention of “the constructor” here is strange. For the “object of class type” case, access and ambiguity control are done when we perform initialization in paragraph 17, and we might not be calling a constructor anyway (for aggregate initialization). This seems wrong.

For the “array of objects of class type” case, it makes slightly more sense (we need to check the trailing array elements can be default-initialized) but again (a) we aren't necessarily using a constructor, (b) we should say which constructor — and we may need overload resolution to find it, and (c) shouldn't this be part of initialization, so we can distinguish between the cases where we should copy-initialize from `{}` and the cases where we should default-initialize?

#### 2281. Consistency of aligned `operator delete` replacement

Section: 7.6.2.8 [expr.new] Status: drafting Submitter: Richard Smith Date: 2016-06-27

We should require that a program that replaces the aligned form of `operator delete` also replaces the sized+aligned form. We only allow a program to replace the non-sized form without replacing the sized form for backwards compatibility. This is not needed for the alignment feature, which is new.

Notes from the March, 2018 meeting:

CWG concurred with the recommendation.

#### 2623. Invoking destroying `operator delete` for constructor failure

Section: 7.6.2.8 [expr.new] Status: drafting Submitter: Blacktea Hamburger Date: 2022-08-25

Subclause 7.6.2.8 [expr.new] paragraph 28 specifies the lookup for the deallocation function that is invoked when the construction of the object in a new-expression exits via an exception. However, a destroying `operator delete` (6.7.5.5.3 [basic.stc.dynamic.deallocation]) should never be used, because the object in question has not yet been fully created.

Suggested resolution [SUPERSEDED]:

Change in 7.6.2.8 [expr.new] paragraph 28 as follows:

> A declaration of a placement deallocation function matches the declaration of a placement allocation function if it has the same number of parameters and, after parameter transformations (9.3.4.6 [dcl.fct]), all parameter types except the first are identical. If the lookup finds a single matching deallocation function, that function will be called; otherwise, no deallocation function will be called. If the lookup finds a usual deallocation function and that function, considered as a placement deallocation function, would have been selected as a match for the allocation function, the program is ill-formed. For a non-placement allocation function, the normal deallocation function lookup is used to find the matching deallocation function (7.6.2.9 [expr.delete]) , except that any destroying operator delete (6.7.5.5.3 [basic.stc.dynamic.deallocation]) is ignored.

#### 2013. Pointer subtraction in large array

Section: 7.6.6 [expr.add] Status: drafting Submitter: Jason Merrill Date: 2014-10-02

The common code sequence used by most implementations for pointer subtraction involves subtracting the pointer values to determine the number of bytes and then shifting to scale for the size of the array element. This produces incorrect results when the difference in bytes is larger than can be represented by a `ptrdiff_t`. For example, assuming a 32-bit `ptrdiff_t`:

      int *a, *b;
      a = malloc(0x21000000 * sizeof(int));
      b = a + 0x21000000;
      printf("%lx\n", (long)(b - a));

This will typically print e1000000 instead of 21000000.

Getting the right answer would require using a more expensive code sequence. It would be better to make this undefined behavior.

#### 2182. Pointer arithmetic in array-like containers

Section: 7.6.6 [expr.add] Status: drafting Submitter: Jonathan Wakely Date: 2015-10-20

The current direction for issue 1776 (see paper P0137) calls into question the validity of doing pointer arithmetic to address separately-allocated but contiguous objects in a container like `std::vector`. A related question is whether there should be some allowance made for allowing pointer arithmetic using a pointer to a base class if the derived class is a standard-layout class with no non-static data members. It is possible that `std::launder` could play a part in the resolution of this issue.

Notes from the February, 2016 meeting:

This issue is expected to be resolved by the resolution of issue 1776. The major problem is when the elements of the vector contain constant or reference members; 6.7.3 [basic.life] paragraph 7 implies that pointer arithmetic leading to such an object produces undefined behavior, and CWG expects this to continue. Some changes to the interface of `std::vector` may be required, perhaps using `std::launder` as part of iterator processing.

#### 2023. Composite reference result type of conditional operator

Section: 7.6.16 [expr.cond] Status: drafting Submitter: Daniel Krügler Date: 2014-10-16

The conditional operator converts pointer operands to their composite pointer type (7.6.16 [expr.cond] bullets 6.3 and 6.4). Similar treatment should be afforded to operands of reference type.

See also issue 2018.

#### 2316. Simplifying class conversions in conditional expressions

Section: 7.6.16 [expr.cond] Status: drafting Submitter: S. B. Tam Date: 2016-08-16

According to 7.6.16 [expr.cond] paragraph 4,

> Attempts are made to form an implicit conversion sequence from an operand expression `E1` of type `T1` to a target type related to the type `T2` of the operand expression `E2` as follows:

> - ...

> - If `E2` is a prvalue or if neither of the conversion sequences above can be formed and at least one of the operands has (possibly cv-qualified) class type:

> - if `T1` and `T2` are the same class type (ignoring cv-qualification) and `T2` is at least as cv-qualified as `T1`, the target type is `T2`,

> - otherwise, if `T2` is a base class of `T1`, the target type is cv1 `T2`, where cv1 denotes the cv-qualifiers of `T1`,

> - otherwise, the target type is the type that `E2` would have after applying the lvalue-to-rvalue (7.3.2 [conv.lval]), array-to-pointer (7.3.3 [conv.array]), and function-to-pointer (7.3.4 [conv.func]) standard conversions.

It seems that to satisfy the conditions in the first two sub-bullets, `T2` must be a class type, in which case `T2` is the same as the type described in the third sub-bullet, since the lvalue-to-rvalue conversion does not change types and the other two conversions do not apply to a class type. Thus, this bullet and sub-bullets could be simplified to:

> - if `E2` is a prvalue or if neither of the conversion sequences above can be formed and at least one of the operands has (possibly cv-qualified) class type, the target type is the type that E2 would have after applying the lvalue-to-rvalue (7.3.2 [conv.lval]), array-to-pointer (7.3.3 [conv.array]), and function-to-pointer (7.3.4 [conv.func]) standard conversions.

Notes from the August, 2020 teleconference:

This issue and suggested resolution predate the resolution of issue 2321, which added the second sub-bullet (the citation above reflects the wording after adoption of issue 2321), giving the result the cv-qualification of `T1` instead of that of `T2`. The suggested resolution would revert that accepted resolution.

#### 1542. Compound assignment of braced-init-list

Section: 7.6.19 [expr.ass] Status: drafting Submitter: Mike Miller Date: 2012-08-21

The specification of 7.6.19 [expr.ass] paragraph 9 is presumably intended to allow use of a braced-init-list as the operand of a compound assignment operator as well as a simple assignment operator, although the normative wording does not explicitly say so. (The example in that paragraph does include

      complex<double> z;
      z += { 1, 2 };      // meaning z.operator+=({1,2})

for instance, which could be read to imply compound assignment operators for scalar types as well.)

However, the details of how this is to be implemented are not clear. Paragraph 7 says,

> The behavior of an expression of the form `E1` op `=` `E2` is equivalent to `E1 = E1` op `E2` except that `E1` is evaluated only once.

Applying this pattern literally to a braced-init-list yields invalid code: `x += {1}` would become `x = x + {1}`, which is non-syntactic.

Another problem is how to apply the prohibition against narrowing conversions to a compound assignment. For example,

      char c;
      c += {1};

would presumably always be a narrowing error, because after integral promotions, the type of `c+1` is `int`. The similar issue 1078 was classified as "NAD" because the workaround was simply to add a cast to suppress the error; however, there is no place to put a similar cast in a compound assignment.

Notes from the October, 2012 meeting:

The incorrect description of the meaning of a compound assignment with a braced-init-list should be fixed by CWG. The question of whether it makes sense to apply narrowing rules to such assignments is better addressed by EWG.

See also issue 2399.

#### 2166. Unclear meaning of “undefined `constexpr` function”

Section: 7.7 [expr.const] Status: drafting Submitter: Howard Hinnant Date: 2015-08-05

According to 7.7 [expr.const] bullet 2.3, an expression is a constant expression unless (among other reasons) it would evaluate

- an invocation of an undefined `constexpr` function or an undefined `constexpr` constructor;

This does not address the question of the point at which a `constexpr` function must be defined. The intent, in order to allow mutually-recursive `constexpr` functions, was that the function must be defined prior to the outermost evaluation that eventually results in the invocation, but this is not clearly stated.

#### 2186. Unclear point that “preceding initialization” must precede

Section: 7.7 [expr.const] Status: drafting Submitter: Hubert Tong Date: 2015-10-24

Similar to the concern of issue 2166, the requirement of 7.7 [expr.const] bullet 2.7.1 for

- a non-volatile glvalue of integral or enumeration type that refers to a complete non-volatile const object with a preceding initialization, initialized with a constant expression, or

does not specify the point at which the determination of “preceding initialization” is made: is it at the point at which the reference to the variable appears lexically, or is it the point at which the outermost constant evaluation occurs? There is implementation divergence on this point.

#### 1680. Including `<initializer_list>` for range-based `for`

Section: 8.6.5 [stmt.ranged] Status: drafting Submitter: Richard Smith Date: 2013-05-13

A simple example like

      int main() {
        int k = 0;
        for (auto x : { 1, 2, 3 })
          k += x;
        return k;
      }

requires that the `<initializer_list>` header be included, because the expansion of the range-based `for` involves a declaration of the form

      auto &&__range = { 1, 2, 3 };

and a braced-init-list causes `auto` to be deduced as a specialization of `std::initializer_list`. This seems unnecessary and could be eliminated by specifying that `__range` has an array type for cases like this.

(It should be noted that EWG is considering a proposal to change `auto` deduction for cases involving braced-init-lists, so resolution of this issue should be coordinated with that effort.)

Notes from the September, 2013 meeting:

CWG felt that this issue should be resolved by using the array variant of the range-based `for` implementation.

#### 2115. Order of implicit destruction vs release of automatic storage

Section: 8.7 [stmt.jump] Status: drafting Submitter: Richard Smith Date: 2015-04-16

The relative ordering between destruction of automatic variables on exit from a block and the release of the variables' storage is not specified by the Standard: are all the destructors executed first and then the storage released, or are they interleaved?

Notes from the February, 2016 meeting:

CWG agreed that the storage should persist until all destructions are complete, although the “as-if” rule would allow for unobservable optimizations of this ordering.

#### 1223. Syntactic disambiguation and trailing-return-types

Section: 8.9 [stmt.ambig] Status: drafting Submitter: Michael Wong Date: 2010-11-08

Because the restriction that a trailing-return-type can appear only in a declaration with “the single type-specifier `auto`” (9.3.4.6 [dcl.fct] paragraph 2) is a semantic, not a syntactic, restriction, it does not influence disambiguation, which is “purely syntactic” (8.9 [stmt.ambig] paragraph 3). Consequently, some previously unambiguous expressions are now ambiguous. For example:

    struct A {
      A(int *);
      A *operator()(void);
      int B;
    };

    int *p;
    typedef struct BB { int C[2]; } *B, C;

    void foo() {
    // The following line becomes invalid under C++0x:
      A (p)()->B;  // ill-formed function declaration

    // In the following,
    // - B()->C is either type-id or class member access expression
    // - B()->C[1] is either type-id or subscripting expression
    // N3126 subclause 8.2 [dcl.ambig.res] does not mention an ambiguity
    // with these forms of expression
      A a(B ()->C);  // function declaration or object declaration
      sizeof(B ()->C[1]);  // sizeof(type-id) or sizeof on an expression
    }

Notes from the March, 2011 meeting:

CWG agreed that the presence of `auto` should be considered in disambiguation, even though it is formally handled semantically rather than syntactically.

#### 2117. Explicit specializations and `constexpr` function templates

Section: 9.2.6 [dcl.constexpr] Status: drafting Submitter: Faisal Vali Date: 2015-04-26

According to 9.2.6 [dcl.constexpr] paragraph 6,

> If no specialization of the template would satisfy the requirements for a constexpr function or constexpr constructor when considered as a non-template function or constructor, the template is ill-formed; no diagnostic required.

This should say “instantiated template specialization” instead of just “specialization” to clarify that an explicit specialization is not in view here.

#### 1348. Use of `auto` in a trailing-return-type

Section: 9.2.9.6 [dcl.spec.auto] Status: drafting Submitter: Richard Smith Date: 2011-08-16

It is not clear whether the `auto` specifier can appear in a trailing-return-type.

#### 1670. `auto` as conversion-type-id

Section: 9.2.9.6 [dcl.spec.auto] Status: drafting Submitter: Richard Smith Date: 2013-04-26
N3690 comment FI 4

The current wording allows something like

      struct S {
        operator auto() { return 0; }
      } s;

If it is intended to be permitted, the details of its handling are not clear. Also, a similar syntax has been discussed as a possible future extension for dealing with proxy types in deduction which, if adopted, could cause confusion.

Additional note, November, 2013:

Doubt was expressed during the 2013-11-25 drafting review teleconference as to the usefulness of this provision. It is therefore being left open for further consideration after C++14 is finalized.

Notes from the February, 2014 meeting:

CWG continued to express doubt as to the usefulness of this construct but felt that if it is permitted, the rules need clarification.

Additional note (December, 2021):

See duplicate issue 2493 for additional details.

#### 1868. Meaning of “placeholder type”

Section: 9.2.9.6 [dcl.spec.auto] Status: drafting Submitter: Dawn Perchik Date: 2014-02-13

9.2.9 [dcl.type] paragraph 2 describes the `auto` specifier as “a placeholder for a type to be deduced.” Elsewhere, the Standard refers to the type represented by the `auto` specifier as a “placeholder type.” This usage has been deemed confusing by some, requiring either a definition of one or both terms or rewording to avoid them.

#### 1488. abstract-pack-declarators in type-ids

Section: 9.3.2 [dcl.name] Status: drafting Submitter: Richard Smith Date: 2012-03-28

The grammar for type-id in 11.3 [class.name] paragraph 1 has two problems. First, the fact that we allow an abstract-pack-declarator makes some uses of type-id (template arguments, alignment specifiers, exception-specifications) ambiguous: `T...` could be parsed either as a type-id, including the ellipsis, or as the type-id `T` with a following ellipsis. There does not appear to be any rule to disambiguate these parses.

The other problem is that we do not allow parentheses in an abstract-pack-declarator, which makes

      template<typename...Ts> void f(Ts (&...)[4]);

ill-formed because `(&...)()` is not an abstract-pack-declarator. There is implementation variance on this point.

#### 453. References may only bind to “valid” objects

Section: 9.3.4.3 [dcl.ref] Status: drafting Submitter: Gennaro Prota Date: 18 Jan 2004

9.3.4.3 [dcl.ref] paragraph 4 says:

> A reference shall be initialized to refer to a valid object or function. [Note: in particular, a null reference cannot exist in a well-defined program, because the only way to create such a reference would be to bind it to the "object" obtained by dereferencing a null pointer, which causes undefined behavior ...]

What is a "valid" object? In particular the expression "valid object" seems to exclude uninitialized objects, but the response to Core Issue 363 clearly says that's not the intent. This is an example (overloading construction on constness of *this) by John Potter, which I think is supposed to be legal C++ though it binds references to objects that are not initialized yet:

     struct Fun {
        int x, y;
        Fun (int x, Fun const&) : x(x), y(42) { }
        Fun (int x, Fun&) : x(x), y(0) { }
      };
      int main () {
        const Fun f1 (13, f1);
        Fun f2 (13, f2);
        cout << f1.y << " " << f2.y << "\n";
      }

Suggested resolution: Changing the final part of 9.3.4.3 [dcl.ref] paragraph 4 to:

> A reference shall be initialized to refer to an object or function. From its point of declaration on (see 6.4.2 [basic.scope.pdecl]) its name is an lvalue which refers to that object or function. The reference may be initialized to refer to an uninitialized object but, in that case, it is usable in limited ways (6.7.3 [basic.life], paragraph 6) [Note: On the other hand, a declaration like this:

>         int & ref = *(int*)0;

> is ill-formed because ref will not refer to any object or function ]

I also think a "No diagnostic is required." would better be added (what about something like int& r = r; ?)

Proposed Resolution (October, 2004):

(Note: the following wording depends on the proposed resolution for issue 232.)

Change 9.3.4.3 [dcl.ref] paragraph 4 as follows:

> A reference shall be initialized to refer to a valid object or function. If an lvalue to which a reference is directly bound designates neither an existing object or function of an appropriate type (9.4.4 [dcl.init.ref]), nor a region of memory of suitable size and alignment to contain an object of the reference's type (6.7.2 [intro.object], 6.7.3 [basic.life], 6.8 [basic.types]), the behavior is undefined. [Note: in particular, a null reference cannot exist in a well-defined program, because the only way to create such a reference would be to bind it to the “object” empty lvalue obtained by dereferencing a null pointer, which causes undefined behavior. As does not designate an object or function. Also, as described in 11.4.10 [class.bit], a reference cannot be bound directly to a bit-field. ]

> The name of a reference shall not be used in its own initializer. Any other use of a reference before it is initialized results in undefined behavior. [Example:

>       int& f(int&);
>       int& g();

>       extern int& ir3;
>       int* ip = 0;

>       int& ir1 = *ip;     // undefined behavior: null pointer
>       int& ir2 = f(ir3);  // undefined behavior: ir3 not yet initialized
>       int& ir3 = g();
>       int& ir4 = f(ir4);  // ill-formed: ir4 used in its own initializer

> —end example]

Rationale: The proposed wording goes beyond the specific concerns of the issue. It was noted that, while the current wording makes cases like `int& r = r;` ill-formed (because `r` in the initializer does not "refer to a valid object"), an inappropriate initialization can only be detected, if at all, at runtime and thus "undefined behavior" is a more appropriate treatment. Nevertheless, it was deemed desirable to continue to require a diagnostic for obvious compile-time cases.

It was also noted that the current Standard does not say anything about using a reference before it is initialized. It seemed reasonable to address both of these concerns in the same wording proposed to resolve this issue.

Notes from the April, 2005 meeting:

The CWG decided that whether to require an implementation to diagnose initialization of a reference to itself should be handled as a separate issue (504) and also suggested referring to “storage” instead of “memory” (because 6.7.2 [intro.object] defines an object as a “region of storage”).

Proposed Resolution (April, 2005):

(Note: the following wording depends on the proposed resolution for issue 232.)

Change 9.3.4.3 [dcl.ref] paragraph 4 as follows:

> A reference shall be initialized to refer to a valid object or function. If an lvalue to which a reference is directly bound designates neither an existing object or function of an appropriate type (9.4.4 [dcl.init.ref]), nor a region of storage of suitable size and alignment to contain an object of the reference's type (6.7.2 [intro.object], 6.7.3 [basic.life], 6.8 [basic.types]), the behavior is undefined. [Note: in particular, a null reference cannot exist in a well-defined program, because the only way to create such a reference would be to bind it to the “object” empty lvalue obtained by dereferencing a null pointer, which causes undefined behavior. As does not designate an object or function. Also, as described in 11.4.10 [class.bit], a reference cannot be bound directly to a bit-field. ]

> Any use of a reference before it is initialized results in undefined behavior. [Example:

>       int& f(int&);
>       int& g();

>       extern int& ir3;
>       int* ip = 0;

>       int& ir1 = *ip;     // undefined behavior: null pointer
>       int& ir2 = f(ir3);  // undefined behavior: ir3 not yet initialized
>       int& ir3 = g();
>       int& ir4 = f(ir4);  // undefined behavior: ir4 used in its own initializer

> —end example]

Note (February, 2006):

The word “use” in the last paragraph of the proposed resolution was intended to refer to the description in 6.3 [basic.def.odr] paragraph 2. However, that section does not define what it means for a reference to be “used,” dealing only with objects and functions. Additional drafting is required to extend 6.3 [basic.def.odr] paragraph 2 to apply to references.

Additional note (May, 2008):

The proposed resolution for issue 570 adds wording to define “use” for references.

Note, January, 2012:

The resolution should also probably deal with the fact that the “one-past-the-end” address of an array does not designate a valid object (even if such a pointer might “point to” an object of the correct type, per 6.8.4 [basic.compound]) and thus is not suuitable for the lvalue-to-rvalue conversion.

#### 1001. Parameter type adjustment in dependent parameter types

Section: 9.3.4.6 [dcl.fct] Status: drafting Submitter: Jason Merrill Date: 2009-11-08

According to 9.3.4.6 [dcl.fct] paragraph 5, top-level cv-qualifiers on parameter types are deleted when determining the function type. It is not clear how or whether this adjustment should be applied to parameters of function templates when the parameter has a dependent type, however. For example:

        template<class T> struct A {
           typedef T arr[3];
        };

        template<class T> void f(const typename A<T>::arr) { } // #1

        template void f<int>(const A<int>::arr);

        template <class T> struct B {
           void g(T);
        };

        template <class T> void B<T>::g(const T) { } // #2

If the `const` in #1 is dropped, `f<int>` has a parameter type of `A*` rather than the `const A*` specified in the explicit instantiation. If the `const` in #2 is not dropped, we fail to match the definition of `B::g` to its declaration.

Rationale (November, 2010):

The CWG agreed that this behavior is intrinsic to the different ways cv-qualification applies to array types and non-array types.

Notes, January, 2012:

Additional discussion of this issue arose regarding the following example:

        template<class T> struct A {
          typedef double Point[2];
          virtual double calculate(const Point point) const = 0;
        };

        template<class T> struct B : public A<T> {
          virtual double calculate(const typename A<T>::Point point) const {
            return point[0];
          }
        };

        int main() {
          B<int> b;
          return 0;
        }

The question is whether the member function in `B<int>` has the same type as that in `A<int>`: is the parameter-type-list instantiated directly (i.e., using the adjusted types) or regenerated from the individual parameter types?

(See also issue 1322.)

#### 1668. Parameter type determination still not clear enough

Section: 9.3.4.6 [dcl.fct] Status: drafting Submitter: Daniel Krügler Date: 2013-04-25

According to 9.3.4.6 [dcl.fct] paragraph 5,

> The type of a function is determined using the following rules. The type of each parameter (including function parameter packs) is determined from its own decl-specifier-seq and declarator. After determining the type of each parameter, any parameter of type “array of `T`” or “function returning `T`” is adjusted to be “pointer to `T`” or “pointer to function returning `T`,” respectively. After producing the list of parameter types, any top-level cv-qualifiers modifying a parameter type are deleted when forming the function type. The resulting list of transformed parameter types and the presence or absence of the ellipsis or a function parameter pack is the function's parameter-type-list. [Note: This transformation does not affect the types of the parameters. For example, `int(*)(const int p, decltype(p)*)` and `int(*)(int, const int*)` are identical types. —end note]

This is not sufficiently clear to specify the intended handling of an example like

      void f(int a[10], decltype(a) *p );

Should the type of `p` be `int(*)[10]` or `int**`? The latter is the intended result, but the phrase “after determining the type of each parameter” makes it sound as if the adjustments are performed after all the parameter types have been determined from the decl-specifier-seq and declarator instead of for each parameter individually.

See also issue 1444.

#### 2537. Overbroad grammar for parameter-declaration

Section: 9.3.4.6 [dcl.fct] Status: drafting Submitter: Davis Herring Date: 2021-02-25

9.3.4.6 [dcl.fct] paragraph 3 specifies the grammar for parameter-declaration:

      parameter-declaration:
          attribute-specifier-seqopt thisopt decl-specifier-seq declarator
          attribute-specifier-seqopt thisopt decl-specifier-seq declarator = initializer-clause
          attribute-specifier-seqopt thisopt decl-specifier-seq abstract-declaratoropt
          attribute-specifier-seqopt thisopt decl-specifier-seq abstract-declaratoropt = initializer-clause

This is overly permissive; using a defining-type-specifier-seq instead of a decl-specifier-seq is sufficient.

Proposed resolution (November, 2022):

1. Change in 9.2.2 [dcl.stc] paragraph 4 as follows:

   > There can be no static function declarations within a block, nor any static function parameters.

2. Change in 9.2.2 [dcl.stc] paragraph 5 as follows:

   > The `extern` specifier shall not be used in the declaration of a class member or function parameter.

3. Change in 9.2.4 [dcl.typedef] paragraph 1 as follows:

   > The `typedef` specifier shall not be combined in a decl-specifier-seq with any other kind of specifier except a defining-type-specifier, and it shall not be used in the decl-specifier-seq of a parameter-declaration (9.3.4.6 [dcl.fct]) nor in the decl-specifier-seq of a function-definition (9.5 [dcl.fct.def]).

4. Change in 9.2.8 [dcl.inline] paragraph 4 as follows:

   > The `inline` specifier shall not appear on a block scope declaration or on the declaration of a function parameter.

5. Change in 9.3.4.6 [dcl.fct] paragraph 3 as follows:

         parameter-declaration:
             attribute-specifier-seqopt thisopt decl-specifier-seq defining-type-specifier-seq declarator
             attribute-specifier-seqopt thisopt decl-specifier-seq defining-type-specifier-seq declarator = initializer-clause
             attribute-specifier-seqopt thisopt decl-specifier-seq defining-type-specifier-seq abstract-declaratoropt
             attribute-specifier-seqopt thisopt decl-specifier-seq defining-type-specifier-seq abstract-declaratoropt = initializer-clause

CWG 2023-02-07

Additional drafting is needed to address references to decl-specifier-seq in other parts of the standard. A list is here. Furthermore, reducing the grammar to a type-specifier-seq appears to be sufficient.

#### 325. When are default arguments parsed?

Section: 9.3.4.7 [dcl.fct.default] Status: drafting Submitter: Nathan Sidwell Date: 27 Nov 2001

The standard is not precise enough about when the default arguments of member functions are parsed. This leads to confusion over whether certain constructs are legal or not, and the validity of certain compiler implementation algorithms.

9.3.4.7 [dcl.fct.default] paragraph 5 says "names in the expression are bound, and the semantic constraints are checked, at the point where the default argument expression appears"

However, further on at paragraph 9 in the same section there is an example, where the salient parts are

      int b;
      class X {
        int mem2 (int i = b); // OK use X::b
        static int b;
      };

which appears to contradict the former constraint. At the point the default argument expression appears in the definition of X, X::b has not been declared, so one would expect ::b to be bound. This of course appears to violate 6.4.7 [basic.scope.class] paragraph 1(2) "A name N used in a class S shall refer to the same declaration in its context and when reevaluated in the complete scope of S. No diagnostic is required."

Furthermore 6.4.7 [basic.scope.class] paragraph 1(1) gives the scope of names declared in class to "consist not only of the declarative region following the name's declarator, but also of .. default arguments ...". Thus implying that X::b is in scope in the default argument of X::mem2 previously.

That previous paragraph hints at an implementation technique of saving the token stream of a default argument expression and parsing it at the end of the class definition (much like the bodies of functions defined in the class). This is a technique employed by GCC and, from its behaviour, in the EDG front end. The standard leaves two things unspecified. Firstly, is a default argument expression permitted to call a static member function declared later in the class in such a way as to require evaluation of that function's default arguments? I.e. is the following well formed?

      class A {
        static int Foo (int i = Baz ());
        static int Baz (int i = Bar ());
        static int Bar (int i = 5);
     };

If that is well formed, at what point does the non-sensicalness of

      class B {
        static int Foo (int i = Baz ());
        static int Baz (int i = Foo());
      };

become detected? Is it when B is complete? Is it when B::Foo or B::Baz is called in such a way to require default argument expansion? Or is no diagnostic required?

The other problem is with collecting the tokens that form the default argument expression. Default arguments which contain template-ids with more than one parameter present a difficulty in determining when the default argument finishes. Consider,

      template <int A, typename B> struct T { static int i;};
      class C {
        int Foo (int i = T<1, int>::i);
      };

The default argument contains a non-parenthesized comma. Is it required that this comma is seen as part of the default argument expression and not the beginning of another of argument declaration? To accept this as part of the default argument would require name lookup of T (to determine that the '<' was part of a template argument list and not a less-than operator) before C is complete. Furthermore, the more pathological

      class D {
        int Foo (int i = T<1, int>::i);
        template <int A, typename B> struct T {static int i;};
      };

would be very hard to accept. Even though T is declared after Foo, T is in scope within Foo's default argument expression.

Suggested resolution:

Append the following text to 9.3.4.7 [dcl.fct.default] paragraph 8.

> The default argument expression of a member function declared in the class definition consists of the sequence of tokens up until the next non-parenthesized, non-bracketed comma or close parenthesis. Furthermore such default argument expressions shall not require evaluation of a default argument of a function declared later in the class.

This would make the above A, B, C and D ill formed and is in line with the existing compiler practice that I am aware of.

Notes from the October, 2005 meeting:

The CWG agreed that the first example (`A`) is currently well-formed and that it is not unreasonable to expect implementations to handle it by processing default arguments recursively.

Additional notes, May, 2009:

Presumably the following is ill-formed:

        int f(int = f());

However, it is not clear what in the Standard makes it so. Perhaps there needs to be a statement to the effect that a default argument only becomes usable after the complete declarator of which it is a part.

Notes from the August, 2011 meeting:

In addition to default arguments, commas in template argument lists also cause problems in initializers for nonstatic data members:

        struct S {
          int n = T<a,b>(c);  // ill-formed declarator for member b
                              // or template argument?
        };

(This is from #16 of the `IssuesFoundImplementingC0x.pdf` document on the Bloomington wiki.

Additional notes (August, 2011):

See also issues 1352 and 361.

Notes from the February, 2012 meeting:

It was decided to handle the question of parsing an initializer like `T<a,b>(c)` (a template-id or two declarators) in this issue and the remaining questions in issue 361. For this issue, a template-id will only be recognized if there is a preceding declaration of a template.

Additional note (November, 2020):

Paper P1787R6, adopted at the November, 2020 meeting, partially addresses this issue.

#### 1580. Default arguments in explicit instantiations

Section: 9.3.4.7 [dcl.fct.default] Status: drafting Submitter: Daveed Vandevoorde Date: 2012-10-29

It is not clear, either from 9.3.4.7 [dcl.fct.default] or 13.9.3 [temp.explicit], whether it is permitted to add a default argument in an explicit instantiation of a function template:

      template<typename T> void f(T, int) { }
      template void f<int>(int, int=0);  // Permitted?

Notes from the April, 2013 meeting:

The intent is to prohibit default arguments in explicit instantiations.

#### 2327. Copy elision for direct-initialization with a conversion function

Section: 9.4 [dcl.init] Status: drafting Submitter: Richard Smith Date: 2016-09-30

Consider an example like:

      struct Cat {};
      struct Dog { operator Cat(); };

      Dog d;
      Cat c(d);

This goes to 9.4 [dcl.init] bullet 17.6.2:

> Otherwise, if the initialization is direct-initialization, or if it is copy-initialization where the cv-unqualified version of the source type is the same class as, or a derived class of, the class of the destination, constructors are considered. The applicable constructors are enumerated (12.2.2.4 [over.match.ctor]), and the best one is chosen through overload resolution (12.2 [over.match]). The constructor so selected is called to initialize the object, with the initializer expression or expression-list as its argument(s). If no constructor applies, or the overload resolution is ambiguous, the initialization is ill-formed.

Overload resolution selects the move constructor of `Cat`. Initializing the `Cat&&` parameter of the constructor results in a temporary, per 9.4.4 [dcl.init.ref] bullet 5.2.1.2. This precludes the possitiblity of copy elision for this case.

This seems to be an oversight in the wording change for guaranteed copy elision. We should presumably be simultaneously considering both constructors and conversion functions in this case, as we would for copy-initialization, but we'll need to make sure that doesn't introduce any novel problems or ambiguities.

See also issue 2311.

#### 2128. Imprecise rule for reference member initializer

Section: 9.4.2 [dcl.init.aggr] Status: drafting Submitter: Richard Smith Date: 2015-05-19

According to 11.9.3 [class.base.init] paragraph 11,

> A temporary expression bound to a reference member from a brace-or-equal-initializer is ill-formed. [Example:

>       struct A {
>         A() = default;          // OK
>         A(int v) : v(v) { }     // OK
>         const int& v = 42;      // OK
>       };
>       A a1;                     // error: ill-formed binding of temporary to reference
>       A a2(1);                  // OK, unfortunately

> —end example]

The rule is intended to apply only if an actual initialization results in such a binding, but it could be read as applying to the declaration of `A::v` itself. It would be clearer if the restriction were moved into bullet 9.1, e.g.,

- if the entity is a non-static data member that has a brace-or-equal-initializer and either

- the constructor's class is a union (11.5 [class.union]), and no other variant member of that union is designated by a mem-initializer-id or

- the constructor's class is not a union, and, if the entity is a member of an anonymous union, no other member of that union is designated by a mem-initializer-id,

the entity is initialized as specified in 9.4 [dcl.init], and the program is ill-formed if the entity is a reference member and this initialization binds it to a temporary;

#### 2149. Brace elision and array length deduction

Section: 9.4.2 [dcl.init.aggr] Status: drafting Submitter: Vinny Romano Date: 2015-06-25

According to 9.4.2 [dcl.init.aggr] paragraph 4,

> An array of unknown size initialized with a brace-enclosed initializer-list containing `n` initializer-clauses, where `n` shall be greater than zero, is defined as having `n` elements (9.3.4.5 [dcl.array]).

However, the interaction of this with brace elision is not clear. For instance, in the example in paragraph 7,

      struct X { int i, j, k = 42; };
      X a[] = { 1, 2, 3, 4, 5, 6 };
      X b[2] = { { 1, 2, 3 }, { 4, 5, 6 } };

`a` and `b` are said to have the same value, even though there are six initializer-clauses in the initializer list in `a`'s initializer and two in `b`'s initializer.

Similarly, 13.10.3.2 [temp.deduct.call] paragraph 1 says,

> in the `P'[N]` case, if `N` is a non-type template parameter, `N` is deduced from the length of the initializer list

Should that take into account the underlying type of the array? For example,

      template<int N> void f1(const X(&)[N]);
      f1({ 1, 2, 3, 4, 5, 6 }); // Is N deduced to 2 or 6?

      template<int N> void f2(const X(&)[N][2]);
      f2({ 1, 2, 3, 4, 5, 6 }); // Is N deduced to 1 or 6?

#### 1304. Omitted array bound with string initialization

Section: 9.4.3 [dcl.init.string] Status: drafting Submitter: Nikolay Ivchenkov Date: 2011-04-26

The example in 9.4.3 [dcl.init.string] paragraph 1 says,

>       char msg[] = "Syntax error on line %s\n";

> shows a character array whose members are initialized with a string-literal. Note that because `'\n'` is a single character and because a trailing `'\0'` is appended, `sizeof(msg)` is `25`.

However, there appears to be no normative specification of how the size of the array is to be calculated.

#### 1414. Binding an rvalue reference to a reference-unrelated lvalue

Section: 9.4.4 [dcl.init.ref] Status: drafting Submitter: Mike Miller Date: 2011-11-09

Currently an attempt to bind an rvalue reference to a reference-unrelated lvalue succeeds, binding the reference to a temporary initialized from the lvalue by copy-initialization. This appears to be intentional, as the accompanying example contains the lines

        int i3 = 2;
        double&& rrd3 = i3;  // rrd3 refers to temporary with value 2.0

This violates the expectations of some who expect that rvalue references can be initialized only with rvalues. On the other hand, it is parallel with the handling of an lvalue reference-to-const (and is handled by the same wording). It also can add efficiency without requiring existing code to be rewritten: the implicitly-created temporary can be moved from, just as if the call had been rewritten to create a prvalue temporary from the lvalue explicitly.

On a related note, assuming the binding is permitted, the intent of the overload tiebreaker found in 12.2.4.3 [over.ics.rank] paragraph 3 is not clear:

- Standard conversion sequence `S1` is a better conversion sequence than standard conversion sequence `S2` if

- ...

- `S1` and `S2` are reference bindings (9.4.4 [dcl.init.ref]) and neither refers to an implicit object parameter of a non-static member function declared without a ref-qualifier, and `S1` binds an rvalue reference to an rvalue and `S2` binds an lvalue reference.

At question is what “to an rvalue” means here. If it is referring to the value category of the initializer itself, before conversions, then the supposed performance advantage of the binding under discussion does not occur because the competing rvalue and lvalue reference overloads will be ambiguous:

        void f(int&&);    // #1
        void f(const int&);
        void g(double d) {
            f(d);         // ambiguous: #1 does not bind to an rvalue
        }

On the other hand, if “to an rvalue” refers to the actual object to which the reference is bound, i.e., to the temporary in the case under discussion, the phrase would seem to be vacuous because an rvalue reference can never bind directly to an lvalue.

Notes from the February, 2012 meeting:

CWG agreed that the binding rules are correct, allowing creation of a temporary when binding an rvalue reference to a non-reference-related lvalue. The phrase “to an rvalue” in 12.2.4.3 [over.ics.rank] paragraph 3 is a leftover from before binding an rvalue reference to an lvalue was prohibited and should be removed. A change is also needed to handle the following case:

        void f(const char (&)[1]);         // #1
        template<typename T> void f(T&&);  // #2
        void g() {
          f("");                           //calls #2, should call #1
        }

Additional note (October, 2012):

Removing “to an rvalue,” as suggested, would have the effect of negating the preference for binding a function lvalue to an lvalue reference instead of an rvalue reference because the case would now fall under the preceding bullet of 12.2.4.3 [over.ics.rank] bullet 3.1, sub-bullets 4 and 5:

> Two implicit conversion sequences of the same form are indistinguishable conversion sequences unless one of the following rules applies:

> - Standard conversion sequence `S1` is a better conversion sequence than standard conversion sequence `S2` if

> - ...

> - `S1` and `S2` are reference bindings (9.4.4 [dcl.init.ref]) and neither refers to an implicit object parameter of a non-static member function declared without a ref-qualifier, and `S1` binds an rvalue reference to an rvalue and `S2` binds an lvalue reference... or, if not that,

> - `S1` and `S2` are reference bindings (9.4.4 [dcl.init.ref]) and `S1` binds an lvalue reference to a function lvalue and `S2` binds an rvalue reference to a function lvalue.

Presumably if the suggested resolution is adopted, the order of these two bullets should be inverted.

#### 1827. Reference binding with ambiguous conversions

Section: 9.4.4 [dcl.init.ref] Status: drafting Submitter: Hubert Tong Date: 2014-01-07

In the following case,

      struct A {
        operator int &&() const;
        operator int &&() volatile;
        operator long();
      };

      int main() {
        int &&x = A();
      }

the conversion for direct binding cannot be used because of the ambiguity, so indirect binding is used, which allows the use of the conversion to `long` in creating the temporary.

Is this intended? There is implementation variation.

Notes from the February, 2014 meeting:

CWG agreed that an ambiguity like this should make the initialization ill-formed instead of falling through to do indirect binding.

#### 2018. Qualification conversion vs reference binding

Section: 9.4.4 [dcl.init.ref] Status: drafting Submitter: Richard Smith Date: 2014-10-07

Qualification conversions are not considered when doing reference binding, which leads to some unexpected results:

      template<typename T> T make();
      struct B {}; struct D : B {};

      const int *p1 = make<int*>();           // ok, qualification conversion
      const int *const *p2 = make<int**>();   // ok, qualification conversion
      const int **p3 = make<int**>();         // error, not type safe

      const int &r1 = make<int&>();           // ok, binds directly
      const int *const &r2 = make<int*&>();   // weird, binds to a temporary
      const int *&r3 = make<int*&>();         // error

      const int &&x1 = make<int&&>();         // ok, binds directly
      const int *const &&x2 = make<int*&&>(); // weird, binds to a temporary
      const int *&&x3 = make<int*&&>();       // weird, binds to a temporary

It might make sense to say that similar types are reference-related and if there is a qualification conversion they are reference-compatible.

See also issue 2023.

#### 1996. Reference list-initialization ignores conversion functions

Section: 9.4.5 [dcl.init.list] Status: drafting Submitter: Richard Smith Date: 2014-09-04

The specification for list-initialization of a reference does not consider the existence of conversion functions. Consequently, the following example is ill-formed:

      struct S { operator struct D &(); } s;
      D &d{s};

#### 2144. Function/variable declaration ambiguity

Section: 9.5.1 [dcl.fct.def.general] Status: drafting Submitter: Richard Smith Date: 2015-06-19

The following fragment,

      int f() {};

is syntactically ambiguous. It could be either a function-definition followed by an empty-declaration, or it could be a simple-declaration whose init-declarator has the brace-or-equal-initializer `{}`. The same is true of a variable declaration

      int a {};

since function-definition simply uses the term declarator in its production.

#### 1854. Disallowing use of implicitly-deleted functions

Section: 9.5.2 [dcl.fct.def.default] Status: drafting Submitter: Richard Smith Date: 2014-02-11

The resolution of issue 1778 means that whether an explicitly-defaulted function is deleted or not cannot be known until the end of the class definition. As a result, new rules are required to disallow references (in, e.g., `decltype`) to explicitly-defaulted functions that might later become deleted.

Notes from the June, 2014 meeting:

The approach favored by CWG was to make any reference to an explicitly-defaulted function ill-formed if it occurs prior to the end of the class definition.

#### 1485. Out-of-class definition of member unscoped opaque enumeration

Section: 9.7.1 [dcl.enum] Status: drafting Submitter: Richard Smith Date: 2012-03-26

The scope in which the names of enumerators are entered for a member unscoped opaque enumeration is not clear. According to 9.7.1 [dcl.enum] paragraph 10,

> Each enum-name and each unscoped enumerator is declared in the scope that immediately contains the enum-specifier.

In the case of a member opaque enumeration defined outside its containing class, however, it is not clear whether the enumerator names are declared in the class scope or in the lexical scope containing the definition. Declaring them in the class scope would be a violation of 11.4 [class.mem] paragraph 1:

> The member-specification in a class definition declares the full set of members of the class; no member can be added elsewhere.

Declaring the names in the lexical scope containing the definition would be contrary to the example in 13.7.2.6 [temp.mem.enum] paragraph 1:

      template<class T> struct A {
        enum E : T;
      };
      A<int> a;
      template<class T> enum A<T>::E : T { e1, e2 };
      A<int>::E e = A<int>::e1;

There also appear to be problems with the rules for dependent types and members of the current instantiation.

Notes from the October, 2012 meeting:

CWG agreed that an unscoped opaque enumeration in class scope should be forbidden.

#### 2131. Ambiguity with opaque-enum-declaration

Section: 9.7.1 [dcl.enum] Status: drafting Submitter: Richard Smith Date: 2015-05-28

The declaration

      enum E;

is ambiguous: it could be either a simple-declaration comprising the elaborated-type-specifier `enum E` and no init-declarator-list, or it could be an opaque-enum-declaration with an omitted enum-base (both of which are ill-formed, for different reasons).

(See also issue 2363.)

#### 2505. Nested unnamed namespace of inline unnamed namespace

Section: 9.8.2.2 [namespace.unnamed] Status: drafting Submitter: Nathan Sidwell Date: 2021-11-22

According to 9.8.2.2 [namespace.unnamed] paragraph 1,

> An unnamed-namespace-definition behaves as if it were replaced by

> `inline`opt `namespace` unique `{ /* empty body */ }`
> `using namespace` unique `;`
> `namespace` unique `{` namespace-body `}`

> where `inline` appears if and only if it appears in the unnamed-namespace-definition and all occurrences of unique in a translation unit are replaced by the same identifier, and this identifier differs from all other identifiers in the translation unit.

The use of a single identifier for all occurrences of unique within a translation unit leads to problems when an inline unnamed namespace contains a nested unnamed namespace, e.g.,

        inline namespace {
          namespace { }
        }

In this case, the unnamed namespace cannot be reopened because the lookup for unique finds both the outer and inner namespaces and is thus ambiguous.

Suggested resolution:

Change 9.8.2.2 [namespace.unnamed] paragraph 1 as follows:

> ...where `inline` appears if and only if it appears in the unnamed-namespace-definition and all occurrences of unique in each scope in a translation unit are replaced by the same scope-specific identifier, and this identifier differs from all other identifiers in the translation unit.

Notes from the December, 2021 teleconference:

The suggested resolution deals specifically with unnamed namespaces, but there are related examples that do not involve unnamed namespaces. The problem needs to be solved more generally in the specification of lookup.

#### 1817. Linkage specifications and nested scopes

Section: 9.11 [dcl.link] Status: drafting Submitter: Richard Smith Date: 2013-12-04

According to 9.1 [dcl.pre] paragraph 2,

> Unless otherwise stated, utterances in Clause 9 [dcl.dcl] about components in, of, or contained by a declaration or subcomponent thereof refer only to those components of the declaration that are not nested within scopes nested within the declaration.

This contradicts the intent of 9.11 [dcl.link] paragraph 4, which says,

> In a linkage-specification, the specified language linkage applies to the function types of all function declarators, function names with external linkage, and variable names with external linkage declared within the linkage-specification.

Also, one of the comments in the example in paragraph 4 is inconsistent with the intent:

      extern "C" {
        static void f4(); // the name of the function f4 has
                          // internal linkage (not C language
                          // linkage) and the function's type
                          // has C language linkage.
      }

      extern "C" void f5() {
        extern void f4(); // OK: Name linkage (internal)
                          // and function type linkage (C
                          // language linkage) gotten from
                          // previous declaration.
      }

The language linkage for the block-scope declaration of `f4` is presumably determined by the fact that it appears in a C-linkage function, not by the previous declaration.

Proposed resolution (February, 2014):

Change 9.11 [dcl.link] paragraph 4 as follows:

> Linkage specifications nest. When linkage specifications nest, the innermost one determines the language linkage. A linkage specification does not establish a scope. A linkage-specification shall occur only in namespace scope (6.4 [basic.scope]). In a linkage-specification, the specified language linkage applies to the function types of all function declarators, function names with external linkage, and variable names with external linkage declared within the linkage-specification, including those appearing in scopes nested inside the linkage specification and not inside a nested linkage-specification. [Example:

> ...

>       extern "C" {
>         static void f4(); // the name of the function f4 has
>                           // internal linkage (not C language
>                           // linkage) and the function's type
>                           // has C language linkage.
>       }

>       extern "C" void f5() {
>         extern void f4(); // OK: Name linkage (internal)
>                           // and function type linkage (C
>                           // language linkage) gotten from
>                           // previous declaration.; function type
>                           // linkage (C language
>                           // linkage) gotten
>                           // from linkage specification
>       }

Additional note, November, 2014:

The issue has been returned to "drafting" status to clarify the circumstances under which a preceding declaration supplies the language linkage for a declaration (for example, not when the declaration uses a typedef, which carries the language linkage, but only when the declaration uses a function declarator).

#### 1706. `alignas` pack expansion syntax

Section: 9.12.1 [dcl.attr.grammar] Status: drafting Submitter: Daveed Vandevoorde Date: 2013-06-26

The grammar for alignment-specifier in 9.12.1 [dcl.attr.grammar] paragraph 1 is:

alignment-specifier:

`alignas (` type-id `...`opt`)`
`alignas (` assignment-expression `...`opt`)`

where the ellipsis indicates pack expansion. Naively, one would expect that the expansion would result in forms like

        alignas()
        alignas(1, 2)
        alignas(int, double)

but none of those forms is given any meaning by the current wording. Instead, 13.7.4 [temp.variadic] paragraph 4 says,

> In an alignment-specifier (9.12.2 [dcl.align]); the pattern is the alignment-specifier without the ellipsis.

Presumably this means that something like `alignas(T...)` would expand to something like

        alignas(int) alignas(double)

This is counterintuitive and should be reexamined.

See also messages 24016 through 24021.

Notes from the February, 2014 meeting:

CWG decided to change the pack expansion of `alignas` so that the type-id or assignment-expression is repeated inside the parentheses and to change the definition of `alignas` to accept multiple arguments with the same meaning as multiple `alignas` specifiers.

#### 2223. Multiple `alignas` specifiers

Section: 9.12.2 [dcl.align] Status: drafting Submitter: Mike Herrick Date: 2016-01-12

According to 9.12.2 [dcl.align] paragraph 4,

> The alignment requirement of an entity is the strictest non-zero alignment specified by its alignment-specifiers, if any; otherwise, the alignment-specifiers have no effect.

It is not clear whether this applies to specifiers within a single declaration, or if it is intended to apply to the union of all declarations.

Similarly, paragraph 6 says,

> If the defining declaration of an entity has an alignment-specifier, any non-defining declaration of that entity shall either specify equivalent alignment or have no alignment-specifier. Conversely, if any declaration of an entity has an alignment-specifier, every defining declaration of that entity shall specify an equivalent alignment. No diagnostic is required if declarations of an entity have different alignment-specifiers in different translation units.

This only talks about agreement between definitions and non-defining declarations. What about an example where an entity is not defined but is declared with different alignment-specifiers?

      struct alignas(16) A;
      struct alignas(32) A;

If `A` is not defined, is this, or should it be, ill-formed?

Notes from the February, 2017 meeting:

CWG agreed that the intent of the wording is that the “strictest” requirement is intended to apply to a single declaration, and the requirement for compatibility should apply to all declarations, whether the entity is defined or not.

#### 2607. Visibility of enumerator names

Section: 10.2 [module.interface] Status: drafting Submitter: Richard Smith Date: 2022-06-28

Consider:

      // module interface unit
      export module M;
      export enum E : int;
      enum E : int { e };

      // other translation unit
      import M;
      auto a = E::e;  // #1: OK?
      auto b = e;     // #2: OK?

It is unclear whether the enumerator name `e` is or ought to be visible in the other translation unit.

See also issues 2588 (friend declarations) and 2480.

CWG 2022-11-10

See 10.2 [module.interface] paragraph 7.

#### 1890. Member type depending on definition of member function

Section: 11.4 [class.mem] Status: drafting Submitter: Hubert Tong Date: 2014-03-07

Consider an example like:

      struct A {
        struct B {
          auto foo() { return 0; }
        };
        decltype(B().foo()) x;
      };

There does not appear to be a prohibition of cases like this, where the type of a member depends on the definition of a member function.

(See also issues 1360, 1397, and 2335.)

Additional notes (January, 2023):

The following example might be related:

      #include <type_traits>

      struct Bar {
        struct Baz {
          int a = 0;
        };
        static_assert(std::is_default_constructible_v<Baz>);
      };

#### 1623. Deleted default union constructor and member initializers

Section: 11.4.5 [class.ctor] Status: drafting Submitter: Vinny Romano Date: 2013-02-15

According to 11.4.5 [class.ctor] paragraph 5,

> A defaulted default constructor for class `X` is defined as deleted if:

> - `X` is a union-like class that has a variant member with a non-trivial default constructor,

> - ...

> - `X` is a union and all of its variant members are of const-qualified type (or array thereof),

> - `X` is a non-union class and all members of any anonymous union member are of const-qualified type (or array thereof),

> - ...

Because the presence of a non-static data member initializer is the moral equivalent of a mem-initializer, these rules should probably be modified not to define the generated constructor as deleted when a union member has a non-static data member initializer. (Note the non-normative references in 11.5 [class.union] paragraphs 2-3 and 9.2.9.2 [dcl.type.cv] paragraph 2 that would also need to be updated if this restriction is changed.)

It would also be helpful to add a requirement to 11.5 [class.union] requiring either a non-static data member initializer or a user-provided constructor if all the members of the union have const-qualified types.

On a more general note, why is the default constructor defined as deleted just because a member has a non-trivial default constructor? The union itself doesn't know which member is the active one, and default construction won't initialize any members (assuming no brace-or-equal-initializer). It is up to the “owner” of the union to control the lifetime of the active member (if any), and requiring a user-provided constructor is forcing a design pattern that doesn't make sense. Along the same lines, why is the default destructor defined as deleted just because a member has a non-trivial destructor? I would agree with this restriction if it only applied when the union also has a user-provided constructor.

See also issues 1460, 1562, 1587, and 1621.

#### 1808. Constructor templates vs default constructors

Section: 11.4.5 [class.ctor] Status: drafting Submitter: Richard Smith Date: 2013-11-12

It is not clear when, if ever, a constructor template can be considered to provide a default constructor. For example:

      struct A {
        template<typename ...T> A(T...); // #1
        A(std::initializer_list<long>);  // #2
      };
      A a{};

According to 9.4.5 [dcl.init.list] paragraph 3, `A` will be value-initialized if it has a default constructor, and there is implementation divergence whether this example calls #1 or #2.

Similarly, for an example like

      struct B {
        template<typename T=int> B(T = 0);
      };

it is not completely clear whether a default constructor should be implicitly declared or not.

More generally, do utterances in the Standard concerning “constructors” also apply to constructor templates?

Notes from the February, 2014 meeting:

One possibility discussed was that we may need to change places that explicitly refer to a default constructor to use overload resolution, similar to the change that was made a few years ago with regard to copy construction vs “copy constructor.” One additional use of “default constructor” is in determining the triviality of a class, but it might be a good idea to remove the concept of a trivial class altogether. This possibility will be explored.

Notes from the February, 2016 meeting:

CWG reaffirmed the direction from the preceding note and also determined that the presence of a constructor template should suppress implicit declaration of a default constructor.

#### 1092. Cycles in overload resolution during instantiation

Section: 11.4.5.3 [class.copy.ctor] Status: drafting Submitter: Jason Merrill Date: 2010-07-15

Moving to always doing overload resolution for determining exception specifications and implicit deletion creates some unfortunate cycles:

        template<typename T> struct A {
           T t;
        };

        template <typename T> struct B {
           typename T::U u;
        };

        template <typename T> struct C {
           C(const T&);
        };

        template <typename T> struct D {
           C<B<T> > v;
        };

        struct E {
           typedef A<D<E> > U;
        };

        extern A<D<E> > a;
        A<D<E> > a2(a);

If declaring the copy constructor for `A<D<E>>` is part of instantiating the class, then we need to do overload resolution on `D<E>`, and thus `C<B<E>>`. We consider `C(const B<E>&)`, and therefore look to see if there's a conversion from `C<B<E>>` to `B<E>`, which instantiates `B<E>`, which fails because it has a field of type `A<D<E>>` which is already being instantiated.

Even if we wait until `A<D<E>>` is considered complete before finalizing the copy constructor declaration, declaring the copy constructor for `B<E>` will want to look at the copy constructor for `A<D<E>>`, so we still have the cycle.

I think that to avoid this cycle we need to short-circuit consideration of `C(const T&)` somehow. But I don't see how we can do that without breaking

        struct F {
           F(F&);
        };

        struct G;
        struct G2 {
           G2(const G&);
        };

        struct G {
           G(G&&);
           G(const G2&);
        };

        struct H: F, G { };

        extern H h;
        H h2(h);

Here, since `G`'s move constructor suppresses the implicit copy constructor, the defaulted `H` copy constructor calls `G(const G2&)` instead. If the move constructor did not suppress the implicit copy constructor, I believe the implicit copy constructor would always be viable, and therefore a better match than a constructor taking a reference to another type.

So perhaps the answer is to reconsider that suppression and then disqualify any constructor taking (a reference to) a type other than the constructor's class from consideration when looking up a subobject constructor in an implicitly defined constructor. (Or assignment operator, presumably.)

Another possibility would be that when we're looking for a conversion from `C<B<E>>` to `B<E>` we could somehow avoid considering, or even declaring, the `B<E>` copy constructor. But that seems a bit dodgy.

Additional note (October, 2010):

An explicitly declared move constructor/op= should not suppress the implicitly declared copy constructor/op=; it should cause it to be deleted instead. This should prevent a member function taking a (reference to) an un-reference-related type from being chosen by overload resolution in a defaulted member function.

And we should clarify that member functions taking un-reference-related types are not even considered during overload resolution in a defaulted member function, to avoid requiring their parameter types to be complete.

#### 1548. Copy/move construction and conversion functions

Section: 11.4.5.3 [class.copy.ctor] Status: drafting Submitter: Nikolay Ivchenkov Date: 2012-09-02

The current wording of 11.4.5.3 [class.copy.ctor] paragraph 31 refers only to constructors and destructors:

> When certain criteria are met, an implementation is allowed to omit the copy/move construction of a class object, even if the constructor selected for the copy/move operation and/or the destructor for the object have side effects.

However, in some cases (e.g., `auto_ptr`) a conversion function is also involved in the copying, and it could presumably also have visible side effects that would be eliminated by copy elision. (Some additional contexts that may also require changes in this regard are mentioned in the resolution of issue 535.)

Additional note (September, 2012):

The default arguments of an elided constructor can also have side effects and should be mentioned, as well; however, the elision should not change the odr-use status of functions and variables appearing in those default arguments.

#### 1594. Lazy declaration of special members vs overload errors

Section: 11.4.5.3 [class.copy.ctor] Status: drafting Submitter: Richard Smith Date: 2012-12-06

The implicit declaration of a special member function sometimes requires overload resolution, in order to select a special member to use for base classes and non-static data members. This can be required to determine whether the member is or would be deleted, and whether the member is trivial, for instance. The standard appears to require such overload resolution be performed at the end of the definition of the class, but in practice, implementations perform it lazily. This optimization appears to be non-conforming, in the case where overload resolution would hit an error. In order to enable this optimization, such errors should be “no diagnostic required.”

Additional note (March, 2013):

See also issue 1360.

Notes from the September, 2013 meeting:

The problem with this approach is that hard errors (not in the immediate context) can occur, affecting portability. There are some cases, such as a virtual assignment operator in the base class, where lazy evaluation cannot be done, so it cannot be mandated.

#### 2203. Defaulted copy/move constructors and UDCs

Section: 11.4.5.3 [class.copy.ctor] Status: drafting Submitter: Vinny Romano Date: 2015-11-20

Consider:

      struct A
      {
        A();
        A(A&);
        explicit A(int);
        operator int() const;
      };
      struct B
      {
        B(B&& other);
        A a;
      };
      B::B(B&& other) : a(static_cast<B&&>(other).a) {}
      // B::B(B&& other) = default; // ill-formed

      void f(B& b1)
      {
        B b2 = static_cast<B&&>(b1);
      }

The user-defined move constructor is well-formed because `B::a` can be initialized via `A::operator int()` and `A::A(int)`; however, Clang and GCC believe a defaulted one would be ill-formed.

What about the following, which is considered well-formed by compilers and calls `A::A(C&&)`?

      struct C {};

      struct A : C
      {
        A();
        A(A&);
        A(C&&);
      };
      struct B
      {
        B(B&& other);
        A a;
      };

      B::B(B&& other) = default;

#### 2264. Memberwise copying with indeterminate value

Section: 11.4.5.3 [class.copy.ctor] Status: drafting Submitter: Hubert Tong Date: 2016-05-06

It appears that the following example may have unwanted undefined behavior in C++, although not in C:

      struct A { int x, y; };
      A passthrough(A a) { return a; }
      int main(void) {
       A a;
       a.x = 0;
       return passthrough(a).x;
      }

The default memberwise copying operation is not specified to be done in a way that is insensitive to indeterminate values.

#### 1499. Missing case for deleted move assignment operator

Section: 11.4.6 [class.copy.assign] Status: drafting Submitter: John Spicer Date: 2012-04-27

Bullet 4 of 11.4.5.3 [class.copy.ctor] paragraph 23 says that a defaulted copy/move assignment operator is defined as deleted if the class has

> a non-static data member of class type `M` (or array thereof) that cannot be copied/moved because overload resolution (12.2 [over.match]), as applied to `M`'s corresponding assignment operator, results in an ambiguity or a function that is deleted or inaccessible from the defaulted assignment operator

The intent of this is that if overload resolution fails to find a corresponding copy/move assignment operator that can validly be called to copy/move a member, the class's assignment operator will be defined as deleted. However, this wording does not cover an example like the following:

      struct A {
        A();
      };

      struct B {
        B();
        const A a;
      };

      typedef B& (B::*pmf)(B&&);

      pmf p =&B::operator=;

Here, the problem is simply that overload resolution failed to find a callable function, which is not one of the cases listed in the current wording. A similar problem exists for base classes in the fifth bullet.

Additional note (January, 2013):

A similar omission exists in paragraph 11 for copy constructors.

#### 2329. Virtual base classes and generated assignment operators

Section: 11.4.6 [class.copy.assign] Status: drafting Submitter: Daveed Vandevoorde Date: 2016-10-31

An example like the following,

      class A {
      private:
        A& operator=(const A&);
      };

      class B : virtual public A {
      public:
        B& operator = (const B& src);
      };

      class C: public B {
      public:
        void f(const C* psrc) {
          *this = *psrc;
        }
      };

is presumably well-formed, even though the copy assignment operator of `A` is inaccessible in `C`, because 11.4.6 [class.copy.assign] paragraph 12 says that only direct, not virtual, base class object assignment operators are invoked by the generated assignment operator (although there is implementation divergence on this question).

Should the example also be well-formed if `A` were a direct virtual base of `C`? That is, if a direct virtual base also has an indirect derivation path, its direct derivation can be ignored for generated assignment operators.

Possibly relevant to this question is the permission for an implementation to assign virtual base class objects more than once:

> It is unspecified whether subobjects representing virtual base classes are assigned more than once by the implicitly-defined copy/move assignment operator.

#### 1977. Contradictory results of failed destructor lookup

Section: 11.4.7 [class.dtor] Status: drafting Submitter: Gabriel Dos Reis Date: 2014-07-21

According to 11.4.7 [class.dtor] paragraph 12,

> At the point of definition of a virtual destructor (including an implicit definition (11.4.5.3 [class.copy.ctor])), the non-array deallocation function is looked up in the scope of the destructor's class (6.5.2 [class.member.lookup]), and, if no declaration is found, the function is looked up in the global scope. If the result of this lookup is ambiguous or inaccessible, or if the lookup selects a placement deallocation function or a function with a deleted definition (9.5 [dcl.fct.def]), the program is ill-formed. [Note: This assures that a deallocation function corresponding to the dynamic type of an object is available for the delete-expression (11.4.11 [class.free]). —end note]

However, bullet 5.3 of that section says that such a lookup failure causes the destructor to be defined as deleted, rather than making the program ill-formed. It appears that paragraph 12 was overlooked when deleted functions were added to the language. See also 11.4.11 [class.free] paragraph 7.

#### 2158. Polymorphic behavior during destruction

Section: 11.4.7 [class.dtor] Status: drafting Submitter: Richard Smith Date: 2015-07-13

Consider the following example:

      #include <stdio.h>
      struct Base {
        Base *p;
        virtual void f() { puts("base"); }
        ~Base() {
          p->f();
        }
      };
      struct Derived : Base {
        Derived() { p = this; }
        void f() { puts("derived"); }
        void g() {
          p->f();
          delete this;
        }
      };
      void h() {
        Derived *p = new Derived;
        p->g();
      }

Should this have defined behavior? On the one hand, the `Derived` object is in its period of destruction, so the behavior of the `p->f()` call in the `Base` destructor should be to call `Base::f()`. On the other hand, `p` is a pointer to a `Derived` object whose lifetime has ended, and the rules in 6.7.3 [basic.life] don't appear to allow the call. (Calling `this->f()` from the `Base` destructor would be OK — the question is whether you can do that for a pointer that used to point to the derived object, or if you can only do it for a pointer that was “created” after the dynamic type of the object changed to be `Base`.)

If the above is valid, it has severe implications for devirtualization. The purpose of 6.7.3 [basic.life] paragraph 7 appears to be to allow an implementation to assume that if it will perform two loads of a constant field (for instance, a const member, the implicit pointer for a reference member, or a vptr), and the two loads are performed on the “same pointer value”, then they load the same value.

Should there be a rule for destructors similar to that of 11.4.5 [class.ctor] paragraph 12?

> During the construction of a const object, if the value of the object or any of its subobjects is accessed through a glvalue that is not obtained, directly or indirectly, from the constructor's this pointer, the value of the object or subobject thus obtained is unspecified.

#### 1283. Static data members of classes with typedef name for linkage purposes

Section: 11.4.9.3 [class.static.data] Status: drafting Submitter: Mike Miller Date: 2011-03-29

According to 11.4.9.3 [class.static.data] paragraph 4,

> Unnamed classes and classes contained directly or indirectly within unnamed classes shall not contain static data members.

There is no such restriction on member functions, and there is no rationale for this difference, given that both static data members and member functions can be defined outside a unnamed class with a typedef name for linkage purposes. (Issue 406 acknowledged the lack of rationale by removing the specious note in 11.4.9.3 [class.static.data] that attempted to explain the restriction but left the normative prohibition in place.)

It would be more consistent to remove the restriction for classes with a typedef name for linkage purposes.

Additional note (August, 2012):

It was observed that, since no definition of a `const` static data member is required if it is not odr-used, there is no reason to prohibit such members in an unnamed class even without a typedef name for linkage purposes.

#### 1721. Diagnosing ODR violations for static data members

Section: 11.4.9.3 [class.static.data] Status: drafting Submitter: Mike Miller Date: 2013-07-31

Describing the handling of static data members with brace-or-equal-initializers, 11.4.9.3 [class.static.data] paragraph 3 says,

> The member shall still be defined in a namespace scope if it is odr-used (6.3 [basic.def.odr]) in the program and the namespace scope definition shall not contain an initializer.

The word “shall” implies a required diagnostic, but this is describing an ODR violation (the static data member might be defined in a different translation unit) and thus should be “no diagnostic required.”

#### 2335. Deduced return types vs member types

Section: 11.4.9.3 [class.static.data] Status: drafting Submitter: John Spicer Date: 2017-01-29

It is not clear how an example like the following should be treated:

      template <class ...> struct partition_indices {
        static auto compute_right () {}
        static constexpr auto right = compute_right;
      };
      auto foo () -> partition_indices<>;
      void f() {
        foo();
      };

The initialization of `right` is in a context that must be done during the initial parse of the class, but the function body of `compute_right` is not supposed to be evaluated until the class is complete. Current implementations appear to accept the template case but not the equivalent non-template case. It's not clear why those cases should be treated differently.

If you change the example to include a forward dependency in the body of `compute_right`, e.g.,

      template <int> struct X {};
      template <class T> struct partition_indices {
        static auto compute_right () { return X<I>(); }
        static constexpr auto right = compute_right;
        static constexpr int I = sizeof(T);
      };

      auto foo () -> partition_indices<int>;

      void f() {
        foo();
      };

current implementations reject the code, but it's not clear that there is a rationale for the different behavior.

Notes from the March, 2018 meeting:

It was proposed that one direction might be to disallow instantiating member functions while the containing class template is being instantiated. However, overnight implementation experience indicated that this approach breaks seemingly-innocuous and currently-accepted code like:

      template <class T> struct A {
        static constexpr int num() { return 42; }
        int ar[num()];
      };
      A<int> a;

There was divergence of opinion regarding whether the current rules describe the current behavior for the two original examples or whether additional explicit rules are needed to clarify the difference in behavior between template and non-template examples, as well as whether there should be a difference at all..

Notes from the June, 2018 meeting:

The consensus of CWG was to treat templates and classes the same by "instantiating" delayed-parse regions when they are needed instead of at the end of the class.

See also issue 1890.

#### 1404. Object reallocation in unions

Section: 11.5 [class.union] Status: drafting Submitter: Nikolay Ivchenkov Date: 2011-10-19

According to 11.5 [class.union] paragraph 4,

> [Note: In general, one must use explicit destructor calls and placement new operators to change the active member of a union. —end note] [Example: Consider an object `u` of a union type `U` having non-static data members `m` of type `M` and `n` of type `N`. If `M` has a non-trivial destructor and `N` has a non-trivial constructor (for instance, if they declare or inherit virtual functions), the active member of `u` can be safely switched from `m` to `n` using the destructor and placement new operator as follows:

>       u.m.~M();
>       new (&u.n)  N;

> —end example]

This pattern is only “safe” if the original object that is being destroyed does not involve any const-qualified or reference types, i.e., satisfies the requirements of 6.7.3 [basic.life] paragraph 7, bullet 3:

- the type of the original object is not const-qualified, and, if a class type, does not contain any non-static data member whose type is const-qualified or a reference type

Although paragraph 4 of 11.5 [class.union] is a note and an example, it should at least refer to the lifetime issues described in 6.7.3 [basic.life].

Additional note (October, 2013):

See also issue 1776, which suggests possibly changing the restriction in 6.7.3 [basic.life]. If such a change is made, this issue may become moot.

#### 1702. Rephrasing the definition of “anonymous union”

Section: 11.5 [class.union] Status: drafting Submitter: Richard Smith Date: 2013-06-17

11.5 [class.union] paragraph 5 defines an anonymous union as follows:

> A union of the form

> `union {` member-specification `} ;`

> is called an anonymous union; it defines an unnamed object of unnamed type.

It is obviously intended that a declaration like

        static union { int i; float f; };

is a declaration of that form (cf paragraph 6, which requires the `static` keyword for anonymous unions declared in namespace scope). However, it would be clearer if the definition were recast in more descriptive terms, e.g.,

> An anonymous union is an unnamed class that is defined with the class-key `union` in a simple-declaration in which the init-declarator-list is omitted. Such a simple-declaration is treated as if it contained a single declarator declaring an unnamed variable of the union's type.

(Note that this definition would require some additional tweaking to apply to class member anonymous union declarations, since simple-declarations are not included as member-declarations.)

As a related point, it is not clear how the following examples are to be treated, and there is implementation variance on some:

       void f() { thread_local union { int a; }; }
       void g() { extern union { int b; }; }
       thread_local union { int c; }; // static is implied by thread_local
       static thread_local union { int d; };
       static const union { int e = 0; }; // is e const? Clang says yes, gcc says no
       static constexpr union { int f = 0; };

#### 2246. Access of indirect virtual base class constructors

Section: 11.8.3 [class.access.base] Status: drafting Submitter: Vinny Romano Date: 2016-03-08

Consider this example from issue 7:

      class Foo { };
      class A : virtual private Foo { };
      class Bar : public A { };

This example should cause `Bar`'s defaulted default constructor to be deleted, because it does not have access to the injected-class-name `Foo`.

Notes from the December, 2016 teleconference:

The injected-class-name is irrelevant to the example, which is ill-formed. The access should be permitted only if conversion of the `this` pointer to a pointer to the base class would succeed.

#### 2588. friend declarations and module linkage

Section: 11.8.4 [class.friend] Status: drafting Submitter: Nathan Sidwell Date: 2022-05-26 Liaison: EWG

Consider:

      export module Foo;
      class X {
        friend void f(X); // #1 linkage?
      };

Subclause 11.8.4 [class.friend] paragraph 4 gives #1 external linkage:

> A function first declared in a friend declaration has the linkage of the namespace of which it is a member (6.6 [basic.link]).

(There is no similar provision for friend classes first declared in a class.)

However, 6.6 [basic.link] bullet 4.8 gives it module linkage:

> ... otherwise, if the declaration of the name is attached to a named module (10.1 [module.unit]) and is not exported (10.2 [module.interface]), the name has module linkage;

Subclause 10.2 [module.interface] paragraph 2 does not apply:

> A declaration is exported if it is declared within an export-declaration and inhabits a namespace scope or it is
> - a namespace-definition that contains an exported declaration, or
> - a declaration within a header unit (10.3 [module.import]) that introduces at least one name.

Also consider this related example:

      export module Foo;
      export class Y;
      // maybe many lines later, or even a different partition of Foo
      class Y {
        friend void f(Y); // #2 linkage?
      };

- Should the friend's linkage be affected by the linkage of the befriending class? In this example, #2 would therefore have external linkage, as `Y` is exported.
- Or should the friend's linkage be affected by the presence or absence of `export` on the class definition itself? In this example, #2 would thus have module linkage.
- Or should the friend's linkage be determined ignoring any enclosing `export` and ignoring whether the enclosing class is exported, per 11.8.4 [class.friend] paragraph 4 (alone)?
- Or should the friend's linkage be as-if the declaration inhabited its nearest enclosing namespace scope, without the `friend`?

See issue 2607 for a similar question about enumerators.

Additional note (May, 2022):

Forwarded to EWG with paper issue 1253, by decision of the CWG chair.

EWG telecon 2022-06-09

Consensus: "A friend's linkage should be affected by the presence/absence of export on the containing class definition itself, but ONLY if the friend is a definition", pending confirmation by electronic polling.

Proposed resolution (June, 2022):

1. Change in 6.6 [basic.link] paragraph 4 as follows:

   > ... The name of an entity that belongs to a namespace scope that has not been given internal linkage above and that is the name of
   > - a variable; or
   > - a function; or
   > - a named class (11.1 [class.pre]), or an unnamed class defined in a typedef declaration in which the class has the typedef name for linkage purposes (9.2.4 [dcl.typedef]); or
   > - a named enumeration (9.7.1 [dcl.enum]), or an unnamed enumeration defined in a typedef declaration in which the enumeration has the typedef name for linkage purposes (9.2.4 [dcl.typedef]); or
   > - an unnamed enumeration that has an enumerator as a name for linkage purposes (9.7.1 [dcl.enum]); or
   > - a template
   > has its linkage determined as follows:
   > - if the entity is a function or function template first declared in a friend declaration and that declaration is a definition, the name has the same linkage, if any, as the name of the enclosing class (11.8.4 [class.friend]);
   > - otherwise, if the entity is a function or function template declared in a friend declaration and a corresponding non-friend declaration is reachable, the name has the linkage determined from that prior declaration,
   > - otherwise, if the enclosing namespace has internal linkage, the name has internal linkage;
   > - otherwise, if the declaration of the name is attached to a named module (10.1 [module.unit]) and is not exported (10.2 [module.interface]), the name has module linkage;
   > - otherwise, the name has external linkage.

2. Remove 11.8.4 [class.friend] paragraph 4:

   > A function first declared in a friend declaration has the linkage of the namespace of which it is a member (6.6 [basic.link]). Otherwise, the function retains its previous linkage (9.2.2 [dcl.stc]).

EWG electronic poll 2022-06

Consensus for "A friend's linkage should be affected by the presence/absence of export on the containing class definition itself, but ONLY if the friend is a definition (option #2, modified by Jason's suggestion). This resolves CWG2588." See vote.

#### 472. Casting across protected inheritance

Section: 11.8.5 [class.protected] Status: drafting Submitter: Mike Miller Date: 16 Jun 2004

Does the restriction in 11.8.5 [class.protected] apply to upcasts across protected inheritance, too? For instance,

        struct B {
            int i;
        };
        struct I: protected B { };
        struct D: I {
            void f(I* ip) {
                B* bp = ip;    // well-formed?
                bp->i = 5;     // aka "ip->i = 5;"
            }
        };

I think the rationale for the 11.8.5 [class.protected] restriction applies equally well here — you don't know whether `ip` points to a `D` object or not, so `D::f` can't be trusted to treat the protected `B` subobject consistently with the policies of its actual complete object type.

The current treatment of “accessible base class” in 11.8.3 [class.access.base] paragraph 4 clearly makes the conversion from `I*` to `B*` well-formed. I think that's wrong and needs to be fixed. The rationale for the accessibility of a base class is whether “an invented public member” of the base would be accessible at the point of reference, although we obscured that a bit in the reformulation; it seems to me that the invented member ought to be considered a non-static member for this purpose and thus subject to 11.8.5 [class.protected].

(See also issues 385 and 471.).

Notes from October 2004 meeting:

The CWG tentatively agreed that casting across protective inheritance should be subject to the additional restriction in 11.8.5 [class.protected].

Proposed resolution (April, 2011)

Change 11.8.3 [class.access.base] paragraph 4 as follows:

> A base class `B` of `N` is accessible at R, if

> - an invented public member of `B` would be a public member of `N`, or

> - R occurs in a member or friend of class `N`, and an invented public member of `B` would be a private or protected member of `N`, or

> - R occurs in a member or friend of a class `P` derived from `N`, and an invented public member of `B` would be a private or (but not a protected [Footnote: A protected invented member is disallowed here for the same reason the additional check of 11.8.5 [class.protected] is applied to member access: it would allow casting a pointer to a derived class to a protected base class that might be a subobject of an object of a class that is different from the class context in which the reference occurs. —end footnote]) member of `P`, or

> - there exists a class `S` such that `B` is a base class of `S` accessible at R and `S` is a base class of `N` accessible at R.

> [Example:

>         class B {
>         public:
>           int m;
>         };

>         class S: private B {
>           friend class N;
>         };
>         class N: private S {
>           void f() {
>             B* p = this;    // OK because class S satisfies the fourth condition
>                             // above: B is a base class of N accessible in f() because
>                             // B is an accessible base class of S and S is an accessible
>                             // base class of N.
>           }
>         };

>         class N2: protected B { };

>         class P2: public N2 {
>           void f2(N2* n2p) {
>             B* bp = n2p;    // error: invented member would be protected and naming
>                             // class N2 not the same as or derived from the referencing
>                             // class P2
>             n2p->m = 0;     // error (cf 11.8.5 [class.protected]) for the same reason
>           }
>         };

> —end example]

#### 1883. Protected access to constructors in mem-initializers

Section: 11.8.5 [class.protected] Status: drafting Submitter: Daveed Vandevoorde Date: 2014-02-26

According to 11.8.5 [class.protected] paragraph 1, except when forming a pointer to member,

> All other accesses involve a (possibly implicit) object expression (7.6.1.5 [expr.ref]).

It is not clear that this is strictly true for the invocation of a base class constructor from a mem-initializer. A wording tweak may be advisable.

#### 2187. Protected members and access via qualified-id

Section: 11.8.5 [class.protected] Status: drafting Submitter: Hubert Tong Date: 2015-10-16

The following line in the example in 11.8.5 [class.protected] paragraph 1 is no longer allowed following the change from issue 1873:

      class B {
      protected:
        int i;
        static int j;
      };
      // ...
      class D2 : public B {
        friend void fr(B*, D1*, D2*);
        void mem(B*, D1*);
      };
      void fr(B* pb, D1* p1, D2* p2) {
        // ...
        p2->B::i = 4;  // OK (access through a D2, even though naming class is B)
        // ...
      }

The example line ought to work, but none of the bullets in 11.8.3 [class.access.base] paragraph 5 apply:

> A member m is accessible at the point R when named in class N if
> - m as a member of N is public, or
> - m as a member of N is private, and R occurs in a direct member or friend of class N, or
> - m as a member of N is protected, and R occurs in a direct member or friend of class N, or in a member of a class P derived from N, where m as a member of P is public, private, or protected, or
> - there exists a base class B of N that is accessible at R, and m is accessible at R when named in class B.

One aproach might be that 11.8.3 [class.access.base] bullet 5.3 should also consider friends of a class P derived from N where P is the type of the object expression (if any) or a base class thereof, and m as a member of P is public, protected, or private.

#### 2056. Member function calls in partially-initialized class objects

Section: 11.9.3 [class.base.init] Status: drafting Submitter: Richard Smith Date: 2014-12-11

According to 11.9.3 [class.base.init] paragraph 16,

> Member functions (including virtual member functions, 11.7.3 [class.virtual]) can be called for an object under construction. Similarly, an object under construction can be the operand of the `typeid` operator (7.6.1.8 [expr.typeid]) or of a `dynamic_cast` (7.6.1.7 [expr.dynamic.cast]). However, if these operations are performed in a ctor-initializer (or in a function called directly or indirectly from a ctor-initializer) before all the mem-initializers for base classes have completed, the result of the operation is undefined.

The example in that paragraph reads, in significant part,

      class B {
      public:
        int f();
      };

      class C {
      public:
        C(int);
      };

      class D : public B, C {
      public:
        D() : C(f())  // undefined: calls member function
                      // but base \tcode{C} not yet initialized
        {}
      };

However, the construction of `B`, the object for which the member function is being called) has completed its construction, so it is not clear why this should be undefined behavior.

(See also issue 1517.)

#### 2403. Temporary materialization and base/member initialization

Section: 11.9.3 [class.base.init] Status: drafting Submitter: Daveed Vandevoorde Date: 2018-12-11

Given the following example,

      struct Noncopyable {
        Noncopyable();
        Noncopyable(const Noncopyable &) = delete;
      };

      Noncopyable make(int kind = 0);

      struct AsBase : Noncopyable {
        AsBase() : Noncopyable(make()) {} // #1
      };

      struct AsMember {
        Noncopyable nc;
        AsMember() : nc(make()) { }  // #2?
      };

All implementations treat #1 as an error, invoking the deleted copy constructor, while #2 is accepted. It's not clear from the current wording why they should be treated differently.

Additional note (August, 2022):

If there are concerns about reuse of tail padding in #1, requiring a copy for some implementation reason, similar concerns should apply to #2 if the data member is declared with `[[no_unique_address]]`.

Furthermore, the following example using a delegating constructor shows implementation divergence:

    struct Noncopyable {
      Noncopyable();
      Noncopyable(const Noncopyable &) = delete;
      Noncopyable(int) : Noncopyable(Noncopyable()) {} // #3?
    };

#### 1517. Unclear/missing description of behavior during construction/destruction

Section: 11.9.5 [class.cdtor] Status: drafting Submitter: Daniel Krügler Date: 2012-07-07

The current wording of 11.9.5 [class.cdtor] paragraph 4 does not describe the behavior of calling a virtual function in a mem-initializer for a base class, only for a non-static data member. Also, the changes for issue 1202 should have been, but were not, applied to the description of the behavior of `typeid` and `dynamic_cast` in paragraphs 5 and 6.

In addition, the resolution of issue 597 allowing the out-of-lifetime conversion of pointers/lvalues to non-virtual base classes, should have been, but were not, applied to paragraph 3.

(See also issue 2056.)

Proposed resolution (August, 2013):

1. Change 11.9.5 [class.cdtor] paragraph 1 as follows:

> For an object with a non-trivial constructor, referring to any non-static member or virtual base class of the object before the constructor begins execution results in undefined behavior. For an object with a non-trivial destructor, referring to any non-static member or virtual base class of the object after the destructor finishes execution results in undefined behavior. [Example:

>       struct X { int i; };
>       struct Y : X { Y(); };                       // non-trivial
>       struct A { int a; };
>       struct B : public virtual A { int j; Y y; }; // non-trivial

>       extern B bobj;
>       B* pb = &bobj;                               // OK
>       int* p1 = &bobj.a;                           // undefined, refers to base class member
>       int* p2 = &bobj.y.i;                         // undefined, refers to member's member

>       A* pa = &bobj;                               // undefined, upcast to a virtual base class type
>       B bobj;                                      // definition of bobj

>       extern X xobj;
>       int* p3 = &xobj.i;                           //OK, X is a trivial class
>       X xobj;

2. Change 11.9.5 [class.cdtor] paragraphs 3-6 as follows:

> To explicitly or implicitly convert a pointer (a glvalue) referring to an object of class `X` to a pointer (reference) to a direct or indirect virtual base class `B` of `X`, the construction of `X` and the construction of all of its direct or indirect bases that directly or indirectly derive from for which `B` is a direct or indirect virtual base shall have started and the destruction of these classes shall not have completed, otherwise the conversion results in undefined behavior. To form a pointer to (or access the value of) a direct non-static member...

> Member functions, including virtual functions (11.7.3 [class.virtual]), can be called during construction or destruction (11.9.3 [class.base.init]). When a virtual function is called directly or indirectly from a constructor or from a destructor, including during the construction or destruction of the class's non-static data members, and the object to which the call applies is the object (call it `x`) under construction or destruction, the function called is the final overrider in the constructor's or destructor's class and not one overriding it in a more-derived class. If the virtual function call uses an explicit class member access (7.6.1.5 [expr.ref]) and the object expression refers to the complete object of x or one of that object's base class subobjects but not to `x` or one of its base class subobjects, the behavior is undefined. The period of construction of an object or subobject whose type is a class type `C` begins immediately after the construction of all its base class subobjects is complete and concludes when the last constructor of class `C` exits. The period of destruction of an object or subobject whose type is a class type `C` begins when the destructor for `C` begins execution and concludes immediately before beginning the destruction of its base class subobjects. A polymorphic operation is a virtual function call (7.6.1.3 [expr.call]), the `typeid` operator (7.6.1.8 [expr.typeid]) when applied to a glvalue of polymorphic type, or the `dynamic_cast` operator (7.6.1.7 [expr.dynamic.cast]) when applied to a pointer to or glvalue of a polymorphic type. A polymorphic operand is the object expression in a virtual function call or the operand of a polymorphic `typeid` or `dynamic_cast`.

> During the period of construction or period of destruction of an object or subobject whose type is a class type `C` (call it `x`), the effect of performing a polymorphic operation in which the polymorphic operand designates `x` or a base class subobject thereof is as if the dynamic type of the object were class `C`. [Footnote: This is true even if `C` is an abstract class, which cannot be the type of a most-derived object. —end footnote] If a polymorphic operand refers to an object or subobject having class type `C` before its period of construction begins or after its period of destruction is complete, the behavior is undefined. [Note: This includes the evaluation of an expression appearing in a mem-initializer of `C` in which the mem-initializer-id designates `C` or one of its base classes. —end note] [Example:

>       struct V {
>         V();
>         V(int);
>         virtual void f();
>         virtual void g();
>       };

>       struct A : virtual V {
>         virtual void f();
>         virtual int h();
>         A() : V(h()) { }     // undefined behavior: virtual function h called
>                              // before A's period of construction begins
>       };

>       struct B : virtual V {
>         virtual void g();
>         B(V*, A*);
>       };

>       struct D : A, B {
>         virtual void f();
>         virtual void g();
>         D() : B((A*)this, this) { }
>       };

>       B::B(V* v, A* a) {
>         f();                 // calls V::f, not A::f
>         g();                 // calls B::g, not D::g
>         v->g();              // v is base of B, the call is well-defined, calls B::g
>         a->f();              // undefined behavior, a's type not a base of B
>         typeid(*this);       // type_info for B
>         typeid(*v);          // well-defined: *v has type V, a base of B,
>                              // so its period of construction is complete;
>                              // yields type_info for B
>         typeid(*a);          // undefined behavior: A is not a base of B,
>                              // so its period of construction has not begun
>         dynamic_cast<B*>(v); // well-defined: v has type V*, V is a base of B,
>                              // so its period of construction is complete;
>                              // results in this
>         dynamic_cast<B*>(a); // undefined behavior: A is not a base of B,
>                              // so its period of construction has not begun
>       }

> —end example]

> The `typeid` operator (7.6.1.8 [expr.typeid]) can be used during construction or destruction (11.9.3 [class.base.init]). When `typeid` is used in a constructor (including the mem-initializer or brace-or-equal-initializer for a non-static data member) or in a destructor, or used in a function called (directly or indirectly) from a constructor or destructor, if the operand of `typeid` refers to the object under construction or destruction, `typeid` yields the `std::type_info` object representing the constructor or destructor's class. If the operand of `typeid` refers to the object under construction or destruction and the static type of the operand is neither the constructor or destructor's class nor one of its bases, the result of `typeid` is undefined.

> `dynamic_cast`s (7.6.1.7 [expr.dynamic.cast]) can be used during construction or destruction (11.9.3 [class.base.init]). When a `dynamic_cast` is used in a constructor (including the mem-initializer or brace-or-equal-initializer for a non-static data member) or in a destructor, or used in a function called (directly or indirectly) from a constructor or destructor, if the operand of the `dynamic_cast` refers to the object under construction or destruction, this object is considered to be a most derived object that has the type of the constructor or destructor's class. If the operand of the `dynamic_cast` refers to the object under construction or destruction and the static type of the operand is not a pointer to or object of the constructor or destructor's own class or one of its bases, the `dynamic_cast` results in undefined behavior. [Example:

>       struct V {
>         virtual void f();
>       };

>       struct A : virtual V { };

>       struct B : virtual V {
>         B(V*, A*);
>       };

>       struct D : A, B {
>         D() : B((A*)this, this) { }
>       };

>       B::B(V* v, A* a) {
>         typeid(*this);       // type_info for B
>         typeid(*v);          // well-defined: *v has type V, a base of B
>                              // yields type_info for B
>         typeid(*a);          // undefined behavior: type A not a base of B
>         dynamic_cast<B*>(v); // well-defined: v of type V*, V base of B
>                              // results in B*
>         dynamic_cast<B*>(a); // undefined behavior,
>                              // a has type A*, A not a base of B

> —end example]

#### 1278. Incorrect treatment of contrived object

Section: 12.2.2.2.2 [over.call.func] Status: drafting Submitter: Nikolay Ivchenkov Date: 2011-03-27

Footnote 127 of 12.2.2.2.2 [over.call.func] paragraph 3 reads,

> An implied object argument must be contrived to correspond to the implicit object parameter attributed to member functions during overload resolution. It is not used in the call to the selected function. Since the member functions all have the same implicit object parameter, the contrived object will not be the cause to select or reject a function.

It is not true that “the member functions all have the same implicit object parameter.” This statement does not take into account member functions brought into the class by using-declarations or cv-qualifiers and ref-qualifiers on the non-static member functions:

        struct B
        {
          char f();         // B &
        };

        struct D : B
        {
          using B::f;
          long f();         // D &

          char g() const;   // D const &
          long g();         // D &

          char h() &;       // D &
          long h() &&;      // D &&
        };

        int main()
        {
          // D::f() has better match than B::f()
          decltype(D().f()) *p1 = (long *)0;

          // D::g() has better match than D::g() const
          decltype(D().g()) *p2 = (long *)0;

          // D::h() & is not viable function
          // D::h() && is viable function
          decltype(D().h()) *p3 = (long *)0;
        }

The value category of a contrived object expression is not specified by the rules and, probably, cannot be properly specified in presence of ref-qualifiers, so the statement “the contrived object will not be the cause to select or reject a function” should be normative rather than informative:

        struct X
        {
          static void f(double) {}
          void f(int) & {}
          void f(int) && {}
        };

        int main()
        {
          X::f(0); // ???
        }

#### 2089. Restricting selection of builtin overloaded operators

Section: 12.2.2.3 [over.match.oper] Status: drafting Submitter: Hubert Tong Date: 2015-02-26

The candidates selected by 12.2.2.3 [over.match.oper] include built-in candidates that will result in an error if chosen; this was affirmed by issue 1687. As a result, `t+u` is ill-formed because it is resolved to the built-in `operator+(int*,std::ptrdiff_t)`, although most implementations do not (yet) agree:

      struct Adaptor { Adaptor(int); };

      struct List { };
      void operator +(List &, Adaptor);

      struct DataType {
        operator int *() const = delete;
        operator List &() const;
      };

      struct Yea;
      struct Nay { int theNaysHaveIt; };

      template <typename T, typename U>
      Yea addCheck(int, T &&t, U &&u, char (*)[sizeof(t + u, 0)] = 0);

      template <typename T, typename U>
      Nay addCheck(void *, T &&t, U &&u);

      void test(DataType &data) { (void)sizeof(addCheck(0, data,
      0.).theNaysHaveIt); }

It might be better to adjust the candidate list in 12.2.2.4 [over.match.ctor] bullet 3.3.3 to allow conversion only on class types and exclude the second standard conversion sequence.

#### 2028. Converting constructors in rvalue reference initialization

Section: 12.2.2.7 [over.match.ref] Status: drafting Submitter: Mitsuru Kariya Date: 2014-10-25

Consider the following example:

      struct T {
        T() {}
        T(struct S&) {}
      };

      struct S {
        operator T() { return T(); }
      };

      int main()
      {
        S s;
        T&& t(s);  // #1
      }

Because there are two possible conversions from `S` to `T`, one by conversion function and the other by converting constructor, one might expect that the initialization at #1 would be ambiguous. However, 12.2.2.7 [over.match.ref] (used in the relevant bullet of 9.4.4 [dcl.init.ref], paragraph 5.2.1.2) only deals with conversion functions and ignores converting constructors.

Notes from the November, 2014 meeting:

CWG agreed that 9.4.4 [dcl.init.ref] should be changed to consider converting constructors in this case.

#### 2108. Conversions to non-class prvalues in reference initialization

Section: 12.2.2.7 [over.match.ref] Status: drafting Submitter: Hubert Tong Date: 2015-03-24

In 12.2.2.7 [over.match.ref], candidates that produce non-class prvalues are considered, although that seems to contradict what 9.4.4 [dcl.init.ref] says. See also issue 2077.

#### 2194. Impossible case in list initialization

Section: 12.2.2.8 [over.match.list] Status: drafting Submitter: Robert Haberlach Date: 2015-11-04

According to 12.2.2.8 [over.match.list] paragraph 1 says,

> If the initializer list has no elements and `T` has a default constructor, the first phase is omitted.

However, this case cannot occur. If `T` is a non-aggregate class type with a default constructor and the initializer is an empty initializer list, the object will be value-constructed, per 9.4.5 [dcl.init.list] bullet 3.4. Overload resolution is only necessary if default-initialization (or a check of its semantic constraints) is implied, with the relevant section concerning candidates for overload resolution being 12.2.2.4 [over.match.ctor].

See also issue 1518.

Proposed resolution (January, 2017):

Change 12.2.2.8 [over.match.list] paragraph 1 as follows:

> When objects of non-aggregate class type `T` are list-initialized such that 9.4.5 [dcl.init.list] specifies that overload resolution is performed according to the rules in this section, overload resolution selects the constructor in two phases:

> - Initially, the candidate functions are the initializer-list constructors (9.4.5 [dcl.init.list]) of the class `T` and the argument list consists of the initializer list as a single argument.

> - If no viable initializer-list constructor is found, overload resolution is performed again, where the candidate functions are all the constructors of the class `T` and the argument list consists of the elements of the initializer list.

> If the initializer list has no elements and `T` has a default constructor, the first phase is omitted. In copy-list-initialization, if an explicit constructor is chosen...

Additional notes, February, 2017:

The statement of the issue is incorrect. In an example like

      struct A { A(); A(initializer_list<int>); };
      void f(A a);
      int main() { f({}); }

the rule in question is not used for the initialization of the parameter. However, it is used to determine whether a valid implicit conversion sequence exists for `a`. It is unclear whether an additional change to resolve this discrepancy is needed or not.

#### 2467. CTAD for alias templates and the deducible check

Section: 12.2.2.9 [over.match.class.deduct] Status: drafting Submitter: Richard Smith Date: 2019-08-12

Given the declarations

      template<typename T = int> using X = vector<int>;
      X x = {1, 2, 3};

      template<typename...> using Y = vector<int>;
      Y y = {1, 2, 3};

CTAD deduces `vector<int>`. Then we are asked to perform a check that the arguments of `X` and `Y` are deducible from `vector<int>`.

I think this check should succeed, deducing `T = int` in the first case and `<`pack`> = <`empty`>` in the second case, so both declarations should be valid. That seems consistent with what would happen for a non-alias with template parameters that CTAD can't deduce, where there is either a default template argument or the parameter is a pack. But what actually happens is that we're asked to form

      template<typename T> struct AA;
      template<typename T = int> struct AA<X<T>>;

and

      template<typename T> struct AA;
      template<typename ...Ts> struct AA<Y<Ts...>>;

However, both of those partial specializations are ill-formed: a partial specialization can't have default template arguments, and neither of these is more specialized than the primary template, because `T` / `Ts` are not used in deducible contexts.

I think we have the wrong model here, and should instead be considering (effectively) whether function template argument deduction would succeed for

      template<typename T> struct AA {};
      template<typename T = int> void f(AA<X<T>>);

and

      template<typename T> struct AA {};
      template<typename ...Ts> void f(AA<Y<Ts...>>);

respectively, when given an argument of type `AA<`deduced return type`>`. That is, get rid of the weird class template partial specialization restrictions, and instead add in the rules from function templates to use default template arguments and to default non-deduced packs to empty packs.

#### 2471. Nested class template argument deduction

Section: 12.2.2.9 [over.match.class.deduct] Status: drafting Submitter: John Spicer Date: 2021-01-26

Consider the following example:

      template<class T> struct S {
        template<class U> struct N {
          N(T) {}
          N(T, U) {}
          template<class V> N(V, U) {}
        };
      };
      S<int>::N x{2.0, 1};

The description of CTAD in 12.2.2.9 [over.match.class.deduct] doesn't really specify how nested classes work. If you are supposed to deduce all the enclosing class template arguments, the example is ill-formed because there is no way to deduce `T`. If you are supposed to consider `S<int>::N` as having a new constructor template, then it should probably be well-formed.

Notes from the March, 2021 teleconference:

CWG agreed that the intent is to use the partially-instantiated inner template with the explicitly-specified template argument `int`.

#### 2319. Nested brace initialization from same type

Section: 12.2.4.2 [over.best.ics] Status: drafting Submitter: Richard Smith Date: 2016-09-06

Consider:

      struct A { A(); } a;
      A a1 = {a}, a2 = {{a}}, a3 = {{{a}}};

`a1` and `a2` are valid, `a3` is ill-formed, because 12.2.4.2 [over.best.ics] bullet 4.5 allows one pair of braces and 12.2.4.2.6 [over.ics.list] paragraph 2 allows a second pair of braces. The implicit conversion sequence from `{{a}}` to `A` is a user-defined conversion.

Prior to the list-initialization-from-same-type changes via issues 1467 and 2076, `a2` was ill-formed like `a3`.

Is this intended, or did DR2076 not go far enough in reintroducing the restriction? Perhaps a more extreme rule, such as saying that a copy/move constructor is simply not a candidate for list-initialization from a list that contains one element that is itself a list, would work better?

Notes from the July, 2017 meeting:

CWG agreed that the `a2` example should be ill-formed but that the `a1` example must remain for C compatibility.

#### 2525. Incorrect definition of implicit conversion sequence

Section: 12.2.4.2.1 [over.best.ics.general] Status: drafting Submitter: Jim X Date: 2021-09-25

According to 12.2.4.2.1 [over.best.ics.general] paragraphs 1 and 9,

> An implicit conversion sequence is a sequence of conversions used to convert an argument in a function call to the type of the corresponding parameter of the function being called. The sequence of conversions is an implicit conversion as defined in 7.3 [conv], which means it is governed by the rules for initialization of an object or reference by a single expression (9.4 [dcl.init], 9.4.4 [dcl.init.ref]).

> If no sequence of conversions can be found to convert an argument to a parameter type, an implicit conversion sequence cannot be formed.

However, 7.3.1 [conv.general] paragraph 3 says,

> An expression E can be implicitly converted to a type `T` if and only if the declaration `T t=E;` is well-formed, for some invented temporary variable `t` (9.4 [dcl.init]).

This definition is too restrictive in the context of overload resolution's implicit conversion sequences. The intent, as stated in 12.2.1 [over.match.general] note 1, is that overload resolution ignores some factors that would make such an initialization ill-formed, and these are applied only after the best match is determined:

> [Note 1: The function selected by overload resolution is not guaranteed to be appropriate for the context. Other restrictions, such as the accessibility of the function, can make its use in the calling context ill-formed. —end note]

For example,

      struct A{
        A(int) = delete;
      };
      struct B{
         B(int) {}
      };
      void fun(A); // #1
      void fun(B); // #2
      int main() {
        fun(0);    // #3
      }

The intent is that overload #1 be viable with a valid implicit conversion sequence, making the call at #3 ambiguous, even though the hypothetical declaration

      A t = 1;

would be ill-formed.

Proposed resolution (approved by CWG 2022-12-02):

Change 12.2.4.2.1 [over.best.ics.general] paragraph 1, merging it with paragraph 2,as follows:

> An implicit conversion sequence is a sequence of conversions used to convert an argument in a function call to the type of the corresponding parameter of the function being called. The sequence of conversions is an implicit conversion as defined in 7.3 [conv], which means; it is governed by thus based on the rules for initialization of an object or reference by a single expression (9.4 [dcl.init], 9.4.4 [dcl.init.ref]). Implicit, except that implicit conversion sequences are concerned only with the type, cv-qualification, and value category of the argument and how these are converted to match the corresponding properties of the parameter. [Note: ... ]

CWG 2023-02-06

Additional drafting is needed to cover e.g. conversions from literal 0 to null pointer constants.

#### 2077. Overload resolution and invalid rvalue-reference initialization

Section: 12.2.4.2.5 [over.ics.ref] Status: drafting Submitter: Richard Smith Date: 2015-01-29

The resolution of issue 1604 broke the following example:

      struct A {};
      struct B { operator const A() const; };
      void f(A const&);
      void f(A&&);

      int main() {
        B a;
        f(a);
      }

Overload resolution selects the `A&&` overload, but then initialization fails. This seems like a major regression; we're now required to reject

       std::vector<A> va;
       B b;
       va.push_back(b);

Should we update 12.2.4.2.5 [over.ics.ref] to match the changes made to 9.4.4 [dcl.init.ref]?

See also issue 2108.

#### 1536. Overload resolution with temporary from initializer list

Section: 12.2.4.2.6 [over.ics.list] Status: drafting Submitter: Mike Miller Date: 2012-08-14

In determining the implicit conversion sequence for an initializer list argument passed to a reference parameter, the intent is that a temporary of the appropriate type will be created and bound to the reference, as reflected in 12.2.4.2.6 [over.ics.list] paragraph 5:

> Otherwise, if the parameter is a reference, see 12.2.4.2.5 [over.ics.ref]. [Note: The rules in this section will apply for initializing the underlying temporary for the reference. —end note]

However, 12.2.4.2.5 [over.ics.ref] deals only with expression arguments, not initializer lists:

> When a parameter of reference type binds directly (9.4.4 [dcl.init.ref]) to an argument expression, the implicit conversion sequence is the identity conversion, unless the argument expression has a type that is a derived class of the parameter type, in which case the implicit conversion sequence is a derived-to-base Conversion (12.2.4.2 [over.best.ics])... If the parameter binds directly to the result of applying a conversion function to the argument expression, the implicit conversion sequence is a user-defined conversion sequence (12.2.4.2.3 [over.ics.user]), with the second standard conversion sequence either an identity conversion or, if the conversion function returns an entity of a type that is a derived class of the parameter type, a derived-to-base Conversion.

> When a parameter of reference type is not bound directly to an argument expression, the conversion sequence is the one required to convert the argument expression to the underlying type of the reference according to 12.2.4.2 [over.best.ics]. Conceptually, this conversion sequence corresponds to copy-initializing a temporary of the underlying type with the argument expression. Any difference in top-level cv-qualification is subsumed by the initialization itself and does not constitute a conversion.

(Note in particular that the reference binding refers to 9.4.4 [dcl.init.ref], which also does not handle initializer lists, and not to 9.4.5 [dcl.init.list].)

Either 12.2.4.2.5 [over.ics.ref] needs to be revised to handle binding references to initializer list arguments or 12.2.4.2.6 [over.ics.list] paragraph 5 needs to be clearer on how the expression specification is intended to be applied to initializer lists.

#### 2110. Overload resolution for base class conversion and reference/non-reference

Section: 12.2.4.3 [over.ics.rank] Status: drafting Submitter: Alexander Kulpin Date: 2015-03-27

There are overload tiebreakers that order reference/nonreference and base/derived conversions, but how they relate is not specified. For example:

      struct A { A(); };
      struct B : A {};
      struct C : B {};

      void f1(B&);
      void f1(A);

      void f2(B);
      void f2(A&);

      int main()
      {
         C v;
         f1(v); // all compilers choose f1(B&)
         f2(v); // all compilers choose f2(B)
      }

The Standard does not appear to specify what happens in this case.

#### 1989. Insufficient restrictions on parameters of postfix operators

Section: 12.4 [over.oper] Status: drafting Submitter: Richard Smith Date: 2014-08-30

According to 12.4.7 [over.inc] paragraph 1,

> The user-defined function called `operator++` implements the prefix and postfix `++` operator. If this function is a non-static member function with no parameters, or a non-member function with one parameter, it defines the prefix increment operator `++` for objects of that type. If the function is a non-static member function with one parameter (which shall be of type `int`) or a non-member function with two parameters (the second of which shall be of type `int`), it defines the postfix increment operator `++` for objects of that type.

According to 12.4 [over.oper] paragraph 8,

> Operator functions cannot have more or fewer parameters than the number required for the corresponding operator, as described in the rest of this subclause.

This does not rule out an `operator++` with more than two parameters, however, since there is no corresponding operator.

One possibility might be to add a sentence like,

> A function named `operator++` shall declare either a prefix or postfix increment operator.

#### 205. Templates and static data members

Section: Clause 13 [temp] Status: drafting Submitter: Mike Miller Date: 11 Feb 2000

Static data members of template classes and of nested classes of template classes are not themselves templates but receive much the same treatment as template. For instance, Clause 13 [temp] paragraph 1 says that templates are only "classes or functions" but implies that "a static data member of a class template or of a class nested within a class template" is defined using the template-declaration syntax.

There are many places in the clause, however, where static data members of one sort or another are overlooked. For instance, Clause 13 [temp] paragraph 6 allows static data members of class templates to be declared with the `export` keyword. I would expect that static data members of (non-template) classes nested within class templates could also be exported, but they are not mentioned here.

Paragraph 8, however, overlooks static data members altogether and deals only with "templates" in defining the effect of the `export` keyword; there is no description of the semantics of defining a static data member of a template to be exported.

These are just two instances of a systematic problem. The entire clause needs to be examined to determine which statements about "templates" apply to static data members, and which statements about "static data members of class templates" also apply to static data members of non-template classes nested within class templates.

(The question also applies to member functions of template classes; see issue 217, where the phrase "non-template function" in 9.3.4.7 [dcl.fct.default] paragraph 4 is apparently intended not to include non-template member functions of template classes. See also issue 108, which would benefit from understanding nested classes of class templates as templates. Also, see issue 249, in which the usage of the phrase "member function template" is questioned.)

Notes from the 4/02 meeting:

Daveed Vandevoorde will propose appropriate terminology.

#### 1463. `extern "C"` alias templates

Section: 13.1 [temp.pre] Status: drafting Submitter: Daveed Vandevoorde Date: 2011-08-19 Liaison: EWG

Currently 13.1 [temp.pre] paragraph 6 forbids any template from having C linkage. Should alias templates be exempt from this prohibition, since they do not have any linkage?

Additional note, April, 2013:

It was suggested that relaxing this restriction for alias templates could provide a way of addressing the long-standing lack of a way of specifying a language linkage for a dependent function type (see issue 13).

Rationale (April, 2013):

CWG felt that this suggested use of alias templates should be considered in a broader context and thus was more appropriate for EWG.

EWG 2022-11-11

`extern "C"` on a template should be allowed, and should affect only calling convention, but not mangling. This is tracked in github issue cplusplus/papers#1373.

#### 1444. Type adjustments of non-type template parameters

Section: 13.2 [temp.param] Status: drafting Submitter: Johannes Schaub Date: 2012-01-15

The type adjustment of template non-type parameters described in 13.2 [temp.param] paragraph 8 appears to be underspecified. For example, implementations vary in their treatment of

      template<typename T, T[T::size]> struct A {};
      int dummy;
      A<int, &dummy> a;

and

      template<typename T, T[1]> struct A;
      template<typename T, T*> struct A {};
      int dummy;
      A<int, &dummy> a;

See also issues 1322 and 1668.

Additional note, February, 2021:

See the discussion regarding top-level cv-qualifiers on template parameters when determining the type in this compiler bug report.

#### 1635. How similar are template default arguments to function default arguments?

Section: 13.2 [temp.param] Status: drafting Submitter: Richard Smith Date: 2013-03-06

Default function arguments are instantiated only when needed. Is the same true of default template arguments? For example, is the following well-formed?

      #include <type_traits>

      template<class T>
      struct X {
        template<class U = typename T::type>
        static void foo(int){}
        static void foo(...){}
      };

      int main(){
        X<std::enable_if<false>>::foo(0);
      }

Also, is the effect on lookup the same? E.g.,

      struct S {
        template<typename T = U> void f();
        struct U {};
      };

Additional note (November, 2020):

Paper P1787R6, adopted at the November, 2020 meeting, partially addresses this issue.

#### 2395. Parameters following a pack expansion

Section: 13.2 [temp.param] Status: drafting Submitter: Richard Smith Date: 2018-12-03

The Standard is not clear, and there is implementation divergence, for an example like the following:

      template<class ...Types> struct Tuple_ { // _VARIADIC_TEMPLATE
        template<Types ...T, int I> int f() {
          return sizeof...(Types);
        }
      };
      int main() {
        Tuple_<char,int> a;
        int b = a.f<1, 2, 3>();
      }

The question is whether the `3` is accepted as the argument for `I` or an error, exceeding the number of arguments for `T`, which is set as 2 by the template arguments for `Tuple_`. See also issue 2383 for a related example.

#### 2450. braced-init-list as a template-argument

Section: 13.3 [temp.names] Status: drafting Submitter: Marek Polacek Date: 2019-01-07

Since non-type template parameters can now have class types, it would seem to make sense to allow a braced-init-list as a template-argument, but the grammar does not permit it.

See also issues 2049 and 2459.

#### 2043. Generalized template arguments and array-to-pointer decay

Section: 13.4.3 [temp.arg.nontype] Status: drafting Submitter: Richard Smith Date: 2014-11-13

According to 13.4.3 [temp.arg.nontype] paragraph 1 (newly revised by the adoption of paper N4268),

> For a non-type template-parameter of reference or pointer type, the value of the constant expression shall not refer to (or for a pointer type, shall not be the address of):

> - a subobject (6.7.2 [intro.object]),

> - ...

This change breaks an example like

       template<int *p> struct X {};
       int arr[32];
       X<arr> x;

because the array-to-pointer decay produces a pointer to the first element, which is a subobject.

Suggested resolution:

Change the referenced bullet to read:

- a subobject (6.7.2 [intro.object]) that is not the first element of a complete object of array type,

Note that this resolution also allows an example like

        template<char &p> struct S { };
        char arr[2];
        S<arr[0]> s_arr;

which may not be exactly what we want.

See also issue 2401.

#### 2049. List initializer in non-type template default argument

Section: 13.4.3 [temp.arg.nontype] Status: drafting Submitter: Ville Voutilainen Date: 2014-11-20

According to 13.4.3 [temp.arg.nontype] paragraph 1,

> A template-argument for a non-type template-parameter shall be a converted constant expression (7.7 [expr.const]) of the type of the template-parameter.

This does not permit an example like:

      template <int* x = {}> struct X {};

which seems inconsistent.

See also issues 2450 and 2459.

#### 2401. Array decay vs prohibition of subobject non-type arguments

Section: 13.4.3 [temp.arg.nontype] Status: drafting Submitter: John Spicer Date: 2019-02-06

Consider an example like:

      template <const char *N> struct A { static const int val; };

      template <const char *N> const int A<N>::val = 0;

      static const char c[2] = "";

      int main() {
        A<c> a;
        return A<c>::val;
      }

Formally, this appears to violate the prohibition of using the address of a subobject as a non-type template argument, since the array reference `c` in the argument decays to a pointer to the first element of the array. However, at least some implementations accept this example, and at least conceptually the template argument designates the complete object. Should an exception be made for the result of array decay?

See also issue 2043.

Notes from the July, 2019 meeting

CWG felt that the example should be allowed if the parameter type is a pointer to object type (thus prohibiting `void*`).

#### 2459. Template parameter initialization

Section: 13.4.3 [temp.arg.nontype] Status: drafting Submitter: Davis Herring Date: 2020-09-21

The initialization of template parameters is severely underspecified. The only descriptions in the existing wording that apply are that the argument is “[converted] to the type of the template-parameter” (13.6 [temp.type] bullet 1.3) and, in 13.4.3 [temp.arg.nontype] paragraph 2,

> A template-argument for a non-type template-parameter shall be a converted constant expression (7.7 [expr.const]) of the type of the template-parameter.

This omission is particularly important for template parameters of class type with lvalue template parameter objects whose addresses can be examined during construction. See also issue 2450.

Suggested resolution:

To avoid address-based paradoxes, template arguments for a template parameter of class type `C` that are not of that type or a derived type are converted to `C` to produce an exemplar. No restrictions are imposed on the conversion from a template argument to a constructor parameter, since explicit and list-initialization may already be used to limit conversions in a similar fashion. Template arguments that are of such a type are used directly as the exemplar (potentially after a materialization conversion); the effect is as if the template parameter were of type `const C&` (except that temporaries are allowed). (In the latter case, we must impose some restrictions on glvalue template parameters to interpret them.) Each exemplar is used to copy-initialize the template parameter object to which it is (to be) template-argument-equivalent; the initialization is required to produce a template-argument-equivalent value. The multiple initializations of the template parameter object are (required to be) all equivalent and produce no side effects, so it is unobservable which happen.

#### 2057. Template template arguments with default arguments

Section: 13.4.4 [temp.arg.template] Status: drafting Submitter: Jonathan Caves Date: 2014-12-12

It is not clear how to handle an example like:

      template<typename T1, typename T2 = char> class A { };

      template<template<typename... T> class X> class S {
        X<int> x;
      };

      S<A> a;

Issue 184 dealt with a similar question but did so in the era before variadic templates. This usage should be permitted in modern C++.

Notes from the February, 2016 meeting:

CWG felt that this usage should be permitted, but only for template template parameters with a parameter pack.. Furthermore, if the template template parameter has a default argument followed by a parameter pack, the parameter's default argument would be used, followed by any remaining default arguments from the template template argument.

#### 2398. Template template parameter matching and deduction

Section: 13.4.4 [temp.arg.template] Status: drafting Submitter: Jason Merrill Date: 2016-12-03

Do the changes from P0522R0 regarding template template parameter matching apply to deduction? For example:

      template<class T, class U = T> class B { /* ... */ };
      template<template<class> class P, class T> void f(P<T>);

      int main()  {
        f(B<int>());       // OK?
        f(B<int,float>()); // ill-formed, T deduced to int and float
      }

In deduction we can determine that `P` is more specialized than `B`, then substitute `B` into `P<T>`, and then compare `B<T,T>` to `B<int,int>`. This will allow deduction to succeed, whereas comparing `<T>` to `<int,int>` without this substitution would fail. I suppose this is similar to deducing a type parameter, substituting it into the type of a non-type parameter, then deducing the value of the non-type parameter

Does this make sense? Do we need more wording?

Consider also this example;

      template<typename> struct match;

      template<template<typename> class t,typename T>
      struct match<t<T> > { typedef int type; };      // #1

      template<template<typename,typename> class t,typename T0,typename T1>
      struct match<t<T0,T1> > { typedef int type; };  // #2

      template<typename,typename = void> struct other { };
      typedef match<other<void,void> >::type type;

Before this change, partial specialization #1 was not a candidate; now it is, and neither partial specialization is at least as specialized as the other, so we get an ambiguity. It seems that the consistent way to address this would be to use `other` during partial ordering, so we'd be comparing

      template<typename T>
      void fn (match<other<T>>); // i.e. other<T,void>
      template<typename T0, typename T1>
      void fn (match<other<T0,T1>>);

So #1 is more specialized, whereas before this change we chose #2.

#### 2037. Alias templates and template declaration matching

Section: 13.6 [temp.type] Status: drafting Submitter: Richard Smith Date: 2014-11-06

For the following example,

      template<int N> struct A {};
      template<short N> using B = A<N>;
      template<int N> void f(B<N>) {} // #1
      template<int N> void f(A<N>) {} // #2

There is implementation variance as to whether there is one `f` or two. As with previously-discussed cases, these have different SFINAE effects, perhaps equivalent but not functionally equivalent. Should the argument to #1 be treated as something like `A<(int)(short)N>` and not just `A<N>`.

See also issues 1668 and 1979.

#### 1730. Can a variable template have an unnamed type?

Section: 13.7 [temp.decls] Status: drafting Submitter: Larisse Voufo Date: 2013-08-05

Is it permitted for a variable template to have an unnamed type?

#### 1647. Type agreement of non-type template arguments in partial specializations

Section: 13.7.6 [temp.spec.partial] Status: drafting Submitter: John Spicer Date: 2013-04-04

The Standard appears to be silent on whether the types of non-type template arguments in a partial specialization must be the same as those of the primary template or whether conversions are permitted. For example,

      template<char...> struct char_values {};
      template<int C1, char C3>
      struct char_values<C1, 12, C3> {
        static const unsigned value = 1;
      };
      int check0[char_values<1, 12, 3>::value == 1? 1 : -1];

The closest the current wording comes to dealing with this question is 13.7.6.1 [temp.spec.partial.general] bullet 9.1:

- A partially specialized non-type argument expression shall not involve a template parameter of the partial specialization except when the argument expression is a simple identifier.

In this example, one might think of the first template argument in the partial specialization as `(char)C1`, which would violate the requirement, but that reasoning is tenuous.

It would be reasonable to require the types to match in cases like this. If this kind of usage is allowed it could get messy if the primary template were `int...` and the partial specialization had a parameter that was `char` because not all of the possible values from the primary template could be represented in the parameter of the partial specialization. A similar issue exists if the primary template takes `signed char` and the partial specialization takes `unsigned int`.

There is implementation variance in the treatment of this example.

(See also issues 1315, 2033, and 2127.)

#### 2127. Partial specialization and nullptr

Section: 13.7.6 [temp.spec.partial] Status: drafting Submitter: Faisal Vali Date: 2015-05-18

An example like the following would seem to be plausible:

      template<class T, T*> struct X { };
      // We want to partially specialize for all nullptrs...
      template<class T> struct X<T, nullptr> { ... }; // NOT OK

This is disallowed by the rule in bullet 9.2 of 13.7.6.1 [temp.spec.partial.general]:

- The type of a template parameter corresponding to a specialized non-type argument shall not be dependent on a parameter of the specialization.

(See also issues 1315, 1647, and 2033.)

#### 2179. Required diagnostic for partial specialization after first use

Section: 13.7.6.1 [temp.spec.partial.general] Status: drafting Submitter: John Spicer Date: 2015-10-12

According to 13.7.6.1 [temp.spec.partial.general] paragraph 1,

> A partial specialization shall be declared before the first use of a class template specialization that would make use of the partial specialization as the result of an implicit or explicit instantiation in every translation unit in which such a use occurs; no diagnostic is required.

There are two problems with this wording. First, the “no diagnostic required” provision is presumably to avoid mandating cross-translation-unit analysis, but there is no reason not to require the diagnostic if the rule is violated within a single translation unit. Also, “would make use” is imprecise; it could be interpreted as applying only when the partial specialization would have been selected by a previous specialization, but it should also apply to cases where the partial specialization would have made a previous specialization ambiguous.

Making these two changes would guarantee that a diagnostic is issued for the following example:

       template <class T1, class T2> class A;
       template <class T> struct A<T, void> { void f(); };
       template <class T> void g(T) { A<char, void>().f(); }   // #1
       template<typename T> struct A<char, T> {};
       A<char, void> f;   // #2

It is unspecified whether the reference to `A<char, void>` at #1 is the “first use” or not. If so, `A<char, void>` is bound to the first partial specialization and, under the current wording, an implementation is not required to diagnose the ambiguity resulting from the second partial specialization. If #2 is the “first use,” it is clearly ambiguous and must result in a diagnostic. There is implementation divergence on the handling of this example that would be addressed by the suggested changes.

#### 549. Non-deducible parameters in partial specializations

Section: 13.7.6.2 [temp.spec.partial.match] Status: drafting Submitter: Martin Sebor Date: 18 November 2005

In the following example, the template parameter in the partial specialization is non-deducible:

        template <class T> struct A { typedef T U; };
        template <class T> struct C { };
        template <class T> struct C<typename A<T>::U> { };

Several compilers issue errors for this case, but there appears to be nothing in the Standard that would make this ill-formed; it simply seems that the partial specialization will never be matched, so the primary template will be used for all specializations. Should it be ill-formed?

(See also issue 1246.)

Notes from the April, 2006 meeting:

It was noted that there are similar issues for constructors and conversion operators with non-deducible parameters, and that they should probably be dealt with similarly.

Additional note, December, 2021:

The original issue, but not the *#8220;similar issues *#8221; pointed out in the 2006-04 note, was resolved by the changes for issue 1315 and paper P0127R2.

#### 1755. Out-of-class partial specializations of member templates

Section: 13.7.6.4 [temp.spec.partial.member] Status: drafting Submitter: Richard Smith Date: 2013-09-19

According to 13.7.6.4 [temp.spec.partial.member] paragraph 2,

> If a member template of a class template is partially specialized, the member template partial specializations are member templates of the enclosing class template; if the enclosing class template is instantiated (13.9.2 [temp.inst], 13.9.3 [temp.explicit]), a declaration for every member template partial specialization is also instantiated as part of creating the members of the class template specialization.

Does this imply that only partial specializations of member templates that are declared before the enclosing class is instantiated are considered? For example, in

      template<typename A> struct X { template<typename B> struct Y; };
      template struct X<int>;
      template<typename A> template<typename B> struct X<A>::Y<B*> { int n; };
      int k = X<int>::Y<int*>().n;

is the last line valid? There is implementation variance on this point. Similarly, for an example like

      template<typename A> struct Outer {
       template<typename B, typename C> struct Inner;
      };
      Outer<int> outer;
      template<typename A> template<typename B>
        struct Outer<A>::Inner<typename A::error, B> {};

at what point, if at all, is the declaration of the partial specialization instantiated? Again, there is implementation variance in the treatment of this example.

Notes from the February, 2014 meeting:

CWG decided that partial specialization declarations should be instantiated only when needed to determine whether the partial specialization matches or not.

Additional note, November, 2014:

See also paper N4090.

#### 1286. Equivalence of alias templates

Section: 13.7.8 [temp.alias] Status: drafting Submitter: Gabriel Dos Reis Date: 2011-04-03

Issue 1244 was resolved by changing the example in 13.6 [temp.type] paragraph 1 from

      template<template<class> class TT> struct X { };
      template<class> struct Y { };
      template<class T> using Z = Y<T>;
      X<Y> y;
      X<Z> z;

to

      template<class T> struct X { };
      template<class> struct Y { };
      template<class T> using Z = Y<T>;
      X<Y<int> > y;
      X<Z<int> > z;

In fact, the original intent was that the example should have been correct as written; however, the normative wording to make it so was missing. The current wording of 13.7.8 [temp.alias] deals only with the equivalence of a specialization of an alias template with the type-id after substitution. Wording needs to be added specifying under what circumstances an alias template itself is equivalent to a class template.

Proposed resolution (September, 2012):

1. Add the following as a new paragraph following 13.7.8 [temp.alias] paragraph 2:

> When the type-id in the declaration of alias template (call it `A`) consists of a simple-template-id in which the template-argument-list consists of a list of identifiers naming each template-parameter of `A` exactly once in the same order in which they appear in `A`'s template-parameter-list, the alias template is equivalent to the template named in the simple-template-id (call it `T`) if `A` and `T` have the same number of template-parameters. [Footnote: This rule is transitive: if an alias template `A` is equivalent to another alias template `B` that is equivalent to a class template `C`, then `A` is also equivalent to `C`, and `A` and `B` are also equivalent to each other. —end footnote] [Example:

>       template<typename T, U = T> struct A;

>       template<typename V, typename W>
>         using B = A<V, W>;                // equivalent to A

>       template<typename V, typename W>
>         using C = A<V>;                   // not equivalent to A:
>                                           // not all parameters used

>       template<typename V>
>         using D = A<V>;                   // not equivalent to A:
>                                           // different number of parameters

>       template<typename V, typename W>
>         using E = A<W, V>;                // not equivalent to A:
>                                           // template-arguments in wrong order

>       template<typename V, typename W = int>
>         using F = A<V, W>;                // equivalent to A:
>                                           // default arguments not considered

>       template<typename V, typename W>
>         using G = A<V, W>;                // equivalent to A and B

>       template<typename V, typename W>
>         using H = E<V, W>;                // equivalent to E

>       template<typename V, typename W>
>         using I = A<V, typename W::type>; // not equivalent to A:
>                                           // argument not identifier

> —end example]

2. Change 13.6 [temp.type] paragraph 1 as follows:

> Two template-ids refer to the same class or function if

> - ...

> - their corresponding template template-arguments refer to the same or equivalent (13.7.8 [temp.alias]) templates.

> [Example:

> ...declares `x2` and `x3` to be of the same type. Their type differs from the types of `x1` and `x4`.

>       template<class T template<class> class TT> struct X { };
>       template<class> struct Y { };
>       template<class T> using Z = Y<T>;
>       X<Y<int> Y> y;
>       X<Z<int> Z> z;

> declares `y` and `z` to be of the same type. —end example]

Additional note, November, 2014:

Concern has been expressed over the proposed resolution with regard to its handling of default template arguments that differ between the template and its alias, e.g.,

       template<typename T, typename U = int> struct A {};
       template<typename T, typename U = char> using B = A<T, U>;
       template<template<typename...> typename C> struct X { C<int> c; };

Notes from the May, 2015 meeting:

See also issue 1979, which CWG is suggesting to be resolved by defining a “simple” alias, one in which the SFINAE conditions are the same as the referenced template and that uses all template parameters.

#### 1554. Access and alias templates

Section: 13.7.8 [temp.alias] Status: drafting Submitter: Jason Merrill Date: 2012-09-17

The interaction of alias templates and access control is not clear from the current wording of 13.7.8 [temp.alias]. For example:

      template <class T> using foo = typename T::foo;

      class B {
        typedef int foo;
        friend struct C;
      };

      struct C {
        foo<B> f;    // Well-formed?
      };

Is the substitution of `B::foo` for `foo<B>` done in the context of the befriended class `C`, making the reference well-formed, or is the access determined independently of the context in which the alias template specialization appears?

If the answer to this question is that the access is determined independently from the context, care must be taken to ensure that an access failure is still considered to be “in the immediate context of the function type” (13.10.3 [temp.deduct] paragraph 8) so that it results in a deduction failure rather than a hard error.

Notes from the October, 2012 meeting:

The consensus of CWG was that instantiation (lookup and access) for alias templates should be as for other templates, in the definition context rather than in the context where they are used. They should still be expanded immediately, however.

Additional note (February, 2014):

A related problem is raised by the definition of `std::enable_if_t` (21.3.3 [meta.type.synop]):

      template <bool b, class T = void>
      using enable_if_t = typename enable_if<b,T>::type;

If `b` is `false`, there will be no `type` member. The intent is that such a substitution failure is to be considered as being “in the immediate context” where the alias template specialization is used, but the existing wording does not seem to accomplish that goal.

Additional note, November, 2014:

Concern has been expressed that the intent to analyze access in the context of the alias template definition is at odds with the fact that friendship cannot be granted to alias templates; if it could, the access violation in the original example could be avoided by making `foo` a friend of class `B`, but that is not possible.

Additional node, February, 2016:

The issue has been returned to "open" status to facilitate further discussion by CWG as to whether the direction in the October, 2012 note is still desirable.

Notes from the February, 2016 meeting:

CWG reaffirmed the direction described in the October, 2012 note above. With regard to the November, 2014 note regarding granting of friendship, it was observed that the same problem occurs with enumerators, which might refer to inaccessible names in the enumerator volue. The solution in both cases is to embed the declaration in a class and grant the class friendship. See issue 1844, dealing with the definition of “immediate context.”

#### 1979. Alias template specialization in template member definition

Section: 13.7.8 [temp.alias] Status: drafting Submitter: Gabriel Dos Reis Date: 2014-07-31

In an example like

      template<typename T> struct A {
        struct B {
          void f();
        };
      };

      template<typename T> using X = typename A<T>::B;

      template<typename T> void X<T>::f() { }       // #1

should #1 be considered a definition of `A<T>::B::f()`?

Analogy with alias-declarations would suggest that it should, but alias template specializations involve issues like SFINAE on unused template parameters (see issue 1558) and possibly other complications.

(See also issues 1980, 2021, 2025, and 2037.)

Notes from the May, 2015 meeting:

CWG felt that this kind of usage should be permitted only via a “simple” alias, in which the SFINAE is the same as the template to which it refers and all the template parameters are used. See also issue 1286.

#### 1980. Equivalent but not functionally-equivalent redeclarations

Section: 13.7.8 [temp.alias] Status: drafting Submitter: Richard Smith Date: 2014-08-04

In an example like

      template<typename T, typename U> using X = T;
      template<typename T> X<void, typename T::type> f();
      template<typename T> X<void, typename T::other> f();

it appears that the second declaration of `f` is a redeclaration of the first but distinguishable by SFINAE, i.e., equivalent but not functionally equivalent.

Notes from the November, 2014 meeting:

CWG felt that these two declarations should not be equivalent.

#### 2236. When is an alias template specialization dependent?

Section: 13.7.8 [temp.alias] Status: drafting Submitter: Maxim Kartashev Date: 2016-03-01

There is implementation divergence for this example:

      struct A { typedef int type; };
      template <typename T> using ALIAS = A;

      template <typename T> void foo()
      {
         ALIAS<T>::type t; // Is typename required here?
      }

      int main()
      {
        foo<A>();
      }

See also issues 1558, 1979, and 2037.

#### 2462. Problems with the omission of the `typename` keyword

Section: 13.8.1 [temp.res.general] Status: drafting Submitter: Mark Hall Date: 2020-12-03

According to 13.8.2 [temp.local] paragraph 5,

> A qualified-id is assumed to name a type if

> - it is a qualified name in a type-id-only context (see below), or

> - it is a decl-specifier of the decl-specifier-seq of a

> - simple-declaration or a function-definition in namespace scope,

> - member-declaration,

> - parameter-declaration in a member-declaration140, unless that parameter-declaration appears in a default argument,

> - parameter-declaration in a declarator of a function or function template declaration whose declarator-id is qualified, unless that parameter-declaration appears in a default argument,

> - ...

There are two possible problems with this specification. First, consider an example like

       template<typename T> struct S {
         static void (*pfunc)(T::name);                               // Omitted typename okay because it is a
                                                                      // member-declaration
       };
       template<typename T> void (*S<T>::pfunc)(T::name) = nullptr;   // Omitted typename ill-formed because not a function
                                                                      // or function template declaration

Should bullet 5.2.4 be extended to include function pointer and member function pointer declarations, as well as function and function template declarations?

Second, given an example like

       template<typename T> struct Y {};
       template<typename T> struct S {
         Y<int(T::type)> m;  // Omitted typename okay because it is in a member-declaration?
      };

Should bullet 5.2.3 be restricted to parameter-declarations of the member being declared, rather than simply “in” such a member-declaration?

Notes from the December, 2020 teleconference:

The second issue was split off into issue 2468 to allow the resolutions to proceed independently.

#### 2468. Omission of the `typename` keyword in a member template parameter list

Section: 13.8.1 [temp.res.general] Status: drafting Submitter: Mark Hall Date: 2020-12-03

According to 13.8.2 [temp.local] paragraph 5,

> A qualified-id is assumed to name a type if

> - it is a qualified name in a type-id-only context (see below), or

> - it is a decl-specifier of the decl-specifier-seq of a

> - simple-declaration or a function-definition in namespace scope,

> - member-declaration,

> - parameter-declaration in a member-declaration140, unless that parameter-declaration appears in a default argument,

> - ...

This specification would appear to allow an example like:

       template<typename T> struct Y {};
       template<typename T> struct S {
         Y<int(T::type)> m;  // Omitted typename okay because it is in a member-declaration?
      };

The affected parameter-declarations should be only those of the member declarator, not in a member template's template parameter list.

(Note: this issue was spun off from issue 2462 to allow the resolutions to proceed independently.)

#### 1390. Dependency of alias template specializations

Section: 13.8.3.2 [temp.dep.type] Status: drafting Submitter: Johannes Schaub Date: 2011-09-04

According to 13.8.3.2 [temp.dep.type] paragraph 8, a type is dependent (among other things) if it is

- a simple-template-id in which either the template name is a template parameter or any of the template arguments is a dependent type or an expression that is type-dependent or value-dependent

This applies to alias template specializations, even if the resulting type does not depend on the template argument:

        struct B { typedef int type; };
        template<typename> using foo = B;
        template<typename T> void f() {
          foo<T>::type * x;  //error: typename required
        }

Is a change to the rules for cases like this warranted?

Notes from the October, 2012 meeting:

CWG agreed that no `typename` should be required in this case. In some ways, an alias template specialization is like the current instantiation and can be known at template definition time.

#### 1524. Incompletely-defined class template base

Section: 13.8.3.2 [temp.dep.type] Status: drafting Submitter: Jason Merrill Date: 2012-07-17

The correct handling of an example like the following is unclear:

      template<typename T> struct A {
        struct B: A { };
      };

A type used as a base must be complete (11.7 [class.derived] paragraph 2) . The fact that the base class in this example is the current instantiation could be interpreted as indicating that it should be available for lookup, and thus the normal rule should apply, as members declared after the nested class would not be visible.

On the other hand, 13.8.3 [temp.dep] paragraph 3 says,

> In the definition of a class or class template, if a base class depends on a template-parameter, the base class scope is not examined during unqualified name lookup either at the point of definition of the class template or member or during an instantiation of the class template or member.

This wording refers not to a dependent type, which would permit lookup in the current instantiation, but simply to a type that “depends on a template-parameter,” and the current instantiation is such a type.

Implementations vary on the handling of this example.

(See also issue 1526 for another case related to the distinction between a “dependent type” and a “type that depends on a template-parameter.”)

Notes from the October, 2012 meeting:

CWG determined that the example should be ill-formed.

#### 2074. Type-dependence of local class of function template

Section: 13.8.3.2 [temp.dep.type] Status: drafting Submitter: Richard Smith Date: 2015-01-20

According to 13.8.3.2 [temp.dep.type] paragraph 9, a local class in a function template is dependent if and only if it contains a subobject of a dependent type. However, given an example like

      template<typename T> void f() {
        struct X {
          typedef int type;
      #ifdef DEPENDENT
          T x;
      #endif
        };
      X::type y;    // #1
      }
      void g() { f<int>(); }

there is implementation variance in the treatment of #1, but whether or not `DEPENDENT` is defined appears to make no difference.

In a related question, should a value-dependent `alignas` specifier cause a type to be dependent? Given

      template<int N> struct Y { typedef int type; };
      template<int N> void h() {
        struct alignas(N) X {};
        Y<alignof(X)>::type z;   // #2
      }
      void i() { h<4>(); }

Most/all implementations issue an error for a missing `typename` in #2.

Perhaps the right answer is that the types should be dependent but a member of the current instantiation, permitting name lookup without `typename`.

Additional notes (September, 2022):

At present, the term "current instantiation" is defined for class templates only, and thus does not apply to function templates.

Moreover, the resolution for this issue should also handle local enums, with particular attention to 9.7.2 [enum.udecl] paragraph 1:

> The elaborated-enum-specifier shall not name a dependent type and...

This rule, without amendment, would disallow the following reasonable example if local enums were made dependent types:

    template <class T>
    void f() {
      enum class E { e1, e2 };
      using enum E;
    }

#### 2275. Type-dependence of function template

Section: 13.8.3.3 [temp.dep.expr] Status: drafting Submitter: Jason Merrill Date: 2016-06-21

Consider:

      struct B { template <class T> void h(); };
      template <class T> struct A {
        template <class U> static U f(U);
        void g() {
         f(B()).h<int>(); // OK, f(B()) is non-type-dependent with type B.
        }
      };

A member template ought to be dependent only if it depends on template parameters of the current scope, but 13.8.3.3 [temp.dep.expr] paragraph 3 is silent on the matter.

#### 2487. Type dependence of function-style cast to incomplete array type

Section: 13.8.3.3 [temp.dep.expr] Status: drafting Submitter: Richard Smith Date: 2021-03-12

Consider:

      using T = int[];
      using U = int[2];
      template<auto M, int ...N> void f() {
        auto &&arr1 = T(N...);
        auto &&arr2 = T{N...};
        auto &&arr3 = U(M, M);
        auto &&arr4 = U{M, M};
      };

I think here `T(N...)` is not type-dependent, per 13.8.3.3 [temp.dep.expr] paragraph 3, but should be. (I think `T{N...}` is type-dependent.) Conversely, I think `U{M, M}` is type-dependent, per 13.8.3.3 [temp.dep.expr] paragraph 6, but should not be. (`U(M, M)` is not type-dependent.)

I think we should say that

simple-type-specifier `(` expression-listopt `)`
typename-specifier `(` expression-listopt `)`
simple-type-specifier braced-init-list
typename-specifier braced-init-list

are type-dependent if the type specifier names a dependent type, or if it names an array of unknown bound and the braced-init-list or expression-list is type-dependent.

(I think we could be a little more precise than that in the case where there is no top-level pack expansion: `T{M, M}` needs to be type-dependent for a general array of unknown bound `T` due to brace elision, but not in the case where the array element type is a scalar type. And `T(M, M)` does not need to be type-dependent because direct aggregate initialization can't perform brace elision. But I think the simpler rule is probably good enough.)

Notes from the August, 2021 teleconference:

CWG agreed with the suggested change. There was some support for the “more precise” approach mentioned in the description.

#### 2090. Dependency via non-dependent base class

Section: 13.8.3.5 [temp.dep.temp] Status: drafting Submitter: Maxim Kartashev Date: 2015-02-27

According to 13.8.3.5 [temp.dep.temp] paragraph 3,

> a non-type template-argument is dependent if the corresponding non-type template-parameter is of reference or pointer type and the template-argument designates or points to a member of the current instantiation or a member of a dependent type.

Members of non-dependent base classes are members of the current instantiation, but using one as a non-type template argument should not be considered dependent.

#### 2. How can dependent names be used in member declarations that appear outside of the class template definition?

Section: 13.8.4 [temp.dep.res] Status: drafting Submitter: unknown Date: unknown

        template <class T> class Foo {

           public:
           typedef int Bar;
           Bar f();
        };
        template <class T> typename Foo<T>::Bar Foo<T>::f() { return 1;}
                           --------------------

In the class template definition, the declaration of the member function is interpreted as:

       int Foo<T>::f();

In the definition of the member function that appears outside of the class template, the return type is not known until the member function is instantiated. Must the return type of the member function be known when this out-of-line definition is seen (in which case the definition above is ill-formed)? Or is it OK to wait until the member function is instantiated to see if the type of the return type matches the return type in the class template definition (in which case the definition above is well-formed)?

Suggested resolution: (John Spicer)

My opinion (which I think matches several posted on the reflector recently) is that the out-of-class definition must match the declaration in the template. In your example they do match, so it is well formed.

I've added some additional cases that illustrate cases that I think either are allowed or should be allowed, and some cases that I don't think are allowed.

        template <class T> class A { typedef int X; };

        template <class T> class Foo {
         public:
           typedef int Bar;
           typedef typename A<T>::X X;
           Bar f();
           Bar g1();
           int g2();
           X h();
           X i();
           int j();
         };

         // Declarations that are okay
         template <class T> typename Foo<T>::Bar Foo<T>::f()
                                                         { return 1;}
         template <class T> typename Foo<T>::Bar Foo<T>::g1()
                                                         { return 1;}
         template <class T> int Foo<T>::g2() { return 1;}
         template <class T> typename Foo<T>::X Foo<T>::h() { return 1;}

         // Declarations that are not okay
         template <class T> int Foo<T>::i() { return 1;}
         template <class T> typename Foo<T>::X Foo<T>::j() { return 1;}

In general, if you can match the declarations up using only information from the template, then the declaration is valid.

Declarations like `Foo::i` and `Foo::j` are invalid because for a given instance of `A<T>`, `A<T>::X` may not actually be int if the class is specialized.

This is not a problem for `Foo::g1` and `Foo::g2` because for any instance of `Foo<T>` that is generated from the template you know that Bar will always be int. If an instance of `Foo` is specialized, the template member definitions are not used so it doesn't matter whether a specialization defines `Bar` as int or not.

#### 287. Order dependencies in template instantiation

Section: 13.8.4.1 [temp.point] Status: drafting Submitter: Martin Sebor Date: 17 May 2001

Implementations differ in their treatment of the following code:

        template <class T>
        struct A {
    	typename T::X x;
        };

        template <class T>
        struct B {
    	typedef T* X;
    	A<B> a;
        };

        int main ()
        {
    	B<int> b;
        }

Some implementations accept it. At least one rejects it because the instantiation of `A<B<int> >` requires that `B<int>` be complete, and it is not at the point at which `A<B<int> >` is being instantiated.

Erwin Unruh:

In my view the programm is ill-formed. My reasoning:

- you need a complete type `B<int>` because you declare a variable in `main`
- `B<int>` contains a member of type `A<B<int> >`, so you need that complete.
- `A<B<int> >` tries to access `B<int>::X`, which in turn needs `B<int>` being complete.

So each class needs the other to be complete.

The problem can be seen much easier if you replace the typedef with

        typedef T (*X) [sizeof(B::a)];

Now you have a true recursion. The compiler cannot easily distinguish between a true recursion and a potential recursion.

John Spicer:

Using a class to form a qualified name does not require the class to be complete, it only requires that the named member already have been declared. In other words, this kind of usage is permitted:

        class A {
            typedef int B;
            A::B ab;
        };

In the same way, once `B` has been declared in `A`, it is also visible to any template that uses `A` through a template parameter.

The standard could be more clear in this regard, but there are two notes that make this point. Both 6.5.5.2 [class.qual] and _N4567_.5.1.1 [expr.prim.general] paragraph 7 contain a note that says "a class member can be referred to using a qualified-id at any point in its potential scope (6.4.7 [basic.scope.class])." A member's potential scope begins at its point of declaration.

In other words, a class has three states: incomplete, being completed, and complete. The standard permits a qualified name to be used once a name has been declared. The quotation of the notes about the potential scope was intended to support that.

So, in the original example, class `A` does not require the type of `T` to be complete, only that it have already declared a member `X`.

Bill Gibbons:

The template and non-template cases are different. In the non-template case the order in which the members become declared is clear. In the template case the members of the instantiation are conceptually all created at the same time. The standard does not say anything about trying to mimic the non-template case during the instantiation of a class template.

Mike Miller:

I think the relevant specification is 13.8.4.1 [temp.point] paragraph 3, dealing with the point of instantiation:

> For a class template specialization... if the specialization is implicitly instantiated because it is referenced from within another template specialization, if the context from which the specialization is referenced depends on a template parameter, and if the specialization is not instantiated previous to the instantiation of the enclosing template, the point of instantiation is immediately before the point of instantiation of the enclosing template. Otherwise, the point of instantiation for such a specialization immediately precedes the namespace scope declaration or definition that refers to the specialization.

That means that the point of instantiation of `A<B<int> >` is before that of `B<int>`, not in the middle of `B<int>` after the declaration of `B::X`, and consequently a reference to `B<int>::X` from `A<B<int> >` is ill-formed.

To put it another way, I believe John's approach requires that there be an instantiation stack, with the results of partially-instantiated templates on the stack being available to instantiations above them. I don't think the Standard mandates that approach; as far as I can see, simply determining the implicit instantiations that need to be done, rewriting the definitions at their respective points of instantiation with parameters substituted (with appropriate "forward declarations" to allow for non-instantiating references), and compiling the result normally should be an acceptable implementation technique as well. That is, the implicit instantiation of the example (using, e.g., `B_int` to represent the generated name of the `B<int>` specialization) could be something like

            struct B_int;

            struct A_B_int {
                B_int::X x;    // error, incomplete type
            };

            struct B_int {
                typedef int* X;
                A_B_int a;
            };

Notes from 10/01 meeting:

This was discussed at length. The consensus was that the template case should be treated the same as the non-template class case it terms of the order in which members get declared/defined and classes get completed.

Proposed resolution:

In 13.8.4.1 [temp.point] paragraph 3 change:

> the point of instantiation is immediately before the point of instantiation of the enclosing template. Otherwise, the point of instantiation for such a specialization immediately precedes the namespace scope declaration or definition that refers to the specialization.

To:

> the point of instantiation is the same as the point of instantiation of the enclosing template. Otherwise, the point of instantiation for such a specialization immediately precedes the nearest enclosing declaration. [Note: The point of instantiation is still at namespace scope but any declarations preceding the point of instantiation, even if not at namespace scope, are considered to have been seen.]

Add following paragraph 3:

> If an implicitly instantiated class template specialization, class member specialization, or specialization of a class template references a class, class template specialization, class member specialization, or specialization of a class template containing a specialization reference that directly or indirectly caused the instantiation, the requirements of completeness and ordering of the class reference are applied in the context of the specialization reference.

and the following example

      template <class T> struct A {
              typename T::X x;
      };

      struct B {
              typedef int X;
              A<B> a;
      };

      template <class T> struct C {
              typedef T* X;
              A<C> a;
      };

      int main ()
      {
              C<int> c;
      }

Notes from the October 2002 meeting:

This needs work. Moved back to drafting status.

See also issues 595 and 1330.

#### 1845. Point of instantiation of a variable template specialization

Section: 13.8.4.1 [temp.point] Status: drafting Submitter: Richard Smith Date: 2014-01-28

The current wording of 13.8.4.1 [temp.point] does not define the point of instantiation of a variable template specialization. Presumably replacing the references to “static data member of a class template” with “variable template” in paragraphs 1 and 8 would be sufficient.

Additional note, July, 2017:

It has also been observed that there is no definition of the point of instantiation for an alias template. It is not clear that there is a need for normative wording for the point of instantiation of an alias template, but if not, a note explaining its absence would be helpful.

#### 2245. Point of instantiation of incomplete class template

Section: 13.8.4.1 [temp.point] Status: drafting Submitter: Richard Smith Date: 2016-03-08

Consider:

      template<typename T> struct X;

      extern X<int> *p;
      void *q = +p; // #1, complete type affects semantics via ADL

      template<typename T> struct X {};
      X<int> x; // #2, ill-formed, X<int> is incomplete

According to the wording of issue 212, this program is ill-formed, because the single point of instantiation for `X<int>` is at #1, thus `X<int>` is an incomplete type even at #2 after the primary template has been completed.

Notes from the December, 2016 teleconference:

The consensus was that references to specializations before the template definition is seen are not points of instantiation.

#### 2497. Points of instantiation for constexpr function templates

Section: 13.8.4.1 [temp.point] Status: drafting Submitter: Richard Smith Date: 2019-07-20

Consider:

      template<typename T> constexpr T f();
      constexpr int g() { return f<int>(); } // #1
      template<typename T> constexpr T f() { return 123; }
      int k[g()];
      // #2

There are two points of instantiation for `f<int>`. At #1, the template isn't defined, so it cannot be instantiated there. At #2, it's too late, as the definition was needed when parsing the type of `k`.

Should we also treat the point of definition of (at least) a constexpr function template as a point of instantiation for all specializations that have a point of instantiation before that point? Note the possible interaction of such a resolution with 13.8.4.1 [temp.point] paragraph 7:

> If two different points of instantiation give a template specialization different meanings according to the one-definition rule (6.3 [basic.def.odr]), the program is ill-formed, no diagnostic required.

Notes from the November, 2021 teleconference:

Another possibility for a point of instantiation, other than the definition of the template, would be the point at which the function is called. Similar questions have been raised regarding the points at which variables are initialized (issue 2186) and constexpr functions are defined (issue 2166).

#### 2202. When does default argument instantiation occur?

Section: 13.9.2 [temp.inst] Status: drafting Submitter: Richard Smith Date: 2015-11-19

According to 13.9.2 [temp.inst] paragraph 11,

> If a function template `f` is called in a way that requires a default argument to be used, the dependent names are looked up, the semantics constraints are checked, and the instantiation of any template used in the default argument is done as if the default argument had been an initializer used in a function template specialization with the same scope, the same template parameters and the same access as that of the function template `f` used at that point, except that the scope in which a closure type is declared (7.5.5.2 [expr.prim.lambda.closure]) — and therefore its associated namespaces — remain as determined from the context of the definition for the default argument. This analysis is called default argument instantiation. The instantiated default argument is then used as the argument of `f`.

Some details are not clear from this description. For example, given

      #include <type_traits>
      template<class T> struct Foo { Foo(T = nullptr) {} };
      bool b = std::is_constructible<Foo<int>>::value;
      int main() {}

does “used” mean odr-used or used in any way? Is a failure of default argument instantiation in the immediate context of the call or is a failure a hard error? And does it apply only to function templates, as it says, or should it apply to member functions of class templates? There is implementation divergence on these questions.

Notes from the March, 2018 meeting:

CWG felt that such errors should be substitution failures, not hard errors.

#### 2222. Additional contexts where instantiation is not required

Section: 13.9.2 [temp.inst] Status: drafting Submitter: CWG Date: 2016-01-11

According to 13.9.2 [temp.inst] paragraph 6,

> If the function selected by overload resolution (12.2 [over.match]) can be determined without instantiating a class template definition, it is unspecified whether that instantiation actually takes place.

There are other contexts in which a smart implementation could presumably avoid instantiations, such as when doing argument-dependent lookup involving a class template specialization when the template definition contains no `friend` declarations or checking base/derived relationships involving incomplete class template definitions. It would be helpful to enumerate such contexts.

#### 2263. Default argument instantiation for `friend`s

Section: 13.9.2 [temp.inst] Status: drafting Submitter: Hubert Tong Date: 2016-05-04

The instantiation of default arguments for friends defined in a templated entity is not covered by 13.7.1 [temp.decls.general] paragraph 3 or 13.9.2 [temp.inst] paragraph 2. Consider:

      template <typename T>
      struct A {
        friend void foo(A &&, int = T::happy) { }
      };

      int main(void) { foo(A<int>(), 0); }

There is implementation divergence in the treatment of this example.

Notes from the December, 2016 teleconference:

This issue should be resolved by the resolution of issue 2174.

#### 2265. Delayed pack expansion and member redeclarations

Section: 13.9.2 [temp.inst] Status: drafting Submitter: Hubert Tong Date: 2016-05-11

It is not clear how to handle parameter packs that are expanded during instantiation in parallel with those that are not yet concrete. In particular, does the following example require a diagnostic?

      template<typename ...T> struct Tuple;
      template<class T, class U> struct Outer;
      template<class ...T, class ...U>
      struct Outer<Tuple<T ...>, Tuple<U ...> > {
        template<class X, class Y> struct Inner;
        template<class ...Y> struct Inner<Tuple<T, Y> ...> { };
        template<class ...Y> struct Inner<Tuple<U, Y> ...> { };
      };
      Outer<Tuple<int, void>, Tuple<int, void> > outer;

Notes from the March, 2018 meeting:

CWG felt that ill-formed, no diagnostic required was the correct approach.

#### 2596. Instantiation of constrained non-template friends

Section: 13.9.2 [temp.inst] Status: drafting Submitter: David Friberg Date: 2022-06-03

Consider:

      struct Base {};

      template<int N>
      struct S : public Base {
        friend int foo(Base&) requires (N == 1) { return 1; }
        friend int foo(Base&) requires (N == 2) { return 3; }
      };

      int main() {
        S<1> s1{};
        S<2> s2{};  // #1
      }

The current wording does not seem to cover what happens for this case. In particular, 13.9.2 [temp.inst] paragraph 17 does not cover constrained non-template friends.

See also the Itanium ABI issue 24.

Suggested resolution:

1. Change in 13.7.5 [temp.friend] paragraph 9 as follows:

   > A non-template friend declaration with a requires-clause shall be a definition. A friend function template with a constraint that depends on a template parameter from an enclosing template shall be a definition. Such a constrained friend function or function template declaration does not declare the same function or function template as a declaration in inhabiting any other scope.

2. Change in 13.9.2 [temp.inst] paragraph 17 as follows:

   > The type-constraints and requires-clause of a template specialization or member templated function are not instantiated along with the specialization or function itself, even for a member function of a local class; substitution into the atomic constraints formed from them is instead performed as specified in 13.5.3 [temp.constr.decl] and 13.5.2.3 [temp.constr.atomic] when determining whether the constraints are satisfied or as specified in 13.5.3 [temp.constr.decl] when comparing declarations.

   > [ Note 7: ... ]

   > [ Example 10: ... ]

   > [ Example:

   >       struct Base {};

   >       template<int N>
   >       struct S : Base {
   >         friend int foo(Base&) requires (N == 1) { return 1; }  // #1
   >         friend int foo(Base&) requires (N == 2) { return 3; }  // #2
   >       };
   >       S<1> s1;
   >       S<2> s2;          // OK, no conflict between #1 and #2
   >       int x = foo(s1);  // OK, selects #1
   >       int y = foo(s2);  // OK, selects #2

   > -- end example ]

   > [ Example 11: ... ]

CWG 2022-11-10

The friend definitions should conflict with friend definitions from other instantiations of the same class template, consistent with how non-constrained friends would work. Note that the enclosing dependent class type does not appear in the friend function's signature, which is unusual.

#### 1665. Declaration matching in explicit instantiations

Section: 13.9.3 [temp.explicit] Status: drafting Submitter: Richard Smith Date: 2013-04-19

Consider a case like

      struct X {
        template<typename T> void f(T);
        void f(int);
      };
      template void X::f(int);

or

      template<typename T> void f(T) {}
      void f(int);
      template void f(int);

Presumably in both these cases the explicit instantiation should refer to the template and not to the non-template; however, 13.7.3 [temp.mem] paragraph 2 says,

> A normal (non-template) member function with a given name and type and a member function template of the same name, which could be used to generate a specialization of the same type, can both be declared in a class. When both exist, a use of that name and type refers to the non-template member unless an explicit template argument list is supplied.

This would appear to give the wrong answer for the first example. It's not clearly stated, but consistency would suggest a similar wrong answer for the second. Presumably a statement is needed somewhere that an explicit instantiation directive applies to a template and not a non-template function if both are visible.

Additional note, January, 2014:

A related example has been raised:

      template<typename T> class Matrix {
      public:
        Matrix(){}
        Matrix(const Matrix&){}
        template<typename U>
          Matrix(const Matrix<U>&);
      };

      template Matrix<int>::Matrix(const Matrix&);

      Matrix<int> m;
      Matrix<int> mm(m);

If the explicit instantiation directive applies to the constructor template, there is no way to explicitly instantiate the copy constructor.

#### 2421. Explicit instantiation of constrained member functions

Section: 13.9.3 [temp.explicit] Status: drafting Submitter: Casey Carter Date: 2019-07-16

An explicit instantiation of a class template specialization also explicitly instantiates member functions of that class template specialization whose constraints are satisfied, even those that are not callable because a more-constrained overload exists which would always be selected by overload resolution. Ideally, we would not explicitly instantiate definitions of such uncallable functions.

Notes from the August, 2020 teleconference:

CWG felt that the concept of “eligible” might form a basis for the resolution of this issue.

#### 2501. Explicit instantiation and trailing requires-clauses

Section: 13.9.3 [temp.explicit] Status: drafting Submitter: Davis Herring Date: 2021-08-09

CWG determined that issue 2488 was not a defect. However, the discussion uncovered an issue regarding the handling of an explicit instantiation of a class template containing such members. According to 13.9.3 [temp.explicit] paragraph 10,

> An explicit instantiation that names a class template specialization is also an explicit instantiation of the same kind (declaration or definition) of each of its direct non-template members that has not been previously explicitly specialized in the translation unit containing the explicit instantiation, provided that the associated constraints, if any, of that member are satisfied by the template arguments of the explicit instantiation (13.5.3 [temp.constr.decl], 13.5.2 [temp.constr.constr]), except as described below.

Paragraph 12 says,

> An explicit instantiation of a prospective destructor (11.4.7 [class.dtor]) shall correspond to the selected destructor of the class.

Perhaps the virtual and constrained members could be handled in an analogous fashion.

Notes from the November, 2021 teleconference:

Issue 2488 is being reopened due to subsequent comments.

CWG 2022-11-10

For each explicit instantiation, there shall be exactly one member whose constraints are more specialized than any other member with the same signature. Use the "address of function" model to determine this member.

#### 529. Use of `template<>` with “explicitly-specialized” class templates

Section: 13.9.4 [temp.expl.spec] Status: drafting Submitter: James Widman Date: 16 August 2005

Paragraph 17 of 13.9.4 [temp.expl.spec] says,

> A member or a member template may be nested within many enclosing class templates. In an explicit specialization for such a member, the member declaration shall be preceded by a `template<>` for each enclosing class template that is explicitly specialized.

This is curious, because paragraph 3 only allows explicit specialization of members of implicitly-instantiated class specializations, not explicit specializations. Furthermore, paragraph 4 says,

> Definitions of members of an explicitly specialized class are defined in the same manner as members of normal classes, and not using the explicit specialization syntax.

Paragraph 18 provides a clue for resolving the apparent contradiction:

> In an explicit specialization declaration for a member of a class template or a member template that appears in namespace scope, the member template and some of its enclosing class templates may remain unspecialized, except that the declaration shall not explicitly specialize a class member template if its enclosing class templates are not explicitly specialized as well. In such explicit specialization declaration, the keyword `template` followed by a template-parameter-list shall be provided instead of the `template<>` preceding the explicit specialization declaration of the member.

It appears from this and the following example that the phrase “explicitly specialized” in paragraphs 17 and 18, when referring to enclosing class templates, does not mean that explicit specializations have been declared for them but that their names in the qualified-id are followed by template argument lists. This terminology is confusing and should be changed.

Proposed resolution (October, 2005):

1. Change 13.9.4 [temp.expl.spec] paragraph 17 as indicated:

> A member or a member template may be nested within many enclosing class templates. In an explicit specialization for such a member, the member declaration shall be preceded by a `template<>` for each enclosing class template that is explicitly specialized specialization. [Example:...

2. Change 13.9.4 [temp.expl.spec] paragraph 18 as indicated:

> In an explicit specialization declaration for a member of a class template or a member template that appears in namespace scope, the member template and some of its enclosing class templates may remain unspecialized, except that the declaration shall not explicitly specialize a class member template if its enclosing class templates are not explicitly specialized as well that is, the template-id naming the template may be composed of template parameter names rather than template-arguments. In For each unspecialized template in such an explicit specialization declaration, the keyword `template` followed by a template-parameter-list shall be provided instead of the `template<>` preceding the explicit specialization declaration of the member. The types of the template-parameters in the template-parameter-list shall be the same as those specified in the primary template definition. In such declarations, an unspecialized template-id shall not precede the name of a template specialization in the qualified-id naming the member. [Example:...

Notes from the April, 2006 meeting:

The revised wording describing “unspecialized” templates needs more work to ensure that the parameter names in the template-id are in the correct order; the distinction between template arguments and parameters is also probably not clear enough. It might be better to replace this paragraph completely and avoid the “unspecialized” wording altogether.

Proposed resolution (February, 2010):

1. Change 13.9.4 [temp.expl.spec] paragraph 17 as follows:

> A member or a member template may be nested within many enclosing class templates. In an explicit specialization for such a member, the member declaration shall be preceded by a `template<>` for each enclosing class template that is explicitly specialized specialization. [Example:...

2. Change 13.9.4 [temp.expl.spec] paragraph 18 as follows:

> In an explicit specialization declaration for a member of a class template or a member template that appears in namespace scope, the member template and some of its enclosing class templates may remain unspecialized, except that the declaration shall not explicitly specialize a class member template if its enclosing class templates are not explicitly specialized as well. In such explicit specialization declaration, the keyword `template` followed by a template-parameter-list shall be provided instead of the `template<>` preceding the explicit specialization declaration of the member. The types of the template-parameters in the template-parameter-list shall be the same as those specified in the primary template definition. that is, the corresponding template prefix may specify a template-parameter-list instead of `template<>` and the template-id naming the template be written using those template-parameters as template-arguments. In such a declaration, the number, kinds, and types of the template-parameters shall be the same as those specified in the primary template definition, and the template-parameters shall be named in the template-id in the same order that they appear in the template-parameter-list. An unspecialized template-id shall not precede the name of a template specialization in the qualified-id naming the member. [Example:...

#### 1840. Non-deleted explicit specialization of deleted function template

Section: 13.9.4 [temp.expl.spec] Status: drafting Submitter: Richard Smith Date: 2014-01-19

The resolution of issue 941 permits a non-deleted explicit specialization of a deleted function template. For example:

      template<typename T> void f() = delete;
      decltype(f<int>()) *p;
      template<> void f<int>();

However, the existing normative wording is not adequate to handle this usage. For one thing, `=delete` is formally, at least, a function definition, and an implementation is not permitted to instantiate a function definition unless it is used; presumably, then, an implementation could not reject the `decltype` above as a reference to a deleted specialization. Furthermore, there should be a requirement that a non-deleted explicit specialization of a deleted function template must precede any reference to that specialization. (I.e., the example should be ill-formed as written but well-formed if the last two lines were interchanged.)

#### 1993. Use of `template<>` defining member of explicit specialization

Section: 13.9.4 [temp.expl.spec] Status: drafting Submitter: Richard Smith Date: 2014-08-31

Issue 531 surveyed existing practice at the time and determined that the most common syntax for defining a member of an explicit specialization used the `template<>` prefix. This approach, however, does not seem consistent, since such a definition is not itself an explicit specialization.

#### 2409. Explicit specializations of constexpr static data members

Section: 13.9.4 [temp.expl.spec] Status: drafting Submitter: Mike Miller Date: 2019-04-29

The status of an example like the following is not clear:

      struct S {
        template <int N> static constexpr inline int m = N;
      };
      template <> constexpr inline int S::m<5>;

Some implementations accept this, apparently on the basis of allowing and ignoring a redeclaration of a constexpr static data member outside its class, although there is implementation divergence. Most or all implementations, however, diagnose an attempt to use such a specialization in a constant context.

Should it be required to have a definition of the explicit specialization in order to declare it outside the class in such cases?

In addition, most or all implementations accept a version of the example in which the explicit specialization contains an initializer, including allowing its use in constant contexts:

      template <> constexpr inline int S::m<5> = 2;

This would seem to be disallowed both by 11.4.9.3 [class.static.data] paragraph 3,

> An inline static data member may be defined in the class definition and may specify a brace-or-equal-initializer. If the member is declared with the `constexpr` specifier, it may be redeclared in namespace scope with no initializer (this usage is deprecated; see _N4778_.D.4 [depr.static_constexpr]).

which prohibits an initializer, and 13.9.4 [temp.expl.spec] paragraph 2,

> An explicit specialization may be declared in any scope in which the corresponding primary template may be defined (_N4868_.9.8.2.3 [namespace.memdef], 11.4 [class.mem], 13.7.3 [temp.mem]).

since the definition of a constexpr static data member is inside the class.

Notes from the May, 2019 teleconference:

These examples should behave in the same way as if the class were templated: instantiate the declaration and the definition of the static data member separately. The first example should be ill-formed, because the explicit specializaation does not have an initializer.

#### 2055. Explicitly-specified non-deduced parameter packs

Section: 13.10.2 [temp.arg.explicit] Status: drafting Submitter: Jonathan Caves Date: 2014-12-09

According to 13.10.2 [temp.arg.explicit] paragraph 3,

> Trailing template arguments that can be deduced (13.10.3 [temp.deduct]) or obtained from default template-arguments may be omitted from the list of explicit template-arguments. A trailing template parameter pack (13.7.4 [temp.variadic]) not otherwise deduced will be deduced to an empty sequence of template arguments. If all of the template arguments can be deduced, they may all be omitted; in this case, the empty template argument list `<>` itself may also be omitted. In contexts where deduction is done and fails, or in contexts where deduction is not done, if a template argument list is specified and it, along with any default template arguments, identifies a single function template specialization, then the template-id is an lvalue for the function template specialization.

It is not clear that this permits an example like:

      template<typename... T> void f(typename T::type...)   {
      }

      int main() {
        f<>();
      }

See also issue 2105.

#### 1172. “instantiation-dependent” constructs

Section: 13.10.3 [temp.deduct] Status: drafting Submitter: Adamczyk Date: 2010-08-05

There are certain constructs that are not covered by the existing categories of “type dependent” and “value dependent.” For example, the expression `sizeof(sizeof(T()))` is neither type-dependent nor value-dependent, but its validity depends on whether `T` can be value-constructed. We should be able to overload on such characteristics and select via deduction failure, but we need a term like “instantiation-dependent” to describe these cases in the Standard. The phrase “expression involving a template parameter” seems to come pretty close to capturing this idea.

Notes from the November, 2010 meeting:

The CWG favored extending the concepts of “type-dependent” and “value-dependent” to cover these additional cases, rather than adding a new concept.

Notes from the March, 2011 meeting:

The CWG reconsidered the direction from the November, 2010 meeting, as it would make more constructs dependent, thus requiring more `template` and `typename` keywords, resulting in worse error messages, etc.

Notes from the August, 2011 meeting:

The following example (from issue 1273) was deemed relevant for this issue:

        template <class T> struct C;

        class A {
           int i;
           friend struct C<int>;
        } a;

        class B {
           int i;
           friend struct C<float>;
        } b;

        template <class T>
        struct C {
           template <class U> decltype (a.i) f() { } // #1
           template <class U> decltype (b.i) f() { } // #2
        };

        int main() {
           C<int>().f<int>();     // calls #1
           C<float>().f<float>(); // calls #2
        }

#### 1322. Function parameter type decay in templates

Section: 13.10.3 [temp.deduct] Status: drafting Submitter: Jason Merrill Date: 2011-05-19

The discussion of issue 1001 seemed to have settled on the approach of doing the 9.3.4.6 [dcl.fct] transformations immediately to the function template declaration, so that the original form need not be remembered. However, the example in 13.10.3 [temp.deduct] paragraph 8 suggests otherwise:

      template <class T> int f(T[5]);
      int I = f<int>(0);
      int j = f<void>(0); // invalid array

One way that might be addressed would be to separate the concepts of the type of the template that participates in overload resolution and function matching from the type of the template that is the source for template argument substitution. (See also the example in paragraph 3 of the same section.)

Notes, January, 2012:

#### 1582. Template default arguments and deduction failure

Section: 13.10.3 [temp.deduct] Status: drafting Submitter: John Spicer Date: 2012-10-31

According to 13.10.3 [temp.deduct] paragraph 5,

> The resulting substituted and adjusted function type is used as the type of the function template for template argument deduction. If a template argument has not been deduced and its corresponding template parameter has a default argument, the template argument is determined by substituting the template arguments determined for preceding template parameters into the default argument. If the substitution results in an invalid type, as described above, type deduction fails.

This leaves the impression that default arguments are used after deduction failure leaves an argument undeduced. For example,

      template<typename T> struct Wrapper;
      template<typename T = int> void f(Wrapper<T>*);
      void g() {
        f(0);
      }

Deduction fails for `T`, so presumably `int` is used. However, some implementations reject this code. It appears that the intent would be better expressed as something like

> ...If a template argument is used only in a non-deduced context and its corresponding template parameter has a default argument...

Rationale (November, 2013):

CWG felt that this issue should be considered by EWG in a broader context before being resolved.

Additional note, April, 2015:

EWG has requested that CWG resolve this issue along the lines discussed above.

Notes from the May, 2015 meeting:

CWG agreed that a default template argument should only be used if the parameter is not used in a deducible context. See also issue 2092.

#### 1513. `initializer_list` deduction failure

Section: 13.10.3.2 [temp.deduct.call] Status: drafting Submitter: Steve Adamczyk Date: 2012-06-28

According to 13.10.3.2 [temp.deduct.call] paragraph 1,

> If removing references and cv-qualifiers from `P` gives `std::initializer_list<P'>` for some `P'` and the argument is an initializer list (9.4.5 [dcl.init.list]), then deduction is performed instead for each element of the initializer list, taking `P'` as a function template parameter type and the initializer element as its argument. Otherwise, an initializer list argument causes the parameter to be considered a non-deduced context (13.10.3.6 [temp.deduct.type]).

It is not entirely clear whether the deduction for an initializer list meeting a `std::initializer_list<T>` is a recursive subcase, or part of the primary deduction. A relevant question is: if the deduction on that part fails, does the entire deduction fail, or is the parameter to be considered non-deduced?

See also issue 2326.

Notes from the October, 2012 meeting:

CWG determined that the entire deduction fails in this case.

#### 1584. Deducing function types from cv-qualified types

Section: 13.10.3.2 [temp.deduct.call] Status: drafting Submitter: Daniel Krügler Date: 2012-11-04

It is not clear whether the following is well-formed or not:

      void foo(){}

      template<class T>
      void deduce(const T*) { }

      int main() {
        deduce(foo);
      }

Implementations vary in their treatment of this example.

Proposed resolution (April, 2013):

Change 13.10.3.6 [temp.deduct.type] paragraph 18 as follows:

> A template-argument can be deduced from a function, pointer to function, or pointer to member function type. [Note: cv-qualification of a deduced function type is ignored; see 9.3.4.6 [dcl.fct]. —end note] [Example:

>       template<class T> void f(void(*)(T,int));
>       template<class T> void f2(const T*);
>       template<class T> void foo(T,int);
>       void g(int,int);
>       void g(char,int);
>       void g2();

>       void h(int,int,int);
>       void h(char,int);
>       int m() {
>         f(&g);     // error: ambiguous
>         f(&h);     // OK: void h(char,int) is a unique match
>         f(&foo);   // error: type deduction fails because foo is a template
>         f2(g2);    // OK: cv-qualification of deduced function type ignored
>       }

> —end example]

Additional note, November, 2014:

Concern was expressed regarding the proposed resolution over its treatment of an example like the following:

      template<typename T> struct tuple_size {};
      template<typename T> struct tuple_size<T const>: tuple_size<T> {};

      tuple_size<void()> t;

In this case `T const` is always considered to be more specialized for `void()`, leading to infinite self-derivation.

The issue has been returned to "open" status for further consideration.

Notes from the May, 2015 meeting:

The consensus of CWG was that the cv-qualification of the argument and parameter must match, so the original example should be rejected.

#### 1486. Base-derived conversion in member pointer deduction

Section: 13.10.3.3 [temp.deduct.funcaddr] Status: drafting Submitter: John Spicer Date: 2012-03-26

The rules for deducing template arguments when taking the address of a function template in 13.10.3.3 [temp.deduct.funcaddr] do not appear to allow for a base-to-derived conversion in a case like:

      struct Base {
        template<class U> void f(U);
      };

      struct Derived : Base { };

      int main() {
        void (Derived::*pmf)(int) = &Derived::f;
      }

Most implementations appear to allow this adjustment, however.

#### 1610. Cv-qualification in deduction of reference to array

Section: 13.10.3.5 [temp.deduct.partial] Status: drafting Submitter: Richard Smith Date: 2013-01-28

Given

       template<class C> void foo(const C* val) {}
       template<int N> void foo(const char (&t)[N]) {}

it is intuitive that the second template is more specialized than the first. However, the current rules make them unordered. In 13.10.3.5 [temp.deduct.partial] paragraph 4, we have P as `const C*` and A as `const char (&)[N]`. Paragraph 5 transforms A to `const char[N]`. Finally, paragraph 7 removes top-level cv-qualification; since a cv-qualified array element type is considered to be cv-qualification of the array (6.8.5 [basic.type.qualifier] paragraph 5, cf issue 1059), A becomes `char[N]`. P remains `const C*`, so deduction fails because of the missing `const` in A.

Notes from the April, 2013 meeting:

CWG agreed that the `const` should be preserved in the array type.

#### 2328. Unclear presentation style of template argument deduction rules

Section: 13.10.3.6 [temp.deduct.type] Status: drafting Submitter: Richard Smith Date: 2016-10-11

The presentation style of 13.10.3.6 [temp.deduct.type] paragraph 8 results in a specification that is unclear, needlessly verbose, and incomplete. Specific problems include:

- What does it mean for `P` and `A` to have one of a set of forms? Do they both have to have that form? (That doesn't happen; typically, only `P` contains template parameters)

- In the introductory sentence, aren't `T`, `TT`, and `i` supposed to be the names of template parameters rather than template arguments?

- In `T[i]`, it appears we can deduce `i`, but not `T` (`T` can only be deduced in the form `T[integer-constant]`

- What is an integer-constant supposed to be?

- What is a cv-list?

- Why can we not deduce `const T` from `T`? (Apparently you only get to deduce if both or neither type have a cv-list, whatever a cv-list is.)

- We have extreme redundancy because, for instance, there is no way to say “in `T (T::*)(T)`, you can deduce any of those `T`s, and it's OK if some of the positions don't have a `T`”. So we have seven (!) forms of that construct, for all cases except the one where none of the three positions contain a `T`.

- We have special case rules for pointers to member functions, even though they're not a special case and should be covered by the rule for pointers to members and the rule for functions.

- We do not allow deducing a template template parameter's value from a template template argument — there is a `TT<T>` form, a `TT<i>` form, a template-name<T> form, and a template-name`<i>` form, but no `TT<TT>` form nor template-name`<TT>` form.

#### 2172. Multiple exceptions with one exception object

Section: 14.4 [except.handle] Status: drafting Submitter: Richard Smith Date: 2015-09-14

During the discussion of issue 2098 it was observed that multiple exceptions may share a single exception object via `std::exception_ptr`. It is not clear that the current wording handles that case correctly.

#### 2219. Dynamically-unreachable handlers

Section: 14.4 [except.handle] Status: drafting Submitter: 2016-01-04 Date: Richard Smith

Consider the following example:

      #include <cstdio>
      #include <cstdlib>

      void f() {
        struct X {
         ~X() {
           std::puts("unwound");
           std::exit(0);
         }
        } x;
        throw 0;
      }

      int main(int argc, char**) {
        try {
          f();
        } catch (int) {
          std::puts("caught");
        }
      }

According to the Standard, this should print `unwound` and exit. Current optimizing implementations call `terminate()`, because:

- `f()` obviously never unwinds (all paths out of it call `exit`)

- Therefore the `catch(int)` clause is unreachable

- Therefore the `catch(int)` clause is removed

- When countering the `throw 0;` statement, the search for a handler finds nothing, so `terminate` is called without unwinding the stack.

More abstractly, before calling `terminate`, we're required to check whether there is an active handler for an exception of type `int`, and in some sense there is not (because the handler in `main` is dynamically unreachable).

There seem to be three possible solutions:

1. Change the standard to say that `terminate()` is a valid response to this situation [this seems problematic, as any non-returning destructor now risks program termination, but is in fact the status quo on multiple implementations and does not seem to have resulted in any bug reports]

2. Always fully unwind before calling terminate() [this significantly harms debugability of exceptions]

3. Teach the compilers to not optimize out unreachable exception handlers [for some implementations, this is “remove, redesign and reimplement middle-end support for EH”-level difficult, and harms the ability to optimize code involving catch handlers]

#### 1718. Macro invocation spanning end-of-file

Section: 15.6 [cpp.replace] Status: drafting Submitter: David Krauss Date: 2013-07-23 Liaison: WG14

Although it seems to be common implementation practice to reject a macro invocation that begins in a header file and whose closing right parenthesis appears in the file that included it, there does not seem to be a prohibition of this case in the specification of function-style macros. Should this be accepted?

Notes from the February, 2014 meeting:

CWG agreed that macro invocations spanning file boundaries should be prohibited. Resolution of this issue should be coordinated with WG14.

#### 2003. Zero-argument macros incorrectly specified

Section: 15.6 [cpp.replace] Status: drafting Submitter: Richard Smith Date: 2014-09-12

According to 15.6 [cpp.replace] paragraph 4,

> If the identifier-list in the macro definition does not end with an ellipsis, the number of arguments (including those arguments consisting of no preprocessing tokens) in an invocation of a function-like macro shall equal the number of parameters in the macro definition.

That is, a sequence of no preprocessing tokens counts as an argument. That phrasing has problems with zero-argument function-like macros, e.g.,

      #define M()
      M();

`M` is defined as having no parameters but the invocation has one (empty) argument, which does not match the number of parameters in the definition.

#### 1709. Stringizing raw string literals containing newline

Section: 15.6.3 [cpp.stringize] Status: drafting Submitter: David Krauss Date: 2013-07-01

Stringizing a raw string literal containing a newline produces an invalid (unterminated) string literal and hence results in undefined behavior. It should be specified that a newline in a string literal is transformed to the two characters `'\' 'n'` in the resulting string literal.

A slightly related case involves stringizing a bare backslash character: because backslashes are only escaped within a string or character literal, a stringized bare backslash becomes `"\"`, which is invalid and hence results in undefined behavior.

#### 1889. Unclear effect of `#pragma` on conformance

Section: 15.9 [cpp.pragma] Status: drafting Submitter: James Widman Date: 2014-03-05

According to 15.9 [cpp.pragma] paragraph 1, the effect of a `#pragma` is to cause

> the implementation to behave in an implementation-defined manner. The behavior might cause translation to fail or cause the translator or the resulting program to behave in a non-conforming manner.

It should be clarified that the extent of the non-conformance is limited to the implementation-defined behavior.

#### 2181. Normative requirements in an informative Annex

Section: Clause Annex B [implimits] Status: drafting Submitter: Sean Hunt Date: 2015-10-18

According to Clause Annex B [implimits] paragraph 1,

> Because computers are finite, C++ implementations are inevitably limited in the size of the programs they can successfully process. Every implementation shall document those limitations where known.

Because Annex Clause Annex B [implimits] is informative, not normative, it should not use “shall.”

#### 1279. Additional differences between C++ 2003 and C++ 2011

Section: C.5 [diff.cpp03] Status: drafting Submitter: Nikolay Ivchenkov Date: 2011-03-27

A number of differences between C++03 and C++11 were omitted from C.5 [diff.cpp03]:

- New keywords. Although these are said in C.5.2 [diff.cpp03.lex] only to invalidate C++03 code, they can also change the meaning, e.g., `thread_local x(y)`, which would declare a variable `thread_local` initialized with `y` in C++03 and a thread-local variable `y` in C++11.

- New deduction rules.

- Removal of the deprecated string literal conversion.

- When a `friend` function defined in a class template is actually defined (i.e., with each instantiation or only when odr-used).

- Removal of access declarations.

- Use of the injected-class-name of a class template as a template template-argument.

Additional note (January, 2012):

In addition to the items previously mentioned, access declarations were removed from C++11 but are not mentioned in C.5 [diff.cpp03].

Proposed (partial) resolution (February, 2012):

Add the following as a new section in C.5 [diff.cpp03]:

> C.2.5 11.8 [class.access]: member access control | pdiff.cpp03.class.access

> Change: Remove access declarations.

> Rationale: Removal of feature deprecated since C++ 1998.

> Effect on original feature: Valid C++ 2003 code that uses access declarations is ill-formed in this International Standard. Instead, using-declarations (9.9 [namespace.udecl]) can be used.

### Issues with "Open" Status

#### 783. Definition of “argument”

Section: Clause 3 [intro.defs] Status: open Submitter: UK Date: 3 March, 2009
N2800 comment UK 3

The definition of an argument does not seem to cover many assumed use cases, and we believe that is not intentional. There should be answers to questions such as: Are lambda-captures arguments? Are type names in a throw-spec arguments? “Argument” to casts, typeid, alignof, alignas, decltype and sizeof? why in x[arg] arg is not an argument, but the value forwarded to operator[]() is? Does not apply to operators as call-points not bounded by parentheses? Similar for copy initialization and conversion? What are deduced template “arguments?” what are “default arguments?” can attributes have arguments? What about concepts, requires clauses and concept_map instantiations? What about user-defined literals where parens are not used?

#### 2632. 'user-declared' is not defined

Section: Clause 3 [intro.defs] Status: open Submitter: Anoop Rana Date: 2022-09-07

The term "user-declared" is used 30 times throughout the standard, but it is not defined.

Possible resolution:

Add a new entry after 3.66 [defns.unspecified] as follows:

> user-declared [defns.user.declared]

> declared in a translation unit using a declarator-id

> [ Note 1 to entry: In contrast, some special member functions can be implicitly declared by the implementation. -- end note]

#### 949. Requirements for freestanding implementations

Section: 4.1 [intro.compliance] Status: open Submitter: Detlef Vollman Date: 2 August, 2009

According to 4.1 [intro.compliance] paragraph 7,

> A freestanding implementation is one in which execution may take place without the benefit of an operating system, and has an implementation-defined set of libraries that includes certain language-support libraries (16.4.2.5 [compliance]).

This definition links two relatively separate topics: the lack of an operating system and the minimal set of libraries. Furthermore, 6.9.3.1 [basic.start.main] paragraph 1 says:

> [Note: in a freestanding environment, start-up and termination is implementation-defined; start-up contains the execution of constructors for objects of namespace scope with static storage duration; termination contains the execution of destructors for objects with static storage duration. —end note]

It would be helpful if the two characteristics (lack of an operating system and restricted set of libraries) were named separately and if these statements were clarified to identify exactly what is implementation-defined.

Notes from the October, 2009 meeting:

The CWG felt that it needed a specific proposal in a paper before attempting to resolve this issue.

#### 1698. Files ending in `\`

Section: 5.2 [lex.phases] Status: open Submitter: David Krauss Date: 2013-06-10

The description of how to handle file not ending in a newline in 5.2 [lex.phases] paragraph 1, phase 2, is:

2. Each instance of a backslash character (`\`) immediately followed by a new-line character is deleted, splicing physical source lines to form logical source lines. Only the last backslash on any physical source line shall be eligible for being part of such a splice. If, as a result, a character sequence that matches the syntax of a universal-character-name is produced, the behavior is undefined. A source file that is not empty and that does not end in a new-line character, or that ends in a new-line character immediately preceded by a backslash character before any such splicing takes place, shall be processed as if an additional new-line character were appended to the file.

This is not clear regarding what happens if the last character in the file is a backslash. In such a case, presumably the result of adding the newline should not be a line splice but rather a backslash preprocessing-token (that will be diagnosed as an invalid token in phase 7), but that should be spelled out.

#### 2573. Undefined behavior when splicing results in a universal-character-name

Section: 5.2 [lex.phases] Status: open Submitter: US Date: 2019-10-23 Liaison: SG12

(From National Body comments US 024 and US 025 on the C++20 DIS.)

Subclause 5.2 [lex.phases] bullet 1.2 specifies:

> Except for splices reverted in a raw string literal, if a splice results in a character sequence that matches the syntax of a universal-character-name, the behavior is undefined.

Undefined behavior during lexing is not acceptable. The behavior ought to be well-defined, ill-formed, or conditionally-supported.

Additional notes (January, 2023):

Forwarded to SG12 with paper issue 1405, by decision of the CWG and SG12 chairs.

#### 2574. Undefined behavior when lexing unmatched quotes

Section: 5.4 [lex.pptoken] Status: open Submitter: US Date: 2019-10-23 Liaison: SG12

(From National Body comment US 027 on the C++20 DIS.)

Subclause 5.4 [lex.pptoken] paragraph 2 specifies:

> If a U+0027 apostrophe or a U+0022 quotation mark character matches the last category, the behavior is undefined.

Undefined behavior during lexing is not acceptable. This ought to be ill-formed.

Additional notes (January, 2023):

Forwarded to SG12 with paper issue 1406, by decision of the CWG and SG12 chairs.

#### 1266. user-defined-integer-literal overflow

Section: 5.13.8 [lex.ext] Status: open Submitter: Michael Wong Date: 2011-03-20

The decimal-literal in a user-defined-integer-literal might be too large for an `unsigned long long` to represent (in implementations with extended integer types). In such cases, the original intent appears to have been to call a raw literal operator or a literal operator template; however, the existing wording of 5.13.8 [lex.ext] paragraph 3 always calls the `unsigned long long` literal operator if it exists, regardless of the value of the decimal-literal.

#### 1209. Is a potentially-evaluated expression in a template definition a “use?”

Section: 6.3 [basic.def.odr] Status: open Submitter: Johannes Schaub Date: 2010-10-08

Consider the following complete program:

        void f();
        template<typename T> void g() { f(); }
        int main() { }

Must `f()` be defined to make this program well-formed? The current wording of 6.3 [basic.def.odr] does not make any special provision for expressions that appear only in uninstantiated template definitions.

(See also issue 1254.)

#### 2488. Overloading virtual functions and functions with trailing requires-clauses

Section: 6.4.1 [basic.scope.scope] Status: open Submitter: Jiang An Date: 2020-08-19

According to 6.4.1 [basic.scope.scope] paragraph 3,

> Two declarations correspond if they (re)introduce the same name, both declare constructors, or both declare destructors, unless

> - ...

> - each declares a function or function template, except when

> - both declare functions with the same parameter-type-list,21 equivalent (13.7.7.2 [temp.over.link]) trailing requires-clauses (if any, except as specified in 13.7.5 [temp.friend]), and, if both are non-static members, the same cv-qualifiers (if any) and ref-qualifier (if both have one), or

> - ...

This would indicate that a virtual function (which cannot have a trailing requires-clause, per 11.7.3 [class.virtual] paragraph 6) can be overloaded with a non-virtual member function with the same parameter type list but with a trailing requires-clause. However, this is not implementable on some ABIs, since the mangling of the two functions would be the same. For example:

      #include <type_traits>
      template<class T>
      struct Foo {
         virtual void fun() const {}
         void fun() const requires std::is_object_v<T> {}
      };
      int main() {
        Foo<int>{}.fun();
      }

Should such overloading be ill-formed or conditionally-supported, or should the current rules be kept?

Rationale (August, 2021):

CWG felt that the current rules are correct; it simply means that only the virtual function can be called, and all other references are simply ambiguous. (See also issue 2501 for a related question dealing with explicit instantiation.

Notes from the November, 2021 teleconference:

The issue has been reopened in response to additional discussion.

CWG 2022-11-11

This is related to issue 2501. CWG solicits a paper to address this issue.

#### 380. Definition of "ambiguous base class" missing

Section: 6.5.2 [class.member.lookup] Status: open Submitter: Jason Merrill Date: 22 Oct 2002

The term "ambiguous base class" doesn't seem to be actually defined anywhere. 6.5.2 [class.member.lookup] paragraph 7 seems like the place to do it.

#### 2567. Operator lookup ambiguity

Section: 6.5.2 [class.member.lookup] Status: open Submitter: Daveed Vandevoorde Date: 2022-04-01

Consider:

      struct B1 {
        bool operator==(B1 const&) const;
      };
      struct B2 {
        bool operator==(B2 const&) const;
      };
      struct D: B1, B2 {} d;
      bool operator==(D const&, D const&);

      auto r = d == d; // ambiguous?

There is implementation divergence in handling this example; some implementations select the non-member operator, others diagnose an ambiguous lookup.

Member name lookup for `operator==` is ambiguous, making the program ill-formed per 6.5.2 [class.member.lookup] paragraph 6:

> The result of the search is the declaration set of S(N, T). If it is an invalid set, the program is ill-formed.

There is no provision for simply failing if the lookup is invoked as part of some larger lookup, as in the case of a lookup for an overloaded operator (12.2.2.3 [over.match.oper] paragraph 3):

> For a unary operator @ with an operand of type cv1 T1, and for a binary operator @ with a left operand of type cv1 T1 and a right operand of type cv2 T2, four sets of candidate functions, designated member candidates, non-member candidates, built-in candidates, and rewritten candidates, are constructed as follows:
> - If T1 is a complete class type or a class currently being defined, the set of member candidates is the result of a search for `operator@` in the scope of T1; otherwise, ....
> - For the operators =, [], or ->, the set of non-member candidates is empty; otherwise, it includes the result of unqualified lookup for `operator@` in the rewritten function call (6.5.3 [basic.lookup.unqual], 6.5.4 [basic.lookup.argdep]), ignoring all member functions. ...
> - For the operator `,`, the unary operator `&`, or the operator `->`, the built-in candidates set is empty. For all other operators, the built-in candidates include all of the candidate operator functions defined in 12.5 [over.built] that ...
> - The rewritten candidate set is determined as follows: ...

It is unclear whether that is intended or desirable.

Suggested resolution:

Change in 6.5.2 [class.member.lookup] paragraph 6 as follows:

> The result of the search is If the declaration set of S(N, T). If it is an invalid set, the program is ill-formed the result of the search is an empty set; otherwise, the result is that set.

#### 2670. Programs and translation units

Section: 6.6 [basic.link] Status: open Submitter: Gabriel dos Reis Date: 2022-12-21

Subclause 6.6 [basic.link] paragraph 1 specifies:

> A program consists of one or more translation units (5.1 [lex.separate]) linked together. A translation unit consists of a sequence of declarations.

Subclause 5.1 [lex.separate] paragraph 1 defines "translation unit":

> A source file together with all the headers (16.4.2.3 [headers]) and source files included (15.3 [cpp.include]) via the preprocessing directive #include, less any source lines skipped by any of the conditional inclusion (15.2 [cpp.cond]) preprocessing directives, is called a translation unit.

Subclause 5.2 [lex.phases] paragraph 7 first mentions the "translation" of translation units:

> Whitespace characters separating tokens are no longer significant. Each preprocessing token is converted into a token (5.6 [lex.token]). The resulting tokens are syntactically and semantically analyzed and translated as a translation unit.

However, 5.2 [lex.phases] paragraph 8 introduces "instantiation units" and 5.2 [lex.phases] paragraph 9 specifies:

> Translated translation units and instantiation units are combined as follows:

and 5.2 [lex.phases] paragraph 9 specifies:

> All external entity references are resolved. Library components are linked to satisfy external references to entities not defined in the current translation. All such translator output is collected into a program image which contains information needed for execution in its execution environment.

The term "linking" in 5.2 [lex.phases] comes after translated translation units and instantiation units are combined, yet 6.6 [basic.link] paragraph 1 states that (untranslated) translation units are linked to form a program. That seems inconsistent.

Additional notes (February, 2023)

See also issue 2518, which introduced the term "preprocessing translation unit" to remove the conflict between 6.6 [basic.link] paragraph 1 and 5.1 [lex.separate] paragraph 1.

#### 2706. Repeated structured binding declarations

Section: 6.6 [basic.link] Status: open Submitter: Jim X Date: 2023-02-13

Consider:

      struct A {
        int a;
      };
      struct B {
        int b;
      };
      auto&& [x] = A{}; //#1
      auto&& [x] = B{}; //#2

A rule is missing to make such repeated structured binding declarations ill-formed.

Suggested resolution:

Change in 6.6 [basic.link] paragraph 11 as follows:

> For any two declarations of an entity E:
> - If one declares E to be a variable or function, the other shall declare E as one of the same type.
> - If one declares E to be an enumerator, the other shall do so.
> - If one declares E to be a structured binding, the program is ill-formed.
> - ...

#### 1953. Data races and common initial sequence

Section: 6.7.1 [intro.memory] Status: open Submitter: Faisal Vali Date: 2014-06-23

According to 6.7.1 [intro.memory] paragraph 3,

> A memory location is either an object of scalar type or a maximal sequence of adjacent bit-fields all having non-zero width. [Note: Various features of the language, such as references and virtual functions, might involve additional memory locations that are not accessible to programs but are managed by the implementation. —end note] Two or more threads of execution (6.9.2 [intro.multithread]) can update and access separate memory locations without interfering with each other.

It is not clear how this relates to the permission granted in 11.4 [class.mem] paragraph 18 to inspect the common initial sequence of standard-layout structs that are members of a standard-layout union. If one thread is writing to the common initial sequence and another is reading from it via a different struct, that should constitute a data race, but the current wording does not clearly state that.

#### 2334. Creation of objects by `typeid`

Section: 6.7.2 [intro.object] Status: open Submitter: Chris Hallock Date: 2017-01-30

The list of ways that an object may be created in 6.7.2 [intro.object] paragraph 1 does not include creation of `type_info` objects by `typeid` expressions, but 7.6.1.8 [expr.typeid] does not appear to require that such objects exist before they are referenced. Should the list in 6.7.2 [intro.object] be extended to include this case?

#### 419. Can cast to virtual base class be done on partially-constructed object?

Section: 6.7.3 [basic.life] Status: open Submitter: Judy Ward Date: 2 June 2003

Consider

      extern "C" int printf (const char *,...);

      struct Base { Base();};
      struct Derived: virtual public Base {
         Derived() {;}
      };

      Derived d;
      extern Derived& obj = d;

      int i;

      Base::Base() {
        if ((Base *) &obj) i = 4;
        printf ("i=%d\n", i);
      }

      int main() { return 0; }

11.9.5 [class.cdtor] paragraph 2 makes this valid, but 6.7.3 [basic.life] paragraph 5 implies that it isn't valid.

Steve Adamczyk: A second issue:

      extern "C" int printf(const char *,...);
      struct A                      { virtual ~A(); int x; };
      struct B : public virtual A   { };
      struct C : public B           { C(int); };
      struct D : public C           { D(); };

      int main()                    { D t; printf("passed\n");return 0; }

      A::~A()                       {}
      C::C(int)                     {}
      D::D() : C(this->x)           {}

Core issue 52 almost, but not quite, says that in evaluating "this->x" you do a cast to the virtual base class A, which would be an error according to 11.9.5 [class.cdtor] paragraph 2 because the base class B constructor hasn't started yet. 7.6.1.5 [expr.ref] should be clarified to say that the cast does need to get done.

James Kanze submitted the same issue via comp.std.c++ on 11 July 2003:

> Richard Smith: Nonsense. You can use "this" perfectly happily in a constructor, just be careful that (a) you're not using any members that are not fully initialised, and (b) if you're calling virtual functions you know exactly what you're doing.

In practice, and I think in intent, you are right. However, the standard makes some pretty stringent restrictions in 6.7.3 [basic.life]. To start with, it says (in paragraph 1):

> The lifetime of an object is a runtime property of the object. The lifetime of an object of type T begins when:
> - storage with the proper alignment and size for type T is obtained, and
> - if T is a class type with a non-trivial constructor, the constructor calls has COMPLETED.
> The lifetime of an object of type T ends when:
> - if T is a class type with a non-trivial destructor, the destructor call STARTS, or
> - the storage which the object occupies is reused or released.

(Emphasis added.) Then when we get down to paragraph 5, it says:

> Before the lifetime of an object has started but after the storage which the object will occupy has been allocated [which sounds to me like it would include in the constructor, given the text above] or, after the lifetime of an object has ended and before the storage which the object occupied is reused or released, any pointer that refers to the storage location where the object will be or was located may be used but only in limited ways. [...] If the object will be or was of a non-POD class type, the program has undefined behavior if:

> [...]

> - the pointer is implicitly converted to a pointer to a base class type, or [...]

I can't find any exceptions for the this pointer.

Note that calling a non-static function in the base class, or even constructing the base class in initializer list, involves an implicit conversion of this to a pointer to the base class. Thus undefined behavior. I'm sure that this wasn't the intent, but it would seem to be what this paragraph is saying.

#### 2258. Storage deallocation during period of destruction

Section: 6.7.3 [basic.life] Status: open Submitter: Richard Smith Date: 2016-04-12

What happens if the storage for an object is deallocated in its period of destruction? Consider:

      struct Base {
        virtual ~Base() {
          ::operator delete(this);
        }
        void operator delete(void*) {}
      };

      struct Derived : Base {};

      int main() {
        delete new Derived;
      }

This ought to be undefined behavior, but the standard is silent on the matter.

Notes from the December, 2016 teleconference:

The consensus view was that this should be undefined behavior.

#### 2551. "Refers to allocated storage" has no meaning

Section: 6.7.3 [basic.life] Status: open Submitter: Andrey Erokhin Date: 2020-09-07

6.7.3 [basic.life] paragraph 6 specifies:

> Before the lifetime of an object has started but after the storage which the object will occupy has been allocated or, after the lifetime of an object has ended and before the storage which the object occupied is reused or released, any pointer that represents the address of the storage location where the object will be or was located may be used but only in limited ways. For an object under construction or destruction, see 11.9.5 [class.cdtor]. Otherwise, such a pointer refers to allocated storage (6.7.5.5.2 [basic.stc.dynamic.allocation]), and using the pointer as if the pointer were of type `void*` is well-defined.

Similarly, 6.7.3 [basic.life] paragraph 7 specifies:

> Similarly, before the lifetime of an object has started but after the storage which the object will occupy has been allocated or, after the lifetime of an object has ended and before the storage which the object occupied is reused or released, any glvalue that refers to the original object may be used but only in limited ways. For an object under construction or destruction, see 11.9.5 [class.cdtor]. Otherwise, such a glvalue refers to allocated storage (6.7.5.5.2 [basic.stc.dynamic.allocation]), and using the properties of the glvalue that do not depend on its value is well-defined.

In either case, it is unclear what "refers to allocated storage" means, beyond the properties ascribed to an object in 6.7.2 [intro.object].

See also issue 1853.

Suggested resolution:

1. Change in 6.7.3 [basic.life] paragraph 6 as follows:

   > For an object under construction or destruction, see 11.9.5 [class.cdtor]. Otherwise, such a pointer refers to allocated storage (6.7.5.5.2 [basic.stc.dynamic.allocation]), and using the such a pointer as if the pointer were of type `void*` is well-defined.

2. Change in 6.7.3 [basic.life] paragraph 7 as follows:

   > For an object under construction or destruction, see 11.9.5 [class.cdtor]. Otherwise, such a glvalue refers to allocated storage (6.7.5.5.2 [basic.stc.dynamic.allocation]), and using the properties of the such a glvalue that do not depend on its value is well-defined.

#### 2676. Replacing a complete object having base subobjects

Section: 6.7.3 [basic.life] Status: open Submitter: Richard Smith Date: 2022-12-06

Base subobjects cannot be transparently replaced with complete objects, as specified in 6.7.3 [basic.life] bullet 8.4:

> An object o1 is transparently replaceable by an object o2 if:
> - ...
> - neither o1 nor o2 is a potentially-overlapping subobject (6.7.2 [intro.object]), and
> - ...

However, that bullet is over-reaching, because it disallows:

      struct A { int n; };
      struct B : A {};
      B b;
      new (&b) B { {5} };  // New A base class does not transparently replace existing A base class due to /8.4.
      int k = b.n;  // UB: member n of A base class is outside its lifetime

See issue 2677 for a suggested resolution.

Additional notes (February, 2023)

Consider this example:

      struct A {
       int n;
       char c;
       // tail padding
      };
      struct B {
       [[no_unique_address]] A a;
       char in_tail_padding[3];
      };

      B b;
      void f() {
       // Transparently replaces old member, potentially overwriting the data in the tail padding!
       new (&b.a) A{};
      }

The suggestions do not address this example.

#### 365. Storage duration and temporaries

Section: 6.7.5 [basic.stc] Status: open Submitter: James Kanze Date: 24 July 2002

There are several problems with 6.7.5 [basic.stc]:

- 6.7.5 [basic.stc] paragraph 2 says that "Static and automatic storage durations are associated with objects introduced by declarations (6.2 [basic.def]) and implicitly created by the implementation (6.7.7 [class.temporary])."

  In fact, objects "implicitly created by the implementation" are the temporaries described in (6.7.7 [class.temporary]), and have neither static nor automatic storage duration, but a totally different duration, described in 6.7.7 [class.temporary].

- 6.7.5 [basic.stc] uses the expression "local object" in several places, without ever defining it. Presumably, what is meant is "an object declared at block scope", but this should be said explicitly.

  In a recent discussion in comp.lang.c++.moderated, on poster interpreted "local objects" as including temporaries. This would require them to live until the end of the block in which they are created, which contradicts 6.7.7 [class.temporary]. If temporaries are covered by this section, and the statement in 6.7.5 [basic.stc] seems to suggest, and they aren't local objects, then they must have static storage duration, which isn't right either.

  I propose adding a fourth storage duration to the list after 6.7.5 [basic.stc] paragraph 1:

  - temporary storage duration

  And rewriting the second paragraph of this section as follows:

  > Temporary storage duration is associated with objects implicitly created by the implementation, and is described in 6.7.7 [class.temporary]. Static and automatic storage durations are associated with objects defined by declarations; in the following, an object defined by a declaration with block scope is a local object. The dynamic storage duration is associated with objects created by the `operator new`.

Steve Adamczyk: There may well be an issue here, but one should bear in mind the difference between storage duration and object lifetime. As far as I can see, there is no particular problem with temporaries having automatic or static storage duration, as appropriate. The point of 6.7.7 [class.temporary] is that they have an unusual object lifetime.

Notes from Ocrober 2002 meeting:

It might be desirable to shorten the storage duration of temporaries to allow reuse of them. The as-if rule allows some reuse, but such reuse requires analysis, including noting whether the addresses of such temporaries have been taken.

Notes from the August, 2011 meeting:

The CWG decided that further consideration of this issue would be deferred until someone produces a paper explaining the need for action and proposing specific changes.

See also issue 1634.

#### 1682. Overly-restrictive rules on function templates as allocation functions

Section: 6.7.5.5.2 [basic.stc.dynamic.allocation] Status: open Submitter: Jason Merrill Date: 2009-03-03

Requirements for allocation functions are given in 6.7.5.5.2 [basic.stc.dynamic.allocation] paragraph 1:

> An allocation function can be a function template. Such a template shall declare its return type and first parameter as specified above (that is, template parameter types shall not be used in the return type and first parameter type). Template allocation functions shall have two or more parameters.

There are a couple of problems with this description. First, it is instances of function templates that can be allocation functions, not the templates themselves (cf 6.7.5.5.3 [basic.stc.dynamic.deallocation] paragraph 2, which uses the correct terminology regarding deallocation functions).

More importantly, this specification was written before template metaprogramming was understood and hence prevents use of SFINAE on the return type or parameter type to select among function template specializations. (The parallel passage for deallocation functions in 6.7.5.5.3 [basic.stc.dynamic.deallocation] paragraph 2 shares this deficit.)

(See also issue 1628.)

#### 523. Can a one-past-the-end pointer be invalidated by deleting an adjacent object?

Section: 6.7.5.5.3 [basic.stc.dynamic.deallocation] Status: open Submitter: comp.std.c++ Date: 8 July 2005

When an object is deleted, 6.7.5.5.3 [basic.stc.dynamic.deallocation] says that the deallocation “[renders] invalid all pointers referring to any part of the deallocated storage.” According to 6.8.4 [basic.compound] paragraph 3, a pointer whose address is one past the end of an array is considered to point to an unrelated object that happens to reside at that address. Does this need to be clarified to specify that the one-past-the-end pointer of an array is not invalidated by deleting the following object? (See also 7.6.2.9 [expr.delete] paragraph 4, which also mentions that the system deallocation function renders a pointer invalid.)

#### 2434. Mandatory copy elision vs non-class objects

Section: 6.7.7 [class.temporary] Status: open Submitter: Richard Smith Date: 2019-09-30

In the following example,

      int f() {
        X x;
        return 4;
      }
      int a = f();

`a` must be directly initialized in the return statement of `f()` because the exception permitting temporaries for function arguments and return types in 6.7.7 [class.temporary] paragraph 3 applies only to certain class types:

> When an object of class type `X` is passed to or returned from a function, if `X` has at least one eligible copy or move constructor (11.4.4 [special]), each such constructor is trivial, and the destructor of `X` is either trivial or deleted, implementations are permitted to create a temporary object to hold the function parameter or result object. The temporary object is constructed from the function argument or return value, respectively, and the function's parameter or return object is initialized as if by using the eligible trivial constructor to copy the temporary (even if that constructor is inaccessible or would not be selected by overload resolution to perform a copy or move of the object). [Note: This latitude is granted to allow objects of class type to be passed to or returned from functions in registers. —end note]

This requirement is observable, since the destructor of `X` in the example could inspect the value of `a`.

The permissions in this paragraph should also apply to all non-class types.

#### 2666. Lifetime extension through `static_cast`

Section: 6.7.7 [class.temporary] Status: open Submitter: Brian Bi Date: 2022-10-17

Consider:

      struct S { S(int) {} };

      const S& r1 = 0; // #1
      const S& r2 = static_cast<const S&>(S(0)); // #2
      const S& r3 = static_cast<const S&>(0); // #3
      // Are r1, r2, and r3 dangling past this point?

For #1, a temporary object is materialized and the reference binds to the result of the materialization conversion. This results in lifetime extension per 6.7.7 [class.temporary] bullet 6.1.

For #2, 6.7.7 [class.temporary] bullet 6.6 requires the cast to convert "without a user-defined conversion", but `S(0)` is such.

For #3, a user-defined conversion is clearly required.

However, implementations treat all three cases the same and do extend the lifetime of the temporary.

Possible resolution:

1. Change in 7.6.1.9 [expr.static.cast] paragraph 1 as follows:

   > The result of the expression `static_cast<T>(v)` is the result of converting the expression v to type T. If T is an lvalue reference type or an rvalue reference to function type, the result is an lvalue; if T is an rvalue reference to object type, the result is an xvalue; otherwise, the result is a prvalue. If `T` is a reference type, the operand shall be a glvalue; otherwise, the operand shall be a prvalue. The `static_cast` operator shall not cast away constness (7.6.1.11 [expr.const.cast]).

2. Change in 7.6.1.9 [expr.static.cast] paragraph 8 as follows:

   > The lvalue-to-rvalue (7.3.2 [conv.lval]), array-to-pointer (7.3.3 [conv.array]), and function-to-pointer (7.3.4 [conv.func]) conversions are applied to the operand. Such a `static_cast` is subject to the restriction that the explicit conversion does not cast away constness (7.6.1.11 [expr.const.cast]), and the following additional rules for specific cases:

3. Change in 7.6.1.10 [expr.reinterpret.cast] paragraph 1 as follows:

   > The result of the expression reinterpret_cast<T>(v) is the result of converting the expression v to type T. If T is an lvalue reference type or an rvalue reference to function type, the result is an lvalue; if T is an rvalue reference to object type, the result is an xvalue; otherwise, the result is a prvalue and the lvalue-to-rvalue (7.3.2 [conv.lval]), array-to-pointer (7.3.3 [conv.array]), and function-to-pointer (7.3.4 [conv.func]) standard conversions are performed on the expression v. If T is a reference type, the operand shall be a glvalue; otherwise, the operand shall be a prvalue. Conversions that can be performed explicitly using reinterpret_cast are listed below. No other conversion can be performed explicitly using reinterpret_cast.

4. Change in 7.6.1.11 [expr.const.cast] paragraph 1 as follows:

   > The result of the expression `const_cast<T>(v)` is of type T. If T is an lvalue reference to object type, the result is an lvalue; if T is an rvalue reference to object type, the result is an xvalue; otherwise, the result is a prvalue and the lvalue-to-rvalue (7.3.2 [conv.lval]), array-to-pointer (7.3.3 [conv.array]), and function-to-pointer (7.3.4 [conv.func]) standard conversions are performed on the expression v. If T is a reference type, the operand shall be a glvalue; otherwise, the operand shall be a prvalue. Conversions that can be performed explicitly using const_cast are listed below. No other conversion shall be performed explicitly using const_cast.

#### 350. `signed char` underlying representation for objects

Section: 6.8 [basic.types] Status: open Submitter: Noah Stein Date: 16 April 2002 Liaison: WG14

Sent in by David Abrahams:

Yes, and to add to this tangent, 6.8.2 [basic.fundamental] paragraph 1 states "Plain char, signed char, and unsigned char are three distinct types." Strangely, 6.8 [basic.types] paragraph 2 talks about how "... the underlying bytes making up the object can be copied into an array of char or unsigned char. If the content of the array of char or unsigned char is copied back into the object, the object shall subsequently hold its original value." I guess there's no requirement that this copying work properly with signed chars!

Notes from October 2002 meeting:

We should do whatever C99 does. 6.5p6 of the C99 standard says "array of character type", and "character type" includes signed char (6.2.5p15), and 6.5p7 says "character type". But see also 6.2.6.1p4, which mentions (only) an array of unsigned char.

Proposed resolution (April 2003):

Change 6.7.3 [basic.life] bullet 5.3 from

- the pointer is used as the operand of a `static_cast` (7.6.1.9 [expr.static.cast]) (except when the conversion is to `void*`, or to `void*` and subsequently to `char*`, or `unsigned` `char*`).

to

- the pointer is used as the operand of a `static_cast` (7.6.1.9 [expr.static.cast]) (except when the conversion is to `void*`, or to `void*` and subsequently to a pointer to byte-character type).

Change 6.7.3 [basic.life] bullet 6.3 from

- the lvalue is used as the operand of a `static_cast` (7.6.1.9 [expr.static.cast]) (except when the conversion is ultimately to `char&` or `unsigned` `char&`), or

to

- the lvalue is used as the operand of a `static_cast` (7.6.1.9 [expr.static.cast]) (except when the conversion is ultimately to a reference to byte-character type), or

Change the beginning of 6.8 [basic.types] paragraph 2 from

> For any object (other than a base-class subobject) of POD type `T`, whether or not the object holds a valid value of type `T`, the underlying bytes (6.7.1 [intro.memory]) making up the object can be copied into an array of `char` or `unsigned` `char`.

to

> For any object (other than a base-class subobject) of POD type `T`, whether or not the object holds a valid value of type `T`, the underlying bytes (6.7.1 [intro.memory]) making up the object can be copied into an array of byte-character type.

Add the indicated text to 6.8.2 [basic.fundamental] paragraph 1:

> Objects declared as characters (`char`) shall be large enough to store any member of the implementation's basic character set. If a character from this set is stored in a character object, the integral value of that character object is equal to the value of the single character literal form of that character. It is implementation-defined whether a `char` object can hold negative values. Characters can be explicitly declared `unsigned` or `signed`. Plain `char`, `signed char`, and `unsigned char` are three distinct types, called the byte-character types. A `char`, a `signed char`, and an `unsigned char` occupy the same amount of storage and have the same alignment requirements (6.8 [basic.types]); that is, they have the same object representation. For byte-character types, all bits of the object representation participate in the value representation. For unsigned byte-character types, all possible bit patterns of the value representation represent numbers. These requirements do not hold for other types. In any particular implementation, a plain `char` object can take on either the same values as a `signed char` or an `unsigned char`; which one is implementation-defined.

Change 7.2.1 [basic.lval] paragraph 15 last bullet from

- a `char` or `unsigned` `char` type.

to

- a byte-character type.

Notes from October 2003 meeting:

It appears that in C99 signed char may have padding bits but no trap representation, whereas in C++ signed char has no padding bits but may have -0. A memcpy in C++ would have to copy the array preserving the actual representation and not just the value.

March 2004: The liaisons to the C committee have been asked to tell us whether this change would introduce any unnecessary incompatibilities with C.

Notes from October 2004 meeting:

The C99 Standard appears to be inconsistent in its requirements. For example, 6.2.6.1 paragraph 4 says:

> The value may be copied into an object of type `unsigned char [`n`]` (e.g., by `memcpy`); the resulting set of bytes is called the object representation of the value.

On the other hand, 6.2 paragraph 6 says,

> If a value is copied into an object having no declared type using `memcpy` or `memmove`, or is copied as an array of character type, then the effective type of the modified object for that access and for subsequent accesses that do not modify the value is the effective type of the object from which the value is copied, if it has one.

Mike Miller will investigate further.

Proposed resolution (February, 2010):

1. Change 6.7.3 [basic.life] bullet 5.4 as follows:

> ...The program has undefined behavior if:

> - ...

> - the pointer is used as the operand of a `static_cast` (7.6.1.9 [expr.static.cast]) (except when the conversion is to cv `void*`, or to cv `void*` and subsequently to `char*`, or `unsigned char*` a pointer to a cv-qualified or cv-unqualified byte-character type (6.8.2 [basic.fundamental])), or

> - ...

2. Change 6.7.3 [basic.life] bullet 6.4 as follows:

> ...The program has undefined behavior if:

> - ...

> - the lvalue is used as the operand of a `static_cast` (7.6.1.9 [expr.static.cast]) except when the conversion is ultimately to cv `char&` or cv `unsigned char&` a reference to a cv-qualified or cv-unqualified byte-character type (6.8.2 [basic.fundamental]) or an array thereof, or

> - ...

3. Change 6.8 [basic.types] paragraph 2 as follows:

> For any object (other than a base-class subobject) of trivially copyable type `T`, whether or not the object holds a valid value of type `T`, the underlying bytes (6.7.1 [intro.memory]) making up the object can be copied into an array of `char` or `unsigned char` a byte-character type (6.8.2 [basic.fundamental]).39 If the content of the that array of `char` or `unsigned char` is copied back into the object, the object shall subsequently hold its original value. [Example:...

4. Change 6.8.2 [basic.fundamental] paragraph 1 as follows:

> ...Characters can be explicitly declared `unsigned` or `signed`. Plain `char`, `signed char`, and `unsigned char` are three distinct types, called the byte-character types. A `char`, a `signed char`, and an `unsigned char` occupy the same amount of storage and have the same alignment requirements (6.7.6 [basic.align]); that is, they have the same object representation. For byte-character types, all bits of the object representation participate in the value representation. For unsigned character types `unsigned char`, all possible bit patterns of the value representation represent numbers...

5. Change 7.2.1 [basic.lval] paragraph 15 final bullet as follows:

> If a program attempts to access the stored value of an object through an lvalue of other than one of the following types the behavior is undefined 52

> - ...

> - a `char` or `unsigned char` byte-character type (6.8.2 [basic.fundamental]).

6. Change 6.7.6 [basic.align] paragraph 6 as follows:

> The alignment requirement of a complete type can be queried using an `alignof` expression (7.6.2.6 [expr.alignof]). Furthermore, the byte-character types (6.8.2 [basic.fundamental]) `char`, `signed char`, and `unsigned char` shall have the weakest alignment requirement. [Note: this enables the byte-character types to be used as the underlying type for an aligned memory area (9.12.2 [dcl.align]). —end note]

7. Change 7.6.2.8 [expr.new] paragraph 10 as follows:

> ...For arrays of `char` and `unsigned char` a byte-character type (6.8.2 [basic.fundamental]), the difference between the result of the new-expression and the address returned by the allocation function shall be an integral multiple of the strictest fundamental alignment requirement (6.7.6 [basic.align]) of any object type whose size is no greater than the size of the array being created. [Note: Because allocation functions are assumed to return pointers to storage that is appropriately aligned for objects of any type with fundamental alignment, this constraint on array allocation overhead permits the common idiom of allocating byte-character arrays into which objects of other types will later be placed. —end note]

Notes from the March, 2010 meeting:

The CWG was not convinced that there was a need to change the existing specification at this time. Some were concerned that there might be implementation difficulties with giving `signed char` the requisite semantics; implementations for which that is true can currently make `char` equivalent to `unsigned char` and avoid those problems, but the suggested change would undermine that strategy.

Additional note, November, 2014:

There is now the term “narrow character type” that should be used instead of “byte-character type”.

#### 146. Floating-point zero

Section: 6.8.2 [basic.fundamental] Status: open Submitter: Andy Sawyer Date: 23 Jul 1999

6.8.2 [basic.fundamental] does not impose a requirement on the floating point types that there be an exact representation of the value zero. This omission is significant in 7.3.14 [conv.fctptr] paragraph 1, in which any non-zero value converts to the `bool` value `true`.

Suggested resolution: require that all floating point types have an exact representation of the value zero.

#### 251. How many signed integer types are there?

Section: 6.8.2 [basic.fundamental] Status: open Submitter: Beman Dawes Date: 18 Oct 2000

6.8.2 [basic.fundamental] paragraph 2 says that

> There are four signed integer types: "`signed char`", "`short int`", "`int`", and "`long int`."

This would indicate that `const int` is not a signed integer type.

Notes from the June, 2016 meeting:

See issue 2185.

#### 2689. Are cv-qualified `std::nullptr_t` fundamental types?

Section: 6.8.2 [basic.fundamental] Status: open Submitter: Anoop Rana Date: 2022-12-08

It is unclear whether cv `std::nullptr_t` is a fundamental type, given that it is declared in a library header and cv-qualifications are not mentioned in 6.8.2 [basic.fundamental] paragraph 15.

Suggested resolution:

Change in 6.8.2 [basic.fundamental] paragraph 15 as follows:

> A type denoted by cv `std::nullptr_t` is a distinct type. A value of type `std::nullptr_t` is a null pointer constant (7.3.12 [conv.ptr]). Such values participate in the pointer and the pointer-to-member conversions (7.3.12 [conv.ptr], 7.3.13 [conv.mem]). `sizeof(std::nullptr_t)` shall be equal to `sizeof(void*)`.

> The types described in this subclause are called fundamental types. [Note 11: Even if the implementation defines two or more fundamental types to have the same value representation, they are nevertheless different types. —end note]

#### 2544. Address of past-the-end of a potentially-overlapping subobject

Section: 6.8.4 [basic.compound] Status: open Submitter: Jiang An Date: 2022-02-20

6.8.4 [basic.compound] paragraph 3 states:

> A value of a pointer type that is a pointer to or past the end of an object represents the address of the first byte in memory (6.7.1 [intro.memory]) occupied by the object [ Footnote: ... ] or the first byte in memory after the end of the storage occupied by the object, respectively.

A potentially-overlapping subobject of type `T` may occupy fewer bytes than indicated by `sizeof(T)`, yet pointer arithmetic will only consider `sizeof(T)`, not the number of actually occupied bytes. For example,

    struct X {
      X() = default;
      int x;
      short y;
    };

    struct S {
      [[no_unique_address]] X x;
      short z;
    };

    static_assert(sizeof(X) == sizeof(S));

On a popular implementation, `z` is actually put into the tail padding of `x`, and thus `&S().x + 1` does not actually point to "the first byte in memory after the end of the storage occupied by" `x`.

Suggested resolution (amended 2022-03-10):

Change in 6.8.4 [basic.compound] paragraph 3 as follows:

> A value V of a pointer type that is a pointer to or past the end of an object represents the address of the first byte in memory (6.7.1 [intro.memory]) occupied by the object A as follows:

> - If V is a pointer to an object, A is the address of the object as specified in 6.7.2 [intro.object] [ Footnote: ... ] or the first byte in memory after the end of the storage occupied by the object, respectively.
> - If V is a pointer past the end of an object that is not a potentially-overlapping subobject, A is the address of the first byte in memory after the end of the storage occupied by the object.
> - If V is a pointer past the end of a potentially-overlapping subobject, A is an implementation-defined choice of an address within the complete object or the address of the first byte in memory after the end of the storage occupied by the complete object.
> - If V is a null pointer value or an invalid pointer value, V does not represent an address.

#### 698. The definition of “sequenced before” is too narrow

Section: 6.9.1 [intro.execution] Status: open Submitter: Jens Maurer Date: 13 July, 2008

According to 6.9.1 [intro.execution] paragraph 14, “sequenced before” is a relation between “evaluations.” However, 6.9.3.3 [basic.start.dynamic] paragraph 3 says,

> If the completion of the initialization of a non-local object with static storage duration is sequenced before a call to `std::atexit` (see `<cstdlib>`, 17.5 [support.start.term]), the call to the function passed to `std::atexit` is sequenced before the call to the destructor for the object. If a call to `std::atexit` is sequenced before the completion of the initialization of a non-local object with static storage duration, the call to the destructor for the object is sequenced before the call to the function passed to `std::atexit`. If a call to `std::atexit` is sequenced before another call to `std::atexit`, the call to the function passed to the second `std::atexit` call is sequenced before the call to the function passed to the first `std::atexit` call.

Except for the calls to `std::atexit`, these events do not correspond to “evaluation” of expressions that appear in the program. If the “sequenced before” relation is to be applied to them, a more comprehensive definition is needed.

#### 1842. Unevaluated operands and “carries a dependency”

Section: 6.9.2 [intro.multithread] Status: open Submitter: Hans Boehm Date: 2014-01-23 Liaison: SG1

According to 6.9.2 [intro.multithread] paragraph 9,

> An evaluation A carries a dependency to an evaluation B if

> - the value of A is used as an operand of B, unless:

> - ...

> - A is the left operand of a built-in logical AND (`&&,` see 7.6.14 [expr.log.and]) or logical OR (`||`, see 7.6.15 [expr.log.or]) operator, or

> - ...

The intent is that this does not apply to the second operands of such operators if the first operand is such that they are not evaluated, but the wording is not clear to that effect. (A similar question applies to the non-selected operand of the conditional operator `?:`.)

Notes from the October, 2015 meeting:

It appears likely that the text involved will be removed by a revision to the `memory_order_consume` specification.

Notes from the February, 2016 meeting:

Action on this issue will be deferred until the specification for `memory_order_consume` is complete; it should not currently be used.

#### 2297. Unclear specification of atomic operations

Section: 6.9.2.2 [intro.races] Status: open Submitter: Kazutoshi Satoda Date: 2016-01-21

It is not sufficiently clear that the only atomic operations are the ones defined in 33.5 [atomics] by the library. The intent is that no accesses are atomic unless the Standard describes them as such.

An additional problem is that, e.g., `new` and `delete` are defined to be synchronization operations, but they are not defined in Clauses 33.5 [atomics] and Clause 33 [thread].

Suggested resolution:

Change 6.9.2.2 [intro.races] paragraph 3 as follows:

> The library defines a number the set of atomic operations (33.5 [atomics]) and operations on mutexes (Clause 33 [thread]) that. Some of these, and some other library operations, such as those on mutexes ( Clause 33 [thread]) are specially identified as synchronization operations. These operations...

Notes from the April, 2017 teleconference:

CWG determined that this issue should be handled editorially; it will be in "review" status until the change has been made and verified. See editorial issue 1611.

Additional notes, October, 2018:

This is also library issue 2506. SG1 has requested a paper to deal with this issue, so it is no longer considered editorial.

#### 2298. Actions and expression evaluation

Section: 6.9.2.2 [intro.races] Status: open Submitter: Kazutoshi Satoda Date: 2016-01-21 Liaison: SG1

Section 6.9.2.2 [intro.races] uses the terms “action” and “expression evaluation” interchangeably. “Sequenced before” is defined on expression evaluations. Probably none of those is correct.

We should really be talking about individual accesses to “memory locations”. Talking about larger “expression evaluations” is incorrect, since they may include internal synchronization. Thus concurrent evaluation of large conflicting expression evaluations may not actually correspond to a data race. I'm not sure what term we should be using instead of “expression evaluation” to denote such individual accesses. Call it X for now.

There is also an issue with the fact that “sequenced before” is defined on expression evaluation. “Sequenced before” should also be defined on Xs. It doesn't make any sense to talk about “sequenced before” ordering on two evaluations when one includes the other. Whenever we say “`A` is sequenced before `B`”, we probably really mean that all Xs in `A` are sequenced before all Xs in `B`. We could probably just include a blanket statement to that effect.

Additional notes (April, 2022)

Forwarded to SG1 with paper issue 1234, reflecting the former "concurrency" status of this issue.

#### 2587. Visible side effects and initial value of an object

Section: 6.9.2.2 [intro.races] Status: open Submitter: Andrey Erokhin Date: 2022-05-10

Subclause 6.9.2.2 [intro.races] paragraph 13 specifies:

> A visible side effect A on a scalar object or bit-field M with respect to a value computation B of M satisfies the conditions:
> - A happens before B and
> - there is no other side effect X to M such that A happens before X and X happens before B.
> The value of a non-atomic scalar object or bit-field M, as determined by evaluation B, shall be the value stored by the visible side effect A.

However, a side effect is defined as 6.9.1 [intro.execution] paragraph 7:

> Reading an object designated by a volatile glvalue (7.2.1 [basic.lval]), modifying an object, calling a library I/O function, or calling a function that does any of those operations are all side effects, which are changes in the state of the execution environment.

It seems that initialization of an object is not a side effect, and thus the value of an scalar object can never be the value obtained during initialization.

#### 371. Interleaving of constructor calls

Section: 6.9.3.2 [basic.start.static] Status: open Submitter: Matt Austern Date: 7 August 2002

Is a compiler allowed to interleave constructor calls when performing dynamic initialization of nonlocal objects? What I mean by interleaving is: beginning to execute a particular constructor, then going off and doing something else, then going back to the original constructor. I can't find anything explicit about this in 6.9.3.2 [basic.start.static].

I'll present a few different examples, some of which get a bit wild. But a lot of what this comes down to is exactly what the standard means when it talks about the order of initialization. If it says that some object x must be initialized before a particular event takes place, does that mean that x's constructor must be entered before that event, or does it mean that it must be exited before that event? If object x must be initialized before object y, does that mean that x's constructor must exit before y's constructor is entered?

(The answer to that question might just be common sense, but I couldn't find an answer in 6.9.3.2 [basic.start.static]. Actually, when I read 6.9.3.2 [basic.start.static] carefully, I find there are a lot of things I took for granted that aren't there.)

OK, so a few specific scenerios.

1. We have a translation unit with nonlocal objects A and B, both of which require dynamic initialization. A comes before B. A must be initialized before B. May the compiler start to construct A, get partway through the constructor, then construct B, and then go back to finishing A?
2. We have a translation unit with nonlocal object A and function f. Construction of A is deferred until after the first statement of main. A must be constructed before the first use of f. Is the compiler permitted to start constructing A, then execute f, then go back to constructing A?
3. We have nonlocal objects A and B, in two different translation units. The order in which A and B are constructed is unspecified by the Standard. Is the compiler permitted to begin constructing A, then construct B, then finish A's constructor? Note the implications of a 'yes' answer. If A's and B's constructor both call some function f, then the call stack might look like this:

          <runtime gunk>
            <Enter A's constructor>
               <Enter f>
                  <runtime gunk>
                     <Enter B's constructor>
                        <Enter f>
                        <Leave f>
                     <Leave B's constructor>
               <Leave f>
            <Leave A's constructor>

   The implication of a 'yes' answer for users is that any function called by a constructor, directly or indirectly, must be reentrant.
4. This last example is to show why a 'no' answer to #3 might be a problem too. New scenerio: we've got one translation unit containing a nonlocal object A and a function f1, and another translation unit containing a nonlocal object B and a function f2. A's constructor calls f2. Initialization of A and B is deferred until after the first statement of main(). Someone in main calls f1. Question: is the compiler permitted to start constructing A, then go off and construct B at some point before f2 gets called, then go back and finish constructing A? In fact, is the compiler required to do that? We've got an unpleasant tension here between the bad implications of a 'yes' answer to #3, and the explicit requirement in 6.9.3.2 [basic.start.static] paragraph 3.

At this point, you might be thinking we could avoid all of this nonsense by removing compilers' freedom to defer initialization until after the beginning of main(). I'd resist that, for two reasons. First, it would be a huge change to make after the standard has been out. Second, that freedom is necessary if we want to have support for dynamic libraries. I realize we don't yet say anything about dynamic libraries, but I'd hate to make decisions that would make such support even harder.

#### 1294. Side effects in dynamic/static initialization

Section: 6.9.3.2 [basic.start.static] Status: open Submitter: Daniel Krügler Date: 2011-04-08

According to 6.9.3.2 [basic.start.static] paragraph 3,

> An implementation is permitted to perform the initialization of a non-local variable with static storage duration as a static initialization even if such initialization is not required to be done statically, provided that

> - the dynamic version of the initialization does not change the value of any other object of namespace scope prior to its initialization, and

> - the static version of the initialization produces the same value in the initialized variable as would be produced by the dynamic initialization if all variables not required to be initialized statically were initialized dynamically.

This does not consider side effects of the initialization in this determination, only the values of namespace-scope variables.

CWG 2022-11-11

The precise normative identification of side effects relevant for the rule remains open. An approach similar to the constexpr model of considering the transitive hull of evaluations might be applicable.

#### 1659. Initialization order of thread_local template static data members

Section: 6.9.3.2 [basic.start.static] Status: open Submitter: Richard Smith Date: 2013-04-14

According to 6.9.3.2 [basic.start.static] paragraph 5,

> It is implementation-defined whether the dynamic initialization of a non-local variable with static or thread storage duration is done before the first statement of the initial function of the thread. If the initialization is deferred to some point in time after the first statement of the initial function of the thread, it shall occur before the first odr-use (6.3 [basic.def.odr]) of any variable with thread storage duration defined in the same translation unit as the variable to be initialized.

This doesn't consider that initialization of instantiations of static data members of class templates (which can be `thread_local`) are unordered. Presumably odr-use of such a static data member should not trigger the initialization of any `thread_local` variable other than that one?

#### 640. Accessing destroyed local objects of static storage duration

Section: 6.9.3.3 [basic.start.dynamic] Status: open Submitter: Howard Hinnant Date: 30 July 2007

6.9.3.3 [basic.start.dynamic] paragraph 2 says,

> If a function contains a local object of static storage duration that has been destroyed and the function is called during the destruction of an object with static storage duration, the program has undefined behavior if the flow of control passes through the definition of the previously destroyed local object.

I would like to turn this behavior from undefined to well-defined behavior for the purpose of achieving a graceful shutdown, especially in a multi-threaded world.

Background: Alexandrescu describes the “phoenix singleton” in Modern C++ Design. This is a class used as a function local static, that will reconstruct itself, and reapply itself to the `atexit` chain, if the program attempts to use it after it is destructed in the `atexit` chain. It achieves this by setting a “destructed flag” in its own state in its destructor. If the object is later accessed (and a member function is called on it), the member function notes the state of the “destructed flag” and does the reconstruction dance. The phoenix singleton pattern was designed to address issues only in single-threaded code where accesses among static objects can have a non-scoped pattern. When we throw in multi-threading, and the possibility that threads can be running after `main` returns, the chances of accessing a destroyed static significantly increase.

The very least that I would like to see happen is to standardize what I believe is existing practice: When an object is destroyed in the `atexit` chain, the memory the object occupied is left in whatever state the destructor put it in. If this can only be reliably done for objects with standard layout, that would be an acceptable compromise. This would allow objects to set “I'm destructed” flags in their state and then do something well-defined if accessed, such as throw an exception.

A possible refinement of this idea is to have the compiler set up a 3-state flag around function-local statics instead of the current 2-state flag:

- Not constructed yet
- Constructed but not destroyed yet
- Destroyed

We have the first two states today. We might choose to add the third state, and if execution passes over a function-local static with “destroyed” state, an exception could be thrown. This would mean that we would not have to guarantee memory stability in destroyed objects of static duration.

This refinement would break phoenix singletons, and is not required for the `~mutex()`/`~condition()` I've described and prototyped. But it might make it easier for Joe Coder to apply this kind of guarantee to his own types.

#### 2684. thread_local dynamic initialization

Section: 6.9.3.3 [basic.start.dynamic] Status: open Submitter: Jason Merrill Date: 2023-01-06

Subclause 6.9.3.3 [basic.start.dynamic] paragraph 7 specifies:

> It is implementation-defined whether the dynamic initialization of a non-block non-inline variable with thread storage duration is sequenced before the first statement of the initial function of a thread or is deferred. If it is deferred, the initialization associated with the entity for thread t is sequenced before the first non-initialization odr-use by t of any non-inline variable with thread storage duration defined in the same translation unit as the variable to be initialized. ...

How does the rule quoted above affect variables declared `constinit`? For example:

      extern thread_local constinit int x;

Clang and gcc do not emit suitable wrapper code to allow for deferred initialization of `x`, which is non-conforming. Should this be allowed?

Suggested resolution:

Change in 6.9.3.3 [basic.start.dynamic] paragraph 7 as follows:

> It is implementation-defined whether the dynamic initialization of a non-block non-inline variable with thread storage duration is sequenced before the first statement of the initial function of a thread or is deferred. If it is deferred, the initialization associated with the entity for thread t is sequenced before the first non-initialization odr-use by t of any non-inline variable with thread storage duration and dynamic initialization defined in the same translation unit as the variable to be initialized. ...

#### 2438. Problems in the specification of qualification conversions

Section: 7.3.6 [conv.qual] Status: open Submitter: Richard Smith Date: 2019-08-14

1. A type has multiple cv-decompositions, and 7.3.6 [conv.qual] paragraph 3 does not say which one to use when determining the cv-combined type. Should this be the longest decomposition that works, i.e., the greatest n for which you can decompose both types? (We used to refer to the cv-qualification signature, which implicitly meant to take the longest decomposition.)

2. When computing the cv-combined types of two types `T1` and `T2`, if `U1` and `U2` are different, shouldn't we add `const` to all layers above that in the type?

3. cv03 is left unspecified by the wording in paragraph 3.

4. We are too eager to replace a Pi3 with “array of unknown bound of”. That should only happen if both Pi1 and Pi2 are array types, or we end up not forming a type T3 that is similar to T1. For example, the cv-combined type of `int**` and `const int (*)[]`, when decomposed with n == 2, is required to be `const int (*)[]` by the bulleted rules, and that type is not similar to the original `T1`.

5. In various places, we have operators that say, “if one operand is of pointer type, apply array-to-pointer conversions, pointer conversions, and qualification conversions to bring the two operands to their composite pointer type,” but that doesn't work, because the definition of composite pointer type can't cope with one operand being a pointer and the other being an array. We either need to define the composite pointer type of a pointer and an array (and, if that's done in terms of computing the cv-combined type, be careful to ensure that computing the cv-combined type actually works in that case) or to perform the array-to-pointer conversion before considering the composite pointer type.

#### 2549. Implicitly moving the operand of a throw-expression in unevaluated contexts

Section: 7.5.4.3 [expr.prim.id.qual] Status: open Submitter: Richard Smith Date: 2022-03-11

Consider:

      void f() {
        X x;
        // Is x an lvalue or an xvalue here?
        void g(int n = (decltype((throw x, 0))()));  // status quo: x is move-eligible here
      }

      void f() {
        X x;
        struct A {
          void g() {
            try {
              struct Y {
                // Is x an lvalue or an xvalue here?
                void h(int n = (decltype((throw x, 0))()));
              };
            } catch (...) { }
          }
        };
      }

11.9.6 [class.copy.elision] paragraph 3 specifies:

> An implicitly movable entity is a variable of automatic storage duration that is either a non-volatile object or an rvalue reference to a non-volatile object type. In the following copy-initialization contexts, a move operation is first considered before attempting a copy operation:
> - ...
> - if the operand of a throw-expression (7.6.18 [expr.throw]) is a (possibly parenthesized) id-expression that names an implicitly movable entity that belongs to a scope that does not contain the compound-statement of the innermost try-block or function-try-block (if any) whose compound-statement or ctor-initializer contains the throw-expression,

Thus, in the first example above, `x` is treated as an xvalue, but it is treated as an lvalue in the second example. This outcome is surprising.

(P2266R2 (Simpler implicit move) moves this wording, introduced by P1825R0 (Merged wording for P0527R1 and P1155R3), from 11.9.6 [class.copy.elision] to 7.5.4.3 [expr.prim.id.qual].)

Suggested resolution (post-P2266R3):

Change in 7.5.4.3 [expr.prim.id.qual] paragraph 3:

> An implicitly movable entity is a variable of with automatic storage duration that is either a non-volatile object or an rvalue reference to a non-volatile object type. In the following contexts, an An id-expression is move-eligible: if
> - it names an implicitly movable entity declared in the body or parameter-declaration-clause of the innermost enclosing function or lambda-expression and
> - If the id-expression (possibly parenthesized) id-expression is the operand of
>   - a `return` or `co_return` statement, and names an implicitly movable entity declared in the body or parameter-declaration-clause of the innermost enclosing function or lambda-expression or
>   - if the id-expression (possibly parenthesized) is the operand of a potentially-evaluated throw-expression, and names an implicitly movable entity that belongs to a scope that does not contain the compound-statement of the innermost lambda-expression, try-block, or function-try-block (if any) whose compound-statement or ctor-initializer encloses the throw-expression and no try-block or function-try-block intervenes between the declaration of the entity and the innermost enclosing scope of the throw-expression.

#### 2561. Conversion to function pointer for lambda with explicit object parameter

Section: 7.5.5.2 [expr.prim.lambda.closure] Status: open Submitter: Barry Revzin Date: 2022-02-14

P0847R7 (Deducing this) (approved October, 2021) added explicit-object member functions. Consider:

      struct C {
        C(auto) { }
      };

      void foo() {
        auto l = [](this C) { return 1; };
        void (*fp)(C) = l;
        fp(1); // same effect as decltype(l){}() or decltype(l){}(1) ?
      }

Subclause 7.5.5.2 [expr.prim.lambda.closure] paragraph 8 does not address explicit object member functions:

> The closure type for a non-generic lambda-expression with no lambda-capture whose constraints (if any) are satisfied has a conversion function to pointer to function with C++ language linkage (9.11 [dcl.link]) having the same parameter and return types as the closure type's function call operator. The conversion is to “pointer to noexcept function” if the function call operator has a non-throwing exception specification. The value returned by this conversion function is the address of a function F that, when invoked, has the same effect as invoking the closure type's function call operator on a default-constructed instance of the closure type. F is a constexpr function if...

Suggested resolution:

1. Change in 7.5.5.2 [expr.prim.lambda.closure] paragraph 8 as follows:

   > ... The value returned by this conversion function is
   > - for a lambda-expression whose parameter-declaration-clause has an explicit object parameter, the address of the function call operator (7.6.2.2 [expr.unary.op];
   > - otherwise, the address of a function F that, when invoked, has the same effect as invoking the closure type's function call operator on a default-constructed instance of the closure type.
   > F is a constexpr function if... is an immediate function.

   > [ Example:

   >       struct C {
   >         C(auto) { }
   >       };

   >       void foo() {
   >         auto a = [](C) { return 0; };
   >         int (*fp)(C) = a;   // OK
   >         fp(1);              // same effect as decltype(a){}(1)
   >         auto b = [](this C) { return 1; };
   >         fp = b;             // OK
   >         fp(1);              // same effect as (&decltype(b)::operator())(1)
   >       }

   > -- end example ]

2. Change in 7.5.5.2 [expr.prim.lambda.closure] paragraph 11 as follows:

   > The value returned by any given specialization of this conversion function template is
   > - for a lambda-expression whose parameter-declaration-clause has an explicit object parameter, the address of the corresponding function call operator template specialization (7.6.2.2 [expr.unary.op]);
   > - otherwise, the address of a function F that, when invoked, has the same effect as invoking the generic lambda's corresponding function call operator template specialization on a default-constructed instance of the closure type.
   > F is a constexpr function if...

#### 2560. Parameter type determination in a requirement-parameter-list

Section: 7.5.7.1 [expr.prim.req.general] Status: open Submitter: Daveed Vandevoorde Date: 2020-01-21

Consider:

      template<typename T>
        requires requires (T p[10]) { (decltype(p))nullptr; }
      int v = 42;
      auto r = v<int>; // well-formed?

This example is only well-formed if the type of the parameter `p` is adjusted to `T*`, but the provisions in 9.3.4.6 [dcl.fct] paragraph 5 cover function parameters only.

One option is to specify application of the same adjustments as for function parameters. Another option is to specify rules that arguably are more useful in a requires-expression.

Suggested resolution:

Change in 7.5.7.1 [expr.prim.req.general] paragraph 3 as follows:

> A requires-expression may introduce local parameters using a parameter-declaration-clause (9.3.4.6 [dcl.fct]). A local parameter of a requires-expression shall not have a default argument. The type of such a parameter is determined as specified for a function parameter in 9.3.4.6 [dcl.fct]. These parameters have no linkage, storage, or lifetime; they are only used as notation for the purpose of defining requirements. The parameter-declaration-clause of a requirement-parameter-list shall not terminate with an ellipsis.
> [Example 2:

>       template<typename T>
>       concept C = requires(T t, ...) {  // error: terminates with an ellipsis
>         t;
>       };
>       concept C2 = requires(T p[2]) {
>         (decltype(p))nullptr;           // OK, p has type "pointer to T"
>       };

> —end example]

#### 2565. Invalid types in the parameter-declaration-clause of a requires-expression

Section: 7.5.7.1 [expr.prim.req.general] Status: open Submitter: Barry Revzin Date: 2022-04-07

Consider:

      template <typename T>
      concept C = requires (typename T::type x) {
        x + 1;
      };

      static_assert(!C<int>);

All implementations accept this translation unit. However, the rule in 7.5.7.1 [expr.prim.req.general] paragraph 5 does not cover the parameter-declaration-clause::

> The substitution of template arguments into a requires-expression may result in the formation of invalid types or expressions in its requirements or the violation of the semantic constraints of those requirements. In such cases, the requires-expression evaluates to `false`; it does not cause the program to be ill-formed. The substitution and semantic constraint checking proceeds in lexical order and stops when a condition that determines the result of the requires-expression is encountered. If substitution (if any) and semantic constraint checking succeed, the requires-expression evaluates to `true`.

Suggested resolution:

Change in 7.5.7.1 [expr.prim.req.general] paragraph 5:

> The substitution of template arguments into a requires-expression may result in the formation of invalid types or expressions in its parameter-declaration-clause (if any) or its requirements or the violation of the semantic constraints of those requirements. In such cases, ...

#### 1642. Missing requirements for prvalue operands

Section: 7.6 [expr.compound] Status: open Submitter: Joseph Mansfield Date: 2013-03-15

Although the note in 7.2.1 [basic.lval] paragraph 1 states that

> The discussion of each built-in operator in Clause 7 [expr] indicates the category of the value it yields and the value categories of the operands it expects

in fact, many of the operators that take prvalue operands do not make that requirement explicit. Possible approaches to address this failure could be a blanket statement that an operand whose value category is not stated is assumed to be a prvalue; adding prvalue requirements to each operand description for which it is missing; or changing the description of the usual arithmetic conversions to state that they imply the lvalue-to-rvalue conversion, which would cover the majority of the omissions.

(See also issue 1685, which deals with an inaccurately-specified value category.)

#### 2284. Sequencing of braced-init-list arguments

Section: 7.6.1.3 [expr.call] Status: open Submitter: Richard Smith Date: 2016-06-30

As of P0400R0 (Wording for Order of Evaluation of Function Arguments), we have in subclause 7.6.1.3 [expr.call] paragraph 8:

> The postfix-expression is sequenced before each expression in the expression-list and any default argument. The initialization of a parameter, including every associated value computation and side effect, is indeterminately sequenced with respect to that of any other parameter.

What about the case when the element in the expression-list is a braced-init-list rather than an expression? The braced-init-list is certainly evaluated left-to-right, but is that required to happen after we evaluate the postfix-expression?

#### 2515. Result of a function call

Section: 7.6.1.3 [expr.call] Status: open Submitter: Andrey Erokhin Date: 2021-11-06

The editorial change referred to in the resolution of issue 2495 updated the terminology used to describe the `return` statement to allow for the fact that the operand could be a braced-init-list instead of an expression. A similar problem exists describing the result of a function call in 7.6.1.3 [expr.call] paragraph 9:

> The result of a function call is the result of the possibly-converted operand of the `return` statement (8.7.4 [stmt.return]) that transferred control out of the called function

It's incorrect to refer to “converting” the operand when it is a braced-init-list.

#### 2660. Confusing term "this parameter"

Section: 7.6.1.3 [expr.call] Status: open Submitter: Anoop Rana Date: 2022-11-23

Subclause 7.6.1.3 [expr.call] paragraph 7 specifies:

> If the function is an implicit object member function, the `this` parameter of the function (7.5.2 [expr.prim.this]) is initialized with a pointer to the object of the call, converted as if by an explicit type conversion (7.6.3 [expr.cast]).

The term "`this` parameter" is undefined.

Suggested resolution:

Change in 7.6.1.3 [expr.call] paragraph 7 as follows:

> If the function is an implicit object member function, the `this` implicit object parameter of the function (7.5.2 [expr.prim.this]) is initialized with a pointer to the object of the call, converted as if by an explicit type conversion (7.6.3 [expr.cast]).

#### 2688. Calling explicit object member functions

Section: 7.6.1.3 [expr.call] Status: open Submitter: Matthew House Date: 2023-01-16

Subclause 7.6.1.3 [expr.call] paragraphs 1 and 2 do not properly handle explicit object member functions. Such a function can be called via a function pointer, via an lvalue obtained by dereferencing a function pointer, and via a class member access expression.

#### 914. Value-initialization of array types

Section: 7.6.1.4 [expr.type.conv] Status: open Submitter: Gabriel Dos Reis Date: 10 June, 2009 Liaison: EWG

Although value-initialization is defined for array types and the `()` initializer is permitted in a mem-initializer naming an array member of a class, the syntax `T()` (where `` is an array type) is explicitly forbidden by 7.6.1.4 [expr.type.conv] paragraph 2. This is inconsistent and the syntax should be permitted.

Rationale (July, 2009):

The CWG was not convinced of the utility of this extension, especially in light of questions about handling the lifetime of temporary arrays. This suggestion needs a proposal and analysis by the EWG before it can be considered by the CWG.

EWG 2022-11-11

This is a defect; a paper is needed. This is tracked in github issue cplusplus/papers#1372.

#### 2705. Accessing ambiguous subobjects

Section: 7.6.1.5 [expr.ref] Status: open Submitter: Davis Herring Date: 2023-02-11

Consider:

      struct A {int i;};
      struct X : A {};
      struct Y : A {};
      struct S : X,Y {};

      void f(S &s) {++s.X::i;}

There is a rule that `&s` be able to "be implicitly converted to a pointer to the naming class of the right operand" (11.8.3 [class.access.base] paragraph 6), which correctly selects `X` here, but 7.6.1.5 [expr.ref] bullet 6.2 merely says that

> If E2 is a non-static data member [...], the expression designates the corresponding member subobject of the object designated by the first expression.

`E2` (i.e. `X::i`) and `Y::i` equally denote `A::i` (since name lookup produces declarations), but there is no rule that indicates which `A::i` is selected as the "corresponding member subobject". (Note that `s.X::A::i` means the same as `s.A::i` and is ambiguous.)

Suggested resolution:

Change 11.8.3 [class.access.base] paragraph 6 to actually perform the conversion it requires, possibly moving it to 7.6.1.5 [expr.ref] since it is not about access control; the definition of "naming class" in 11.8.3 [class.access.base] paragraph 5 would then need to be italicized.

#### 742. Postfix increment/decrement with long bit-field operands

Section: 7.6.1.6 [expr.post.incr] Status: open Submitter: Mike Miller Date: 11 November, 2008

Given the following declarations:

        struct S {
            signed long long sll: 3;
        };
        S s = { -1 };

the expressions `s.sll-- < 0u` and `s.sll < 0u` have different results. The reason for this is that `s.sll--` is an rvalue of type `signed long long` (7.6.1.6 [expr.post.incr]), which means that the usual arithmetic conversions (Clause 7 [expr] paragraph 10) convert `0u` to `signed long long` and the result is `true`. `s.sll`, on the other hand, is a bit-field lvalue, which is promoted (7.3.7 [conv.prom] paragraph 3) to `int`; both operands of `<` have the same rank, so `s.sll` is converted to `unsigned int` to match the type of `0u` and the result is `false`. This disparity seems undesirable.

#### 282. Namespace for `extended_type_info`

Section: 7.6.1.8 [expr.typeid] Status: open Submitter: Jens Maurer Date: 01 May 2001

The original proposed resolution for issue 160 included changing `extended_type_info` (7.6.1.8 [expr.typeid] paragraph 1, footnote 61) to `std::extended_type_info`. There was no consensus on whether this name ought to be part of namespace `std` or in a vendor-specific namespace, so the question was moved into a separate issue.

#### 528. Why are incomplete class types not allowed with `typeid`?

Section: 7.6.1.8 [expr.typeid] Status: open Submitter: Dave Abrahams Date: 18 May 2005

7.6.1.8 [expr.typeid] paragraph 4 says,

> When `typeid` is applied to a type-id, the result refers to a `std::type_info` object representing the type of the type-id. If the type of the type-id is a reference type, the result of the `typeid` expression refers to a `std::type_info` object representing the referenced type. If the type of the type-id is a class type or a reference to a class type, the class shall be completely-defined.

I'm wondering whether this is not overly restrictive. I can't think of a reason to require that `T` be completely-defined in `typeid(T)` when `T` is a class type. In fact, several popular compilers enforce that restriction for `typeid(T)`, but not for `typeid(T&)`. Can anyone explain this?

Nathan Sidwell: I think this restriction is so that whenever the compiler has to emit a typeid object of a class type, it knows what the base classes are, and can therefore emit an array of pointers-to-base-class typeids. Such a tree is necessary to implement `dynamic_cast` and exception catching (in a commonly implemented and obvious manner). If the class could be incomplete, the compiler might have to emit a typeid for incomplete `Foo` in one object file and a typeid for complete `Foo` in another object file. The compilation system will then have to make sure that (a) those compare equal and (b) the complete `Foo` gets priority, if that is applicable.

Unfortunately, there is a problem with exceptions that means there still can be a need to emit typeids for incomplete class. Namely one can throw a pointer-to-pointer-to-incomplete. To implement the matching of pointer-to-derived being caught by pointer-to-base, it is necessary for the typeid of a pointer type to contain a pointer to the typeid of the pointed-to type. In order to do the qualification matching on a multi-level pointer type, one has a chain of pointer typeids that can terminate in the typeid of an incomplete type. You cannot simply NULL-terminate the chain, because one must distinguish between different incomplete types.

Dave Abrahams: So if implementations are still required to be able to do it, for all practical purposes, why aren't we letting the user have the benefits?

Notes from the April, 2006 meeting:

There was some concern expressed that this might be difficult under the IA64 ABI. It was also observed that while it is necessary to handle exceptions involving incomplete types, there is no requirement that the RTTI data structures be used for exception handling.

#### 1954. `typeid` null dereference check in subexpressions

Section: 7.6.1.8 [expr.typeid] Status: open Submitter: David Majnemer Date: 2014-06-23

According to 7.6.1.8 [expr.typeid] paragraph 2,

> If the glvalue expression is obtained by applying the unary `*` operator to a pointer69 and the pointer is a null pointer value (7.3.12 [conv.ptr]), the `typeid` expression throws an exception (14.2 [except.throw]) of a type that would match a handler of type `std::bad_typeid` exception (17.7.5 [bad.typeid]).

The footnote makes clear that this requirement applies without regard to parentheses, but it is unspecified whether it applies when the dereference occurs in a subexpression of the operand (e.g., in the second operand of the comma operator or the second or third operand of a conditional operator). There is implementation divergence on this question.

#### 2048. C-style casts that cast away constness vs `static_cast`

Section: 7.6.1.9 [expr.static.cast] Status: open Submitter: Richard Smith Date: 2014-11-19

According to 7.6.1.9 [expr.static.cast] paragraph 1,

> The `static_cast` operator shall not cast away constness (7.6.1.11 [expr.const.cast]).

However, this phrasing is problematic in the context of a C-style cast like the following:

       const void *p;
       int *q = (int*)p;

The intent of 7.6.3 [expr.cast] is that this should be interpreted as a `static_cast` followed by a `const_cast`. However, because `int*` to `const void*` is a valid standard conversion, and 7.6.1.9 [expr.static.cast] paragraph 7 allows `static_cast` to perform the inverse of a standard conversion sequence, the C-style cast is interpreted as just a `static_cast` without a `const_cast` and is thus ill-formed.

#### 2668. `co_await` in a lambda-expression

Section: 7.6.2.4 [expr.await] Status: open Submitter: Jim X Date: 2022-12-12

Subclause 7.6.2.4 [expr.await] paragraph 2 disallows an await-expression to appear in the body of a lambda-expression:

> An await-expression shall appear only in a potentially-evaluated expression within the compound-statement of a function-body outside of a handler (14.1 [except.pre]). ...

This is probably unintended.

Possible resolution:

Change in 7.6.2.4 [expr.await] paragraph 2 as follows:

> An await-expression shall appear only in as a potentially-evaluated expression within the compound-statement of a function-body or lambda-expression, in either case outside of a handler (14.1 [except.pre]). ...

#### 2609. Padding in class types

Section: 7.6.2.5 [expr.sizeof] Status: open Submitter: Jim X Date: 2022-07-19

Class types may have padding, influencing the result of `sizeof`. It is unclear whether the placement and amount of padding is implementation-defined, unspecified, or something else. If it is unspecified, the limits of permissible behavior are unclear. Empty classes might need special consideration.

Suggested resolution:

Change in 7.6.2.5 [expr.sizeof] paragraph 2

> ... When applied to a class, the result is the number of bytes in an object of that class including any padding required for placing objects of that type in an array. The amount and placement of padding in a class type is unspecified. The result of applying sizeof to a potentially-overlapping subobject is the size of the type, not the size of the subobject. [ Footnote: ... ]

#### 267. Alignment requirement for new-expressions

Section: 7.6.2.8 [expr.new] Status: open Submitter: James Kuyper Date: 4 Dec 2000

Requirements for the alignment of pointers returned by new-expressions are given in 7.6.2.8 [expr.new] paragraph 10:

> For arrays of `char` and `unsigned char`, the difference between the result of the new-expression and the address returned by the allocation function shall be an integral multiple of the most stringent alignment requirement (6.8 [basic.types]) of any object type whose size is no greater than the size of the array being created.

The intent of this wording is that the pointer returned by the new-expression will be suitably aligned for any data type that might be placed into the allocated storage (since the allocation function is constrained to return a pointer to maximally-aligned storage). However, there is an implicit assumption that each alignment requirement is an integral multiple of all smaller alignment requirements. While this is probably a valid assumption for all real architectures, there's no reason that the Standard should require it.

For example, assume that `int` has an alignment requirement of 3 bytes and `double` has an alignment requirement of 4 bytes. The current wording only requires that a buffer that is big enough for an `int` or a `double` be aligned on a 4-byte boundary (the more stringent requirement), but that would allow the buffer to be allocated on an 8-byte boundary — which might not be an acceptable location for an `int`.

Suggested resolution: Change "of any object type" to "of every object type."

A similar assumption can be found in 7.6.1.10 [expr.reinterpret.cast] paragraph 7:

> ...converting an rvalue of type "pointer to `T1`" to the type "pointer to `T2`" (where ... the alignment requirements of `T2` are no stricter than those of `T1`) and back to its original type yields the original pointer value...

Suggested resolution: Change the wording to

> ...converting an rvalue of type "pointer to `T1`" to the type "pointer to `T2`" (where ... the alignment requirements of `T1` are an integer multiple of those of `T2`) and back to its original type yields the original pointer value...

The same change would also be needed in paragraph 9.

Additional note (June, 2022):

Subclause 6.7.6 [basic.align] paragraph 4 specifies:

> ... Every alignment value shall be a non-negative integral power of two.

Thus, the situation that a stricter alignment is not an integral multiple of a weaker alignment does not arise.

#### 473. Block-scope declarations of allocator functions

Section: 7.6.2.8 [expr.new] Status: open Submitter: John Spicer Date: 12 Jul 2004

Looking up `operator new` in a new-expression uses a different mechanism from ordinary lookup. According to 7.6.2.8 [expr.new] paragraph 9,

> If the new-expression begins with a unary `::` operator, the allocation function's name is looked up in the global scope. Otherwise, if the allocated type is a class type `T` or array thereof, the allocation function's name is looked up in the scope of `T`. If this lookup fails to find the name, or if the allocated type is not a class type, the allocation function's name is looked up in the global scope.

Note in particular that the scope in which the new-expression occurs is not considered. For example,

        void f() {
            void* operator new(std::size_t, void*);
            int* i = new int;    // okay?
        }

In this example, the implicit reference to `operator new(std::size_t)` finds the global declaration, even though the block-scope declaration of `operator new` with a different signature would hide it from an ordinary reference.

This seems strange; either the block-scope declaration should be ill-formed or it should be found by the lookup.

Notes from October 2004 meeting:

The CWG agreed that the block-scope declaration should not be found by the lookup in a new-expression. It would, however, be found by ordinary lookup if the allocation function were invoked explicitly.

#### 1628. Deallocation function templates

Section: 7.6.2.8 [expr.new] Status: open Submitter: Richard Smith Date: 2013-02-22

According to 7.6.2.8 [expr.new] paragraphs 18-20, an exception thrown during the initialization of an object allocated by a new-expression will cause a deallocation function to be called for the object's storage if a matching deallocation function can be found. The rules deal only with functions, however; nothing is said regarding a mechanism by which a deallocation function template might be instantiated to free the storage, although 6.7.5.5.3 [basic.stc.dynamic.deallocation] paragraph 2 indicates that a deallocation function can be an instance of a function template.

One possibility for this processing might be to perform template argument deduction on any deallocation function templates; if there is a specialization that matches the allocation function, by the criteria listed in paragraph 20, that function template would be instantiated and used, although a matching non-template function would take precedence as is the usual outcome of overloading between function template specializations and non-template functions.

Another possibility might be to match non-template deallocation functions with non-template allocation functions and template deallocation functions with template allocation functions.

There is a slightly related wording problem in 7.6.2.8 [expr.new] paragraph 21:

> If a placement deallocation function is called, it is passed the same additional arguments as were passed to the placement allocation function, that is, the same arguments as those specified with the new-placement syntax.

This wording ignores the possibility of default arguments in the allocation function, in which case the arguments passed to the deallocation function might be a superset of those specified in the new-placement.

(See also issue 1682.)

#### 2532. Kind of pointer value returned by `new T[0]`

Section: 7.6.2.8 [expr.new] Status: open Submitter: Andrey Erokhin Date: 2022-02-17

A pointer value must have one of the kinds specified in 6.8.4 [basic.compound] paragraph 3:

> Every value of pointer type is one of the following:
> - a pointer to an object or function (the pointer is said to point to the object or function), or
> - a pointer past the end of an object (7.6.6 [expr.add]), or
> - the null pointer value for that type, or
> - an invalid pointer value.

When allocating an array with no elements, 7.6.2.8 [expr.new] paragraph 10 is silent on the kind of pointer value returned:

> When the allocated type is “array of `N T`” (that is, the noptr-new-declarator syntax is used or the new-type-id or type-id denotes an array type), the new-expression yields a prvalue of type “pointer to `T`” that points to the initial element (if any) of the array. Otherwise, let `T` be the allocated type; the new-expression is a prvalue of type “pointer to `T`” that points to the object created.

Related to that, are `p` and `q` allowed to compare equal in the following example?

>     T *p = new T[0];
>     T *q = new T;

Some implementations return a pointer past the array cookie for empty arrays, which can compare equal to a pointer to an object obtained from an unrelated allocation. However, if `new T[0]` is specified to yield a pointer to an object, this behavior violates the rule that pointers to disjoint objects with overlapping lifetimes must not compare equal.

#### 2566. Matching deallocation for uncaught exception

Section: 7.6.2.8 [expr.new] Status: open Submitter: Jim X Date: 2022-04-13

Initialization of an object may terminate via an exception, in which case any dynamically-allocated memory is freed, per 7.6.2.8 [expr.new] paragraph 26:

> If any part of the object initialization described above [ Footnote: ... ] terminates by throwing an exception and a suitable deallocation function can be found, the deallocation function is called to free the memory in which the object was being constructed, after which the exception continues to propagate in the context of the new-expression. If no unambiguous matching deallocation function can be found, propagating the exception does not cause the object's memory to be freed.

However, implementations do not consistently support this provision in case the exception remains uncaught:

      #include <iostream>
      struct C {
        void* operator new(std::size_t n) {
          std::cout << "malloc\n";
          return malloc(n);
        }
        void operator delete(void* ptr) {
          std::cout << "free\n";
          free(ptr);
        }
        C() {
          throw 0;
        }
      };
      int main() {
        auto ptr = new C;
      }

Both clang and GCC do not free the memory in this example; they do so if the exception is caught in `main`.

Maybe a similar provision as used for stack unwinding in 14.4 [except.handle] paragraph 9 is desirable:

> If no matching handler is found, the function `std::terminate` is invoked; whether or not the stack is unwound before this invocation of `std::terminate` is implementation-defined (14.6.2 [except.terminate]).

Suggested resolution:

Integrate freeing dynamically-allocated memory with stack unwinding (14.3 [except.ctor]), since this is what implementations actually do.

#### 2592. Missing definition for placement allocation/deallocation function

Section: 7.6.2.8 [expr.new] Status: open Submitter: Jim X Date: 2022-04-14

Subclause 7.6.2.8 [expr.new] has multiple references to "placement allocation function" and "placement deallocation function", but those terms are never defined. The term "usual deallocation function" is defined in 6.7.5.5.3 [basic.stc.dynamic.deallocation] paragraph 3:

> ... A usual deallocation function is a deallocation function whose parameters after the first are
> - optionally, a parameter of type `std::destroying_delete_t`, then
> - optionally, a parameter of type `std::size_t`, [ Footnote: ... ] then
> - optionally, a parameter of type `std::align_val_t`.

Possible resolution:

1. Split 6.7.5.5.2 [basic.stc.dynamic.allocation] paragraph 1 and change it as follows:

   > ... The value of the first parameter is interpreted as the requested size of the allocation. A usual allocation function is an allocation function with no parameters after the first or with a single parameter of type `std::align_val_t` after the first. A placement allocation function is an allocation function that is not a usual allocation function.

   > An allocation function can be a function template. ...

2. Split 6.7.5.5.3 [basic.stc.dynamic.deallocation] paragraph 3 and change it as follows:

   > ... A destroying operator delete shall be a usual deallocation function. A placement deallocation function is a deallocation function that is not a usual deallocation function.

   > A deallocation function may be an instance of a function template. ...

#### 196. Arguments to deallocation functions

Section: 7.6.2.9 [expr.delete] Status: open Submitter: Matt Austern Date: 20 Jan 2000

7.6.2.8 [expr.new] paragraph 10 says that the result of an array allocation function and the value of the array new-expression from which it was invoked may be different, allowing for space preceding the array to be used for implementation purposes such as saving the number of elements in the array. However, there is no corresponding description of the relationship between the operand of an array delete-expression and the argument passed to its deallocation function.

6.7.5.5.3 [basic.stc.dynamic.deallocation] paragraph 3 does state that

> the value supplied to `operator delete[](void*)` in the standard library shall be one of the values returned by a previous invocation of either `operator new[](std::size_t)` or `operator new[](std::size_t, const std::nothrow_t&)` in the standard library.

This statement might be read as requiring an implementation, when processing an array delete-expression and calling the deallocation function, to perform the inverse of the calculation applied to the result of the allocation function to produce the value of the new-expression. (7.6.2.9 [expr.delete] paragraph 2 requires that the operand of an array delete-expression "be the pointer value which resulted from a previous array new-expression.") However, it is not completely clear whether the "shall" expresses an implementation requirement or a program requirement (or both). Furthermore, there is no direct statement about user-defined deallocation functions.

Suggested resolution: A note should be added to 7.6.2.9 [expr.delete] to clarify that any offset added in an array new-expression must be subtracted in the array delete-expression.

#### 2593. Insufficient base class restriction for pointer-to-member expression

Section: 7.6.4 [expr.mptr.oper] Status: open Submitter: Hubert Tong Date: 2022-06-04

Consider:

      struct A {};
      struct AA : A { int y; };
      struct B : A { int x; };
      struct C : AA, B {};

      constexpr int f(const A &a) {
        int A::*mp = static_cast<int A::*>(&B::x);
        return a.*mp;
      }

      extern char x[f(static_cast<const AA &>(C{{{}, 13}, {{}, 42}}))];
      extern char x[13];

Subclause 7.6.4 [expr.mptr.oper] paragraph 4 specifies:

> Abbreviating pm-expression.*cast-expression as E1.*E2, E1 is called the object expression. If the dynamic type of E1 does not contain the member to which E2 refers, the behavior is undefined.

In the example, the dynamic type of `a` is `C`, which does contain `B::x`, and the undefined behavior provision does not trigger. Thus the call to `f` is required to yield 42; however common implementations produce 13. The behavior for this case ought to be undefined.

Suggested resolution:

Change in 7.6.4 [expr.mptr.oper] paragraph 4 as follows:

> Abbreviating pm-expression.*cast-expression as E1.*E2, E1 is called the object expression. If the dynamic type of E1 does not contain the member to which E2 refers, Where the type of E2 is "pointer to member of T", C is the (unique) class of which the member to which E2 refers is a direct member, and B is the object of type T that either is the result of E1 or is the uniquely so-typed base subobject thereof, if B is neither of type C nor a base class subobject of an object of type C, then the behavior is undefined.

#### 2548. Array prvalues and additive operators

Section: 7.6.6 [expr.add] Status: open Submitter: Andrey Erokhin Date: 2022-03-08

Consider

      int main()
      {
        using IA = int[];
        IA{ 1, 2, 3 } + 0;
      }

This appears to be ill-formed given the current wording, because the operand is already a prvalue, thus 7.2.1 [basic.lval] paragraph 6 does not apply and the array-to-pointer conversion is not applied:

> Whenever a glvalue appears as an operand of an operator that expects a prvalue for that operand, the lvalue-to-rvalue (7.3.2 [conv.lval]), array-to-pointer (7.3.3 [conv.array]), or function-to-pointer (7.3.4 [conv.func]) standard conversions are applied to convert the expression to a prvalue.

This outcome might be an oversight in the resolution for issue 1232.

See also clang issue 54016.

#### 2696. Relational comparisons of pointers to `void`

Section: 7.6.9 [expr.rel] Status: open Submitter: CWG Date: 2023-02-09 Liaison: SG22

While discussing issue 2526, it was noted that C++ yields an unspecified result for comparing unequal pointers to `void` in all cases, whereas C does specify the result if the pointer values point to the same object or (possibly different) subobjects thereof, because C considers `void` an object type.

February, 2023

Requested confirmation of the C status quo from SG22.

#### 1255. Definition problems with `constexpr` functions

Section: 7.7 [expr.const] Status: open Submitter: Nikolay Ivchenkov Date: 2011-03-08

The current wording of the Standard is not sufficiently clear regarding the interaction of class scope (which treats the bodies of member functions as effectively appearing after the class definition is complete) and the use of `constexpr` member functions within the class definition in contexts requiring constant expressions. For example, an array bound cannot use a `constexpr` member function that relies on the completeness of the class or on members that have not yet been declared, but the current wording does not appear to state that.

Additional note (October, 2013):

This question also affects function return type deduction (the `auto` specifier) in member functions. For example, the following should presumably be prohibited, but the current wording is not clear:

      struct S {
        static auto f() {
          return 42;
        }
        auto g() -> decltype(f()) {
          return f();
        }
      };

#### 1256. Unevaluated operands are not necessarily constant expressions

Section: 7.7 [expr.const] Status: open Submitter: Nikolay Ivchenkov Date: 2011-03-08

The current definition of constant expressions appears to make unevaluated operands constant expressions; for example, `new char[10]` would seem to be a constant expression if it appears as the operand of `sizeof`. This seems wrong.

#### 1626. `constexpr` member functions in brace-or-equal-initializers

Section: 7.7 [expr.const] Status: open Submitter: Daveed Vandevoorde Date: 2013-02-19

The Standard should make clear that a `constexpr` member function cannot be used in a constant expression until its class is complete. For example:

      template<typename T> struct C {
        template<typename T2> static constexpr bool _S_chk() {
          return false;
        }
        static const bool __value = _S_chk<int>();
      };

      C<double> c;

Current implementations accept this, although they reject the corresponding non-template case:

      struct C {
        static constexpr bool _S_chk() { return false; }
        static const bool __value = _S_chk();
      };

      C c;

Presumably the template case should be handled consistently with the non-template case.

#### 2192. Constant expressions and order-of-eval undefined behavior

Section: 7.7 [expr.const] Status: open Submitter: Peter Sommerlad Date: 2015-10-27

Notes from the November, 2016 meeting:

CWG did not wish to require implementations to detect this kind of undefined behavior in determining whether an expression is constant or not, but an implementation should be permitted to reject such expressions. These should be indeterminately sequenced, not unsequenced.

#### 2301. Value-initialization and constexpr constructor evaluation

Section: 7.7 [expr.const] Status: open Submitter: Daveed Vandevoorde Date: 2016-04-18

Consider the following example:

      union A {
        constexpr A(int) : x(++x) { }
        int x;
        char* y;
      };
      union B {
        A a = 5;
      };
      int arr[B().a.x];

Value-initialization of the object created by `B()` zero-initializes the object (9.4 [dcl.init] bullet 8.2), which should mean that the `++x` in the mem-initilizer for `A` operates on a zero-initialized object, but current implementations reject this code as non-constant. It is not clear what in the current wording justifies this treatment.

#### 2456. Viable user-defined conversions in converted constant expressions

Section: 7.7 [expr.const] Status: open Submitter: Mike Miller Date: 2020-08-17

Consider an example like the following:

      struct A {
        constexpr A(int i) : val(i) { }
        constexpr operator int() const { return val; }
        constexpr operator float() const { return val; }
      private:
        int val;
      };
      constexpr A a = 42;
      int ary[a];

According to 9.3.4.5 [dcl.array] paragraph 1, the array bound expression

> shall be a converted constant expression of type `std::size_t` (7.7 [expr.const]).

The user-defined conversion to `float` would involve a floating-integral conversion (7.3.11 [conv.fpint]; however, such a conversion is not permitted by the list of acceptable conversions in 7.7 [expr.const] paragraph 10:

> A converted constant expression of type `T` is an expression, implicitly converted to type `T`, where the converted expression is a constant expression and the implicit conversion sequence contains only

> - user-defined conversions,

> - lvalue-to-rvalue conversions (7.3.2 [conv.lval]),

> - array-to-pointer conversions (7.3.3 [conv.array]),

> - function-to-pointer conversions (7.3.4 [conv.func]),

> - qualification conversions (7.3.6 [conv.qual]),

> - integral promotions (7.3.7 [conv.prom]),

> - integral conversions (7.3.9 [conv.integral]) other than narrowing conversions (9.4.5 [dcl.init.list]),

> - null pointer conversions (7.3.12 [conv.ptr]) from `std::nullptr_t`,

> - null member pointer conversions (7.3.13 [conv.mem]) from `std::nullptr_t`, and

> - function pointer conversions (7.3.14 [conv.fctptr]),

> and where the reference binding (if any) binds directly.

It is not clear whether this list is intended to restrict the set of viable user-defined conversions, and there is implementation divergence on this point: clang accepts the example above, while g++ rejects it, presumably on the basis of an ambiguous conversion.

Notes from the August, 2020 teleconference:

No direction was established pending information about why the example is accepted by clang.

Additional note, December, 2020:

The clang behavior turns out to have been an oversight, corrected in the current version, so the example is now rejected by both compilers. However, it is unclear that this is desirable. In particular, given the example above, `a` can be used without error as a bit-field width, as an enumerator value, and as the operand of `alignas`. Presumably the difference between these integral constant expression contexts and an array bound is the fact that the target type is known to be `size_t`. However, both bit-field widths and `alignas` operands are also required to be non-negative. Furthermore, the definition of an “erroneous” array bound in 7.6.2.8 [expr.new] paragraph 9 goes to awkward lengths to check for negative values as the result of user-defined conversions, which might argue in favor of reconsidering the converted constant expression treatment of array bounds.

Notes from the February, 2021 teleconference:

CWG agreed with the considerations in the December, 2020 note, feeling that the difference in treatment between integral constant expressions and a converted constant expression to a specific integral type is somewhat gratuitous. However, it was felt that code like that of the example was unlikely to occur often in real-world code.

#### 2536. Partially initialized variables during constant initialization

Section: 7.7 [expr.const] Status: open Submitter: Barry Revzin Date: 2022-02-21 Liaison: EWG

Consider:

      struct A { int x = 1; int y; };
      constinit A a;                   // static storage duration; #1

The treatment of this example changed with P1331R2 (Permitting trivial default initialization in constexpr contexts), adopted 2019-07. Prior to this paper, the default constructor of `A` was not `constexpr` because it left a data member uninitialized. With paper P1331, the restriction was shifted to reading uninitialized objects during constant evaluation, and the variable `a` now satisfies the requirements for "constant-initialized" in 7.7 [expr.const] paragraph 2:

> A variable or temporary object o is constant-initialized if
> - either it has an initializer or its default-initialization results in some initialization being performed, and
> - the full-expression of its initialization is a constant expression when interpreted as a constant-expression, except that if o is an object, that full-expression may also invoke constexpr constructors for o and its subobjects even if those objects are of non-literal class types.

Zero-initialization is not performed prior to constant-initialization per 6.9.3.2 [basic.start.static] paragraph 2:

> Constant initialization is performed if a variable or temporary object with static or thread storage duration is constant-initialized (7.7 [expr.const]). If constant initialization is not performed, a variable with static storage duration (6.7.5.2 [basic.stc.static]) or thread storage duration (6.7.5.3 [basic.stc.thread]) is zero-initialized (9.4 [dcl.init]). Together, zero-initialization and constant initialization are called static initialization; all other initialization is dynamic initialization.

Thus, #1 is valid and `a` is statically initialized, but `a.y` would remain uninitialized, which is surprising for an object with static storage duration.

Current implementations diagnose an error at #1, because the variable `a` is actually not considered to be constant-initialized.

This issue is closely related to issue 2558.

Suggested resolution [SUPERSEDED]:

Change in 7.7 [expr.const] paragraph 2:

> A variable or temporary object o is constant-initialized if
> - either it has an initializer or its default-initialization results in some initialization being performed, and
> - every non-variant non-static data member and base class subobject is initialized (11.9.3 [class.base.init]), and
> - the full-expression of its initialization is a constant expression when interpreted as a constant-expression, except that if o is an object, that full-expression may also invoke constexpr constructors for o and its subobjects even if those objects are of non-literal class types.

Alternative suggested resolution (March, 2022) [SUPERSEDED]:

Change in 7.7 [expr.const] paragraph 11 as follows:

> A constant expression is either a glvalue core constant expression that refers to an entity that is a permitted result of a constant expression (as defined below), or a prvalue core constant expression whose value satisfies the following constraints:
> - if the value is an object of class type, each non-static data member of reference type refers to an entity that is a permitted result of a constant expression,
> - if the value is of pointer type, it contains the address of an object with static storage duration, the address past the end of such an object (7.6.6 [expr.add]), the address of a non-immediate function, or a null pointer value,
> - if the value is of pointer-to-member-function type, it does not designate an immediate function, and
> - if the value is an object of class or array type, each subobject is initialized (11.9.3 [class.base.init]) and satisfies these constraints for the value.

Notes from the November, 2022 meeting

CWG preferred to zero-initialize `a.y` in the example, and keep #1 well-formed.

Possible resolution:

Change in 6.9.3.2 [basic.start.static] paragraph 2 as follows:

> Constant initialization is performed if a variable or temporary object with static or thread storage duration is constant-initialized (7.7 [expr.const]). If constant initialization is not performed, a A variable with static storage duration (6.7.5.2 [basic.stc.static]) or thread storage duration (6.7.5.3 [basic.stc.thread]) or a subobject thereof is zero-initialized (9.4 [dcl.init]) if constant initialization is not performed or if it does not initialize that subobject. Together, zero-initialization and constant initialization are called static initialization; all other initialization is dynamic initialization. All static initialization strongly happens before (6.9.2.2 [intro.races]) any dynamic initialization.

CWG 2022-12-02

The resolution shown above would leave padding bits uninitialized. In contrast, zero-initialization does set padding bits to 0 to possibly facilitate `memcmp`. Additional example:

      struct C {
        int a;
        int b;
        C() : b(a) {}   // #1
      };
      constinit C x;    // OK when zero-initializing first, because #1 reads zero-initialized a?

2022-12-03

Forwarded to EWG with cplusplus/papers#1380.

Additional notes (January, 2023)

The standard does not guarantee stable results when reading padding bits, i.e. bits that are not part of the value representation of some in-lifetime object. In C, explicit rules keep padding bits stable; they are allowed to change only if a store to any class member occurs.

#### 2545. Transparently replacing objects in constant expressions

Section: 7.7 [expr.const] Status: open Submitter: Richard Smith Date: 2022-03-05

7.7 [expr.const] paragraph 6 specifies that `std::construct_at` can be used during constant evaluation:

> Similarly, the evaluation of a call to `std::construct_at` or `std::ranges::construct_at` does not disqualify `E` from being a core constant expression unless the first argument, of type `T*`, does not point to storage allocated with `std::allocator<T>` or to an object whose lifetime began within the evaluation of `E`, or the evaluation of the underlying constructor call disqualifies `E` from being a core constant expression.

Apparently, implementations are required to track whether an object is transparently replaceable (6.7.3 [basic.life] paragraph 8) during constant evaluation to satisfy 7.7 [expr.const] bullet 5.8, which requires that undefined behavior be detected and rejected during constant evaluation:

> - ...
> - an operation that would have undefined behavior as specified in Clause 4 through Clause 15;
> - ...

For example,

      struct A {
        int x, y;
      };
      struct B {
        float a;
        int b;
      };
      union C {
        A a;
        B b;
      };
      constexpr int f() {
       C c = {};
       std::construct_at(&c.b.b, 5);
       // Does this return 5 if c.a.y and c.b.b are laid out at the same address?
       return c.a.y;
      }

No known implementation diagnoses the violation of the rules for transparently replaceable in the following example, but there is implementation divergence for the results of `f()`:

      #include <memory>

      struct A {
        virtual constexpr char f() { return 'A'; }
      };
      struct B : A {
        constexpr char f() override { return 'B'; }
      };

      constexpr char f() {
        B b;
        A *p = &b;
        std::construct_at(p);
        return p->f();     // alternative: return b.f()
      }

#### 2552. Constant evaluation of non-defining variable declarations

Section: 7.7 [expr.const] Status: open Submitter: Hubert Tong Date: 2022-03-21

Paper P2242 (Non-literal variables (and labels and gotos) in constexpr functions) added 7.7 [expr.const] bullet 5.2:

> - a control flow that passes through a declaration of a variable with static (6.7.5.2 [basic.stc.static]) or thread (6.7.5.3 [basic.stc.thread]) storage duration;

It seems that block-scope extern (i.e. non-defining) declarations are covered by the above bullet, but only definitions should be in view here.

Suggested resolution:

1. Change in 7.7 [expr.const] bullet 5.2 as follows:

   > - a control flow that passes through a declaration definition of a variable with static (6.7.5.2 [basic.stc.static]) or thread (6.7.5.3 [basic.stc.thread]) storage duration;

2. Change in 8.8 [stmt.dcl] paragraph 3 as follows:

   > Dynamic initialization of a block variable with static storage duration (6.7.5.2 [basic.stc.static]) or thread storage duration (6.7.5.3 [basic.stc.thread]) is performed the first time control passes through its declaration definition; such a variable is considered initialized upon the completion of its initialization. If the initialization exits by throwing an exception, the initialization is not complete, so it will be tried again the next time control enters the declaration definition. If control enters the declaration concurrently while the variable is being initialized, the concurrent execution shall wait for completion of the initialization. [Note: ... —end note] If control re-enters the declaration definition recursively while the variable is being initialized, the behavior is undefined.

#### 2559. Defaulted consteval functions

Section: 7.7 [expr.const] Status: open Submitter: Aaron Ballman Date: 2022-03-29

Consider:

      template <typename Ty>
      struct S {
        Ty i;
        consteval S() = default;
      };

      template <typename Ty>
      struct T {
        Ty i;
        consteval T() {}
      };

      S<int> one; // only Clang rejects
      T<int> two; // Clang, GCC, ICC, MSVC reject

      void locals() {
        S<int> three; // only Clang rejects
        T<int> four;  // Clang, GCC, ICC, MSVC reject
      }

A consteval function should always be evaluated at compile time and never fall back to runtime, thus all four cases should be rejected. Issue 2558 is related.

#### 2633. typeid of constexpr-unknown dynamic type

Section: 7.7 [expr.const] Status: open Submitter: Jim X Date: 2022-09-18

Consider the example in 7.7 [expr.const] paragraph 7:

    extern Swim dc;
    constexpr auto& sandeno  = typeid(dc);     // OK, can only be typeid(Swim)

The comment in the example seems not to be backed by normative text. In particular, the dynamic type of `dc` is constexpr-unknown per 7.7 [expr.const] paragraph 7:

> During the evaluation of an expression E as a core constant expression, all id-expressions and uses of `*this` that refer to an object or reference whose lifetime did not begin with the evaluation of E are treated as referring to a specific instance of that object or reference whose lifetime and that of all subobjects (including all union members) includes the entire constant evaluation. For such an object that is not usable in constant expressions, the dynamic type of the object is constexpr-unknown. ...

Thus, `typeid(dc)` is not a core constant expression per 7.7 [expr.const] bullet 5.26:

> - a dynamic_cast (7.6.1.7 [expr.dynamic.cast]) or typeid (7.6.1.8 [expr.typeid]) expression on a glvalue that refers to an object whose dynamic type is constexpr-unknown or that would throw an exception;

#### 2656. Converting consteval lambda to function pointer in non-immediate context

Section: 7.7 [expr.const] Status: open Submitter: Hubert Tong Date: 2022-11-11

Converting a consteval lambda to a function pointer in a non-immediate context should be immediately-escalating.

Currently, this is well-formed:

      auto x = &*[]() consteval { return 42; };

Suggested resolution:

Make the conversion function of a lambda whose call operator is an immediate function also an immediate function.

#### 2702. Constant destruction of reference members

Section: 7.7 [expr.const] Status: open Submitter: Richard Smith Date: 2023-02-13

Issue 2529 says that a constexpr reference bound to a lifetime-extended temporary is required to have constant destruction. However, that rule overlooks constexpr objects with a reference member bound to a lifetime-extended temporary, recursively, and the case for `std::initializer_list` lifetime extension.

#### 2710. Loops in constant expressions

Section: 7.7 [expr.const] Status: open Submitter: Daniel Krügler Date: 2023-03-23

Iteration statements such as `while` and `for` loops are specified by equivalent code involving `goto` (8.6.2 [stmt.while] paragraph 2, 8.6.4 [stmt.for] paragraph 1, 8.6.5 [stmt.ranged] paragraph 1). The `goto` statement cannot be evaluated in constant expressions (7.7 [expr.const] bullet 5.30), thus `while` and `for` loops cannot be evaluated in constant expressions. Similar concerns arise for `continue` (8.7.3 [stmt.cont] paragraph 1).

However, that is neither intended nor existing practice.

Suggested resolution:

Change in 7.7 [expr.const] bullet 5.30 as follows:

> An expression E is a core constant expression unless the evaluation of E, following the rules of the abstract machine (6.9.1 [intro.execution]), would evaluate one of the following:
> - ...
> - a goto statement (8.7.6 [stmt.goto]) unless it is introduced by replacing an iteration-statement (8.6 [stmt.iter]) or a `continue` statement (8.7.3 [stmt.cont]) with its equivalent.

#### 2495. Glvalue result of a function call

Section: 8.7.4 [stmt.return] Status: open Submitter: Jim X Date: 2021-07-04

According to 8.7.4 [stmt.return] paragraph 1,

> A `return` statement with any other operand shall be used only in a function whose return type is not cv `void`; the `return` statement initializes the glvalue result or prvalue result object of the (explicit or implicit) function call by copy-initialization (9.4 [dcl.init]) from the operand.

It is not clear what a “glvalue result” is or what it means to initialize it.

Suggested resolution:

> A `return` statement with any other operand shall be used only in a function whose return type is not cv `void`;. If the function call is a prvalue, the `return` statement initializes the glvalue result or prvalue result object of the (explicit or implicit) function call by copy-initialization (9.4 [dcl.init]) from the operand. Otherwise, the `return` statement is equivalent to the following hypothetical declaration

> `T t =` e`;`

> If the operand of the `return` statement, X, is a comma expression without parentheses, e is `(`X`)`, otherwise e is X. `T` is the return type of the function call; the invented variable `t` is the result of the function call.

Notes from the August, 2021 teleconference:

A simpler approach would be simply to use a phrase like “returned object or reference” in place of the current wording referring to glvalues and prvalues. This change was regarded as editorial. The issue will remain in "review" status until CWG can look over the wording change.

#### 2556. Unusable `promise::return_void`

Section: 8.7.5 [stmt.return.coroutine] Status: open Submitter: Davis Herring Date: 2022-03-24

Subclause 8.7.5 [stmt.return.coroutine] paragraph 3 specifies:

> If `p.return_void()` is a valid expression, flowing off the end of a coroutine's function-body is equivalent to a `co_return` with no operand; otherwise flowing off the end of a coroutine's function-body results in undefined behavior.

However, 9.5.4 [dcl.fct.def.coroutine] paragraph 6 suggests:

> If searches for the names `return_void` and `return_value` in the scope of the promise type each find any declarations, the program is ill-formed. [Note: If `return_void` is found, flowing off the end of a coroutine is equivalent to a `co_return` with no operand. Otherwise, flowing off the end of a coroutine results in undefined behavior (8.7.5 [stmt.return.coroutine]). —end note]

The difference is between the conditions "valid expression" and "found by name lookup". Effectively, it means that undefined behavior might result where the implementation could instead diagnose an ill-formed use of `return_void` (for example, because it is inaccessible, deleted, or the function call requires arguments).

Suggested resolution:

Change in 8.7.5 [stmt.return.coroutine] paragraph 3 as follows:

> If `p.return_void()` is a valid expression a search for the name `return_void` in the scope of the promise type finds any declarations, flowing off the end of a coroutine's function-body is equivalent to a `co_return` with no operand; otherwise flowing off the end of a coroutine's function-body results in undefined behavior.

#### 2123. Omitted constant initialization of local static variables

Section: 8.8 [stmt.dcl] Status: open Submitter: Hubert Tong Date: 2015-02-02

According to 8.8 [stmt.dcl] paragraph 4,

> The zero-initialization (9.4 [dcl.init]) of all block-scope variables with static storage duration (6.7.5.2 [basic.stc.static]) or thread storage duration (6.7.5.3 [basic.stc.thread]) is performed before any other initialization takes place. Constant initialization (6.9.3.2 [basic.start.static]) of a block-scope entity with static storage duration, if applicable, is performed before its block is first entered.

The fact that a variable need not be constant-initialized if its block is not entered appears to allow inspection of the variable after zero-initialization but before constant initialization:

      constexpr int x = 0;

      auto foo() {
        constexpr static const int *p = &x;
        struct A {
          const int *const &getPtr() { return p; }
        } a;
        return a;
      }

      int xcpy = *decltype(foo()){ }.getPtr();

      int main(void) {
        return xcpy;
      }

For a related example, consider:

      // tu1.cpp
      extern const int a = 1;
      inline auto f() {
        static const int b = a;
        struct A { auto operator()() { return &b; } } a;
        return a;
      }

      // tu2.cpp
      extern const int a;
      inline auto f() {
        static const int b = a;
        struct A { auto operator()() { return &b; } } a;
        return a;
      }
      int main() {
        return *decltype(f())()();
      }

Here, `b` may or may not have constant initialization, but we don't have an ODR violation.

If we want to support such code, the nicest option would be to say that the ODR requires us to act as if we pick one of the definitions of the inline function, which requires us to make a consistent choice for all static storage duration variables within a given function. Alternatively, we could say that if multiple definitions of a variable disagree over whether it has constant initialization, then it does not, allowing more implementation simplicity and no functional change outside of pathological cases.

Notes from the February, 2016 meeting:

The second example will be dealt with separately under issue 2242. For the first example, the Standard should require that local types can be used outside their function only via a returned object. It was still to be decided whether this should be undefined behavior or an error on use of such a type. It was also noted that the same issue can arise with static member functions.

#### 157. Omitted typedef declarator

Section: 9.1 [dcl.pre] Status: open Submitter: Daveed Vandevoorde Date: 19 Aug 1999

9.1 [dcl.pre] paragraph 3 reads,

> In a simple-declaration, the optional init-declarator-list can be omitted only when... the decl-specifier-seq contains either a class-specifier, an elaborated-type-specifier with a class-key (11.3 [class.name] ), or an enum-specifier. In these cases and whenever a class-specifier or enum-specifier is present in the decl-specifier-seq, the identifiers in those specifiers are among the names being declared by the declaration... In such cases, and except for the declaration of an unnamed bit-field (11.4.10 [class.bit] ), the decl-specifier-seq shall introduce one or more names into the program, or shall redeclare a name introduced by a previous declaration. [Example:

>         enum { };           // ill-formed
>         typedef class { };  // ill-formed

> —end example]

In the absence of any explicit restrictions in 9.2.4 [dcl.typedef] , this paragraph appears to allow declarations like the following:

        typedef struct S { };    // no declarator
        typedef enum { e1 };     // no declarator

In fact, the final example in 9.1 [dcl.pre] paragraph 3 would seem to indicate that this is intentional: since it is illustrating the requirement that the decl-specifier-seq must introduce a name in declarations in which the init-declarator-list is omitted, presumably the addition of a class name would have made the example well-formed.

On the other hand, there is no good reason to allow such declarations; the only reasonable scenario in which they might occur is a mistake on the programmer's part, and it would be a service to the programmer to require that such errors be diagnosed.

#### 498. Storage class specifiers in definitions of class members

Section: 9.2.2 [dcl.stc] Status: open Submitter: Matt Austern Date: 13 Jan 2005

Suppose we've got this class definition:

        struct X {
           void f();
           static int n;
        };

I think I can deduce from the existing standard that the following member definitions are ill-formed:

        static void X::f() { }
        static int X::n;

To come to that conclusion, however, I have to put together several things in different parts of the standard. I would have expected to find an explicit statement of this somewhere; in particular, I would have expected to find it in 9.2.2 [dcl.stc]. I don't see it there, or anywhere.

Gabriel Dos Reis: Or in 6.6 [basic.link] which is about linkage. I would have expected that paragraph to say that that members of class types have external linkage when the enclosing class has an external linkage. Otherwise 6.6 [basic.link] paragraph 8:

> Names not covered by these rules have no linkage.

might imply that such members do not have linkage.

Notes from the April, 2005 meeting:

The question about the linkage of class members is already covered by 6.6 [basic.link] paragraph 5.

#### 2232. `thread_local` anonymous unions

Section: 9.2.2 [dcl.stc] Status: open Submitter: Mike Herrick Date: 2016-02-23

It is not clear from the current wording whether the `thread_local` specifier can be applied to anonymous unions or not. According to 9.2.2 [dcl.stc] paragraph 3,

> The `thread_local` specifier indicates that the named entity has thread storage duration (6.7.5.3 [basic.stc.thread]). It shall be applied only to the names of variables of namespace or block scope and to the names of static data members.

One might think that an anonymous union object would be a “variable,” but the next paragraph seems to treat variables and anonymous unions as distinct:

> The `static` specifier can be applied only to names of variables and functions and to anonymous unions (11.5.2 [class.union.anon]).

#### 2195. Unsolicited reading of trailing volatile members

Section: 9.2.9.2 [dcl.type.cv] Status: open Submitter: Hubert Tong Date: 2015-11-06

Consider:

      struct A {
        ~A();
        double d;
        float f;
      };

      struct B : A { volatile int i; };

      A foo(B *bp) { return *static_cast<A *>(bp); }

Is it okay for the memory associated with `bp->i` to be accessed by `foo`?

See also 9.2.9.2 [dcl.type.cv] paragraph 5

> The semantics of an access through a volatile glvalue are implementation-defined. If an attempt is made to access an object defined with a volatile-qualified type through the use of a non-volatile glvalue, the behavior is undefined.

Additional notes from the November, 2016 meeting:

See also national body comment CH2, addressed in March, 2017 by P0612R0.

#### 144. Position of `friend` specifier

Section: 9.2.9.4 [dcl.type.elab] Status: open Submitter: Daveed Vandevoorde Date: 22 Jul 1999

9.2.9.4 [dcl.type.elab] paragraph 1 seems to impose an ordering constraint on the elements of friend class declarations. However, the general rule is that declaration specifiers can appear in any order. Should

        class C friend;

be well-formed?

#### 2634. Avoid circularity in specification of scope for friend class declarations

Section: 9.2.9.4 [dcl.type.elab] Status: open Submitter: Jim X Date: 2022-07-04

Consider:

    auto f(struct X* ptr) {
      struct D {
        private:
          int d;
          friend class X;      // #1
      };
      return D{};
    }
    X* b = 0;
    struct X {
      void show() {
        auto t = f(0);
        t.d = 10;              // #2 error: ::X is not a friend of f::D
      }
    };

The target scope for #2 is `f`'s block scope, making `::X` not a friend of `f::D`. Thus the access at #2 is ill-formed. Clang disagrees.

Subclause 9.2.9.4 [dcl.type.elab] paragraph 3 specifies:

> ... If E contains an identifier but no nested-name-specifier and (unqualified) lookup for the identifier finds nothing, E shall not be introduced by the enum keyword and declares the identifier as a class-name. The target scope of E is the nearest enclosing namespace or block scope.

> If an elaborated-type-specifier appears with the friend specifier as an entire member-declaration, the member-declaration shall have one of the following forms:

>     friend class-key nested-name-specifieropt identifier ;
>     ...

> Any unqualified lookup for the identifier (in the first case) does not consider scopes that contain the target scope; no name is bound.

This specification is circular in that the target scope that limits unqualified lookup is defined only if the identifier is actually declared, but the identifier is declared only if lookup finds nothing.

Possible resolution:

Change in 9.2.9.4 [dcl.type.elab] paragraph 4 as follows:

> ... Any unqualified lookup for the identifier (in the first case) does not consider scopes that contain the target nearest enclosing namespace or block scope; no name is bound. [ Note: ... ]

#### 2228. Ambiguity resolution for cast to function type

Section: 9.3.3 [dcl.ambig.res] Status: open Submitter: Richard Smith Date: 2016-02-02 Liaison: EWG

Consider:

      int x = (int()) + 5;

This is ill-formed, because 9.3.3 [dcl.ambig.res] paragraph 2 specifies:

> An ambiguity can arise from the similarity between a function-style cast and a type-id. The resolution is that any construct that could possibly be a type-id in its syntactic context shall be considered a type-id.

and thus `int()` is interpreted as a type-id instead of as a function-style cast, so this is an ill-formed cast to a function type.

This seems to be the wrong disambiguation for all cases where there is a choice between a C-style cast and a parenthesized expression: in all those cases, the C-style cast interpretation results in a cast to a function type, which is always ill-formed.

Further, there is implementation divergence in the handling of this example:

      struct T { int operator++(int); T operator[](int); };
      int a = (T()[3])++; // not a cast

EWG 2022-11-11

This is tracked in github issue cplusplus/papers#1376.

#### 2671. friend named by a template-id

Section: 9.3.4.1 [dcl.meaning.general] Status: open Submitter: Davis Herring Date: 2022-11-18

Subclause 9.3.4.1 [dcl.meaning.general] paragraph 1 specifies:

> ... If the unqualified-id occurring in a declarator-id is a template-id, the declarator shall appear in the declaration of a template-declaration (13.7 [temp.decls]), explicit-specialization (13.9.4 [temp.expl.spec]), or explicit-instantiation (13.9.3 [temp.explicit]).

However, that is too restrictive and prevents befriending template specializations, for example:

      template<typename> void f();

      class A {
        friend void f<A>();
      };

#### 504. Should use of a variable in its own initializer require a diagnostic?

Section: 9.3.4.3 [dcl.ref] Status: open Submitter: Bjarne Stroustrup Date: 14 Apr 2005

Split off from issue 453.

It is in general not possible to determine at compile time whether a reference is used before it is initialized. Nevertheless, there is some sentiment to require a diagnostic in the obvious cases that can be detected at compile time, such as the name of a reference appearing in its own initializer. The resolution of issue 453 originally made such uses ill-formed, but the CWG decided that this question should be a separate issue.

Rationale (October, 2005):

The CWG felt that this error was not likely to arise very often in practice. Implementations can warn about such constructs, and the resolution for issue 453 makes executing such code undefined behavior; that seemed to address the situation adequately.

Note (February, 2006):

Recent discussions have suggested that undefined behavior be reduced. One possibility (broadening the scope of this issue to include object declarations as well as references) was to require a diagnostic if the initializer uses the value, but not just the address, of the object or reference being declared:

        int i = i;        // Ill-formed, diagnostic required
        void* p = &p;     // Okay

#### 2550. Type "reference to cv `void`" outside of a declarator

Section: 9.3.4.3 [dcl.ref] Status: open Submitter: Jens Maurer Date: 2022-03-13

9.3.4.3 [dcl.ref] paragraph 1 specifies:

> A declarator that specifies the type “reference to cv `void`” is ill-formed.

A declarator does not contain the leading decl-specifier-seq of a declaration, so the following example is not covered by the prohibition:

      void f(void& x);

Suggested resolution:

Change in 9.3.4.3 [dcl.ref] paragraph 1 as follows:

> A declarator that specifies Forming the type “reference to cv `void`” is ill-formed.

#### 1790. Ellipsis following function parameter pack

Section: 9.3.4.6 [dcl.fct] Status: open Submitter: Daryle Walker Date: 2013-10-01 Liaison: WG14

Although the current wording permits an ellipsis to immediately follow a function parameter pack, it is not clear that the `<cstdarg>` facilities permit access to the ellipsis arguments.

Rationale (June, 2014):

CWG felt that this is a question of language design and thus should be considered by EWG before any action.

EWG 2022-11-11

C23 removes the requirement that the last parameter be named for `va_start`. This is tracked in github issue cplusplus/papers#1374.

#### 2553. Restrictions on explicit object member functions

Section: 9.3.4.6 [dcl.fct] Status: open Submitter: Jens Maurer Date: 2021-12-10

Subclause 9.3.4.6 [dcl.fct] paragraph 6 specifies

> A member-declarator with an explicit-object-parameter-declaration shall not include a ref-qualifier or a cv-qualifier-seq and shall not be declared `static` or `virtual`.

This does not address the situation when an explicit object member function becomes implicitly virtual by overriding an implicit object member function. That should be prevented.

This also does not address class-specific allocation and deallocation functions, which are implicitly `static`.

Suggested resolution:

1. Change in 9.3.4.6 [dcl.fct] paragraph 6 as follows:

   > A member-declarator with an explicit-object-parameter-declaration shall not include a ref-qualifier or a cv-qualifier-seq and shall not be declared `static` or `virtual`.

2. Change in 9.3.4.6 [dcl.fct] paragraph 7 as follows:

   > ... An implicit object member function is a non-static member function without an explicit object parameter. [ Note: An explicit object member function cannot be static (11.4.9.2 [class.static.mfct]) or virtual (11.7.3 [class.virtual]). -- end note ]

3. Add a new paragraph after 11.4.9.2 [class.static.mfct] paragraph 1:

   > [Note: The rules described in 11.4.2 apply to static member functions. -- end note]

   > A static member function (11.4.1 [class.mem.general], 11.4.11 [class.free]) shall not be an explicit object member function (9.3.4.6 [dcl.fct]).

4. Add a new paragraph before 11.7.3 [class.virtual] paragraph 7 as follows:

   > A virtual function shall not be an explicit object member function (9.3.4.6 [dcl.fct]).

   > [ Example:

   >       struct B {
   >         virtual void g(); // #1
   >       };
   >       struct D : B {
   >         virtual void f(this D&);  // error: explicit object member function cannot be virtual
   >         void g(this D&);          // overrides #1; error: explicit object member function cannot be virtual
   >       };

   > -- end example]

   > The ref-qualifier, or lack thereof, ...

#### 361. Forward reference to default argument

Section: 9.3.4.7 [dcl.fct.default] Status: open Submitter: Steve Clamage Date: 17 June 2002

Is this program well-formed?

      struct S {
        static int f2(int = f1()); // OK?
        static int f1(int = 2);
      };
      int main()
      {
        return S::f2();
      }

A class member function can in general refer to class members that are declared lexically later. But what about referring to default arguments of member functions that haven't yet been declared?

It seems to me that if f2 can refer to f1, it can also refer to the default argument of f1, but at least one compiler disagrees.

Notes from the February, 2012 meeting:

Implementations seem to have come to agreement that this example is ill-formed.

Additional note (March, 2013):

Additional discussion has occurred suggesting the following examples as illustrations of this issue:

      struct B {
        struct A { int a = 0; };
        B(A = A());    // Not permitted?
      };

as well as

      struct C {
       struct A { int a = C().n; }; // can we use the default argument here?
       C(int k = 0);
       int n;
      };

      bool f();
      struct D {
       struct A { bool a = noexcept(B()); }; // can we use the default initializer here?
       struct B { int b = f() ? throw 0 : 0; };
      };

(See also issue 325.)

Additional note (October, 2013):

Issue 1330 treats exception-specifications like default arguments, evaluated in the completed class type. That raises the same questions regarding self-referential `noexcept` clauses that apply to default arguments.

Additional note (November, 2020):

Paper P1787R6, adopted at the November, 2020 meeting, partially addresses this issue.

#### 1609. Default arguments and function parameter packs

Section: 9.3.4.7 [dcl.fct.default] Status: open Submitter: Jonathan Caves Date: 2013-01-25

It is not clear from 9.3.4.7 [dcl.fct.default] whether the following is well-formed or not:

      template<typename... T>
      void f2(int a = 0, T... b, int c = 1);

      f2<>(); // parameter a has the value 0 and parameter c has the value 1

(`T... b` is a non-deduced context per 13.10.3.6 [temp.deduct.type] paragraph 5, so the template arguments must be specified explicitly.)

Notes from the April, 2013 meeting:

CWG agreed that the example should be ill-formed.

Additional note (August, 2013):

9.3.4.7 [dcl.fct.default] paragraph 4 explicitly allows for a function parameter pack to follow a parameter with a default argument:

> In a given function declaration, each parameter subsequent to a parameter with a default argument shall have a default argument supplied in this or a previous declaration or shall be a function parameter pack.

However, any instantiation of such a function template with a non-empty pack expansion would result in a function declaration in which one or more parameters without default arguments (from the pack expansion) would follow a parameter with a default argument and thus would be ill-formed. Such a function template declaration thus violates 13.8 [temp.res] paragraph 8:

> If every valid specialization of a variadic template requires an empty template parameter pack, the template is ill-formed, no diagnostic required.

Although the drafting review teleconference of 2013-08-26 suggested closing the issue as NAD, it is being kept open to discuss and resolve this apparent contradiction.

Notes from the September, 2013 meeting:

CWG agreed that this example should be accepted; the restriction on default arguments applies to the template declaration itself, not to its specializations.

#### 2701. Default arguments in multiple scopes / inheritance of array bounds in the same scope

Section: 9.3.4.7 [dcl.fct.default] Status: open Submitter: Richard Smith Date: 2020-11-20

It appears that P1787R6 has inadvertently changed the rules for where default arguments can be (re-)declared for a parameter. Consider:

    namespace N {
      void f(int a, int b = 1);
    }
    void N::f(int a = 1, int b) {} // valid before P1787R6, invalid afterwards
    void N::f(int a, int b = 2) {} // invalid before P1787R6, valid afterwards,
      // but this default argument will either never be used or will make any call using it ill-formed (unclear which)

Subclause 9.3.4.7 [dcl.fct.default] paragraph 4 specifies:

> For non-template functions, default arguments can be added in later declarations of a function that inhabit the same scope. Declarations that inhabit different scopes have completely distinct sets of default arguments. ...

The problem is that the out-of-line declaration of `N::f` inhabits the global namespace scope, not the scope of namespace `N`.

A similar problem exists for inheriting array bounds per 9.3.4.5 [dcl.array] paragraph 8:

> Furthermore, if there is a reachable declaration of the entity that inhabits the same scope in which the bound was specified, an omitted array bound is taken to be the same as in that earlier declaration, and similarly for the definition of a static data member of a class.

Possible resolution:

1. Change in 6.4.1 [basic.scope.scope] paragraph 2 as follows:

   > ... The host scope of a declaration is the inhabited scope if that scope is a block scope and the target scope otherwise. An entity belongs to a scope S if S is the target scope of a declaration of the entity.

2. Change in 9.3.4.5 [dcl.array] paragraph 8 as follows:

   > Furthermore, if there is a reachable declaration of the entity that inhabits has the same host scope in which the bound was specified, an omitted array bound is taken to be the same as in that earlier declaration, and similarly for the definition of a static data member of a class.

3. Change in 9.3.4.7 [dcl.fct.default] paragraph 4 as follows:

   > For non-template functions, default arguments can be added in later declarations of a function that inhabit have the same host scope. Declarations that inhabit have different host scopes have completely distinct sets of default arguments. ...

4. Change in 9.3.4.7 [dcl.fct.default] paragraph 9 as follows:

   > When an overload set contains a declaration of a function that inhabits a whose host scope is S, any default argument associated with any reachable declaration that inhabits whose host scope is S is available to the call.

5. Change in 12.2.4.1 [over.match.best.general] paragraph 4 as follows:

   > If the best viable function resolves to a function for which multiple declarations were found, and if any two of these declarations inhabit have different host scopes and specify a default argument that made the function viable, the program is ill-formed.

#### 2708. Parenthesized initialization of arrays

Section: 9.4.1 [dcl.init.general] Status: open Submitter: Mike Miller Date: 2023-03-14

Consider:

      const int arr[2](1,2);

This is accepted by all major implementations, yet 9.4.1 [dcl.init.general] paragraph 13 prohibits it:

> If the entity being initialized does not have class type, the expression-list in a parenthesized initializer shall be a single expression.

Presumably, this was an oversight when adding parenthesized aggregate initialization.

Possible resolution:

Change in 9.4.1 [dcl.init.general] paragraph 13 as follows:

> If the entity being initialized does not have class or array type, the expression-list in a parenthesized initializer shall be a single expression.

#### 2709. Parenthesized initialization of reference-to-aggregate

Section: 9.4.1 [dcl.init.general] Status: open Submitter: Mike Miller Date: 2023-03-14

Paper P0960R3 enabled initialization of aggregates (including arrays) from parenthesized expression lists. The rather similar case of initializating a reference to an aggregate was not addressed. For example:

      const int (&ra)[2](1,2);

There is no evidence in 9.4.1 [dcl.init.general] paragraph 16 that such support is intended, nor does 9.4.4 [dcl.init.ref] paragraph 5 admit an initializer other than a (single) expression for a reference.

There is implementation divergence: gcc accepts, others reject. Also, the parenthesized aggregate initialization paper P0960R3 apparently originally envisioned a textual transformation to a braced-init-list, which presumably would have permitted the example. It is not clear whether the reference case was intentionally or accidentially dropped during the multiple revisions of the paper.

Additional notes (March, 2023)

Forwarded to EWG for consideration via cplusplus/papers#1494 by decision of the CWG chair.

#### 233. References vs pointers in UDC overload resolution

Section: 9.4.4 [dcl.init.ref] Status: open Submitter: Matthias Meixner Date: 9 Jun 2000

There is an inconsistency in the handling of references vs pointers in user defined conversions and overloading. The reason for that is that the combination of 9.4.4 [dcl.init.ref] and 7.3.6 [conv.qual] circumvents the standard way of ranking conversion functions, which was probably not the intention of the designers of the standard.

Let's start with some examples, to show what it is about:

        struct Z { Z(){} };

        struct A {
           Z x;

           operator Z *() { return &x; }
           operator const Z *() { return &x; }
        };

        struct B {
           Z x;

           operator Z &() { return x; }
           operator const Z &() { return x; }
        };

        int main()
        {
           A a;
           Z *a1=a;
           const Z *a2=a; // not ambiguous

           B b;
           Z &b1=b;
           const Z &b2=b; // ambiguous
        }

So while both classes `A` and `B` are structurally equivalent, there is a difference in operator overloading. I want to start with the discussion of the pointer case (`const Z *a2=a;`): 12.2.4 [over.match.best] is used to select the best viable function. Rule 4 selects `A::operator const Z*()` as best viable function using 12.2.4.3 [over.ics.rank] since the implicit conversion sequence `const Z*` -> `const Z*` is a better conversion sequence than `Z*` -> `const Z*`.

So what is the difference to the reference case? Cv-qualification conversion is only applicable for pointers according to 7.3.6 [conv.qual]. According to 9.4.4 [dcl.init.ref] paragraphs 4-7 references are initialized by binding using the concept of reference-compatibility. The problem with this is, that in this context of binding, there is no conversion, and therefore there is also no comparing of conversion sequences. More exactly all conversions can be considered identity conversions according to 12.2.4.2.5 [over.ics.ref] paragraph 1, which compare equal and which has the same effect. So binding `const Z*` to `const Z*` is as good as binding `const Z*` to `Z*` in terms of overloading. Therefore `const Z &b2=b;` is ambiguous. [12.2.4.2.5 [over.ics.ref] paragraph 5 and 12.2.4.3 [over.ics.rank] paragraph 3 rule 3 (S1 and S2 are reference bindings ...) do not seem to apply to this case]

There are other ambiguities, that result in the special treatment of references: Example:

        struct A {int a;};
        struct B: public A { B() {}; int b;};

        struct X {
           B x;
           operator A &() { return x; }
           operator B &() { return x; }
        };

        main()
        {
           X x;
           A &g=x; // ambiguous
        }

Since both references of class `A` and `B` are reference compatible with references of class `A` and since from the point of ranking of implicit conversion sequences they are both identity conversions, the initialization is ambiguous.

So why should this be a defect?

- References behave fundamentally different from pointers in combination with user defined conversions, although there is no reason to have this different treatment.
- This difference only shows up in combination with user defined conversion sequences, for all other cases, there are special rules, e.g. 12.2.4.3 [over.ics.rank] paragraph 3 rule 3.

So overall I think this was not the intention of the authors of the standard.

So how could this be fixed? For comparing conversion sequences (and only for comparing) reference binding should be treated as if it was a normal assignment/initialization and cv-qualification would have to be defined for references. This would affect 9.4.4 [dcl.init.ref] paragraph 6, 7.3.6 [conv.qual] and probably 12.2.4.3 [over.ics.rank] paragraph 3.

Another fix could be to add a special case in 12.2.4 [over.match.best] paragraph 1.

#### 2657. Cv-qualification adjustment when binding reference to temporary

Section: 9.4.4 [dcl.init.ref] Status: open Submitter: Brian Bi Date: 2022-11-10

Core issue 2481 was resolved by clarifying that the temporary object in p5.4.2 is cv-qualified if the reference being initialized is cv-qualified. However, this is not the right bullet point for the example given,

      constexpr const int &r = 42;

Such an initialization would actually use bullet 5.3.1 instead. (5.4.2 would be used if the initializer were, for example, 3.14.) We therefore need to make a similar clarification in bullet 5.3, and ideally using the same language.

Suggested resolution:

1. Change in 9.4.4 [dcl.init.ref] bullet 5.3 as follows:

   > Otherwise, if the initializer expression
   > - is an rvalue (but not a bit-field) or function lvalue and “cv1 T1” is reference-compatible with “cv2 T2”, or
   > - has a class type (i.e., T2 is a class type), where T1 is not reference-related to T2, and can be converted to an rvalue or function lvalue of type “cv3 T3”, where “cv1 T1” is reference-compatible with “cv3 T3” (see 12.2.2.7 [over.match.ref]),
   > then the initializer expression in the first case and the converted expression in the second case is called the converted initializer. If the converted initializer is a prvalue, let its type be denoted by `T4`; the temporary materialization conversion (7.3.5 [conv.rval]) is applied, considering the type of the prvalue to be is adjusted to type “cv1 T4” (7.3.6 [conv.qual]) and the temporary materialization conversion (7.3.5 [conv.rval]) is applied. In any case, the reference binds to the resulting glvalue (or to an appropriate base class subobject).

2. Append to the example in 9.4.4 [dcl.init.ref] bullet 5.3 as follows:

         B&& rrb = x;  // binds directly to the result of operator B
         const int& r2 = 0;  // binds directly to temporary of type const int

3. Change the example in 9.4.4 [dcl.init.ref] bullet 5.4 as follows:

         const double& rcd2 = 2;   // rcd2 refers to temporary with type const double and value 2.0

#### 2704. Clarify meaning of "bind directly"

Section: 9.4.4 [dcl.init.ref] Status: open Submitter: Brian Bi Date: 2022-11-11

Currently, 9.4.4 [dcl.init.ref] paragraph 5 specifies:

> In all cases except the last (i.e., implicitly converting the initializer expression to the referenced type), the reference is said to bind directly to the initializer expression.

This is intended to refer to both sub-bullets of the last bullet (see issue 1604), but the presentation is unclear.

Suggested resolution:

1. Change in 9.4.4 [dcl.init.ref] bullet 5.4 as follows:

   > Otherwise, the reference is said to bind indirectly to the initializer expression:
   > - If T1 or T2 is a class type and T1 is not reference-related to T2, user-defined conversions are considered using the rules for copy-initialization of an object of type “cv1 T1” by user-defined conversion (9.4 [dcl.init], 12.2.2.5 [over.match.copy], 12.2.2.6 [over.match.conv]); the program is ill-formed if the corresponding non-reference copy-initialization would be ill-formed. The result of the call to the conversion function, as described for the non-reference copy-initialization, is then used to direct-initialize the reference. For this direct-initialization, user-defined conversions are not considered.
   > - Otherwise, the initializer expression is implicitly converted to a prvalue of type “T1”. The temporary materialization conversion is applied, considering the type of the prvalue to be “cv1 T1”, and the reference is bound to the result.
   > If T1 is reference-related to T2:
   > - cv1 shall be the same cv-qualification as, or greater cv-qualification than, cv2; and
   > - if the reference is an rvalue reference, the initializer expression shall not be an lvalue. [Note 3: This can be affected by whether the initializer expression is move-eligible (7.5.4.2 [expr.prim.id.unqual]). —end note]

2. Change in 9.4.4 [dcl.init.ref] paragraph 5 as follows:

   > In all cases except the last (i.e., implicitly converting the initializer expression to the referenced type), the The reference is said to bind directly to the initializer expression if it does not bind indirectly. [ Example:

   >       struct S {
   >         operator int();
   >       };
   >       struct T {
   >         T(int);
   >       };
   >       int&& r1 = S(); // binds directly
   >       T&& r2 = int(); // binds indirectly
   >       int&& r3 = double(); // binds indirectly

   > -- end example ]

#### 2168. Narrowing conversions and +/- infinity

Section: 9.4.5 [dcl.init.list] Status: open Submitter: Hubert Tong Date: 2015-08-19

The intended treatment of a floating point infinity with respect to narrowing conversions is not clear. Is `std::numeric_limits<double>::infinity()` usable in a constant expression, for example, and should that be different from a calculation that results in an infinity?

Notes from the October, 2015 meeting:

CWG requests the assistance of SG6 in resolving this issue.

Notes from the November, 2016 meeting:

SG6 said that arithmetic operations (not conversions) that produce infinity are not allowed in a constant expression. However, using `std::numeric_limits<T>::infinity()` is okay, but it can't be used as a subexpression. Conversions that produce infinity from non-infinity values are considered to be narrowing conversions.

#### 2638. Improve the example for initializing by initializer list

Section: 9.4.5 [dcl.init.list] Status: open Submitter: Shafik Yaghmour Date: 2022-10-26

Issue 2137 amended the rules for initialization by initializer list, but neglected to add an example.

Suggested resolution:

Change the example in 9.4.5 [dcl.init.list] bullet 3.7 as follows:

    struct S {
      S(std::initializer_list<double>); // #1
      S(std::initializer_list<int>);    // #2
      S(std::initializer_list<S>);      // #3
      S();                              // #3#4
      S(const S&);                      // #5

      // ...
    };
    S s1 = { 1.0, 2.0, 3.0 };  // invoke #1
    S s2 = { 1, 2, 3 };        // invoke #2
    S s3{s2};                  // invoke #3
    S s3s4 = { };              // invoke #3#4
    S s5(s4);                  // invoke #5

#### 1962. Type of `__func__`

Section: 9.5.1 [dcl.fct.def.general] Status: open Submitter: Steve Clamage Date: 2014-07-04 Liaison: EWG

Two questions have arisen regarding the treatment of the type of the `__func__` built-in variable. First, some implementations accept

      void f() {
        typedef decltype(__func__) T;
        T x = __func__;
      }

even though `T` is specified to be an array type.

In a related question, it was noted that `__func__` is implicitly required to be unique in each function, and that not only the value but the type of `__func__` are implementation-defined; e.g., in something like

      inline auto f() { return &__func__; }

the function type is implementation-specific. These concerns could be addressed by making the value a prvalue of type `const char*` instead of an array lvalue.

Notes from the May, 2015 meeting:

CWG agreed with the suggested direction.

Rationale (November, 2018):

See also issue 2362, which asks for the ability to use `__func__` in a constexpr function. These two goals are incompatible, so EWG input is requested.

EWG 2022-11-11

Paper requested. This is tracked in github issue cplusplus/papers#1375.

#### 2362. `__func__` should be `constexpr`

Section: 9.5.1 [dcl.fct.def.general] Status: open Submitter: Anthony Polukhin Date: 2017-10-23 Liaison: EWG

The definition of `__func__` in 9.5.1 [dcl.fct.def.general] paragraph 8 is:

      static const char __func__[] = "function-name";

This prohibits its use in constant expressions, e.g.,

      int main () {
        // error: the value of __func__ is not usable in a constant expression
        constexpr char c = __func__[0];
      }

Notes from the October, 2018 teleconference:

CWG agreed with the proposed change.

Rationale (November, 2018):

See also issue 1962, which asks that the type of `__func__` be `const char*`. These two goals are incompatible, so EWG input is requested.

EWG 2022-11-11

This is tracked in github issue cplusplus/papers#1378.

#### 2547. Defaulted comparison operator function for non-classes

Section: 9.5.2 [dcl.fct.def.default] Status: open Submitter: Jim X Date: 2022-03-07

(See editorial issue 5337.)

Subclause 9.5.2 [dcl.fct.def.default] paragraph 1 specifies:

> A function definition whose function-body is of the form `= default ;` is called an explicitly-defaulted definition. A function that is explicitly defaulted shall
> - be a special member function or a comparison operator function (12.4.3 [over.binary]), and
> - not have default arguments.

There seem to be no further restrictions on which comparison operator functions are allowed to be defaulted. For example,

      enum E { };
      bool operator==(E, E) = default;  // well-formed?

Subclause 11.10.1 [class.compare.default] paragraph 1 applies only to comparison operator functions "for some class":

> A defaulted comparison operator function (12.4.3 [over.binary]) for some class `C` shall be a non-template function that is
> - a non-static const non-volatile member of C having one parameter of type `const C&` and either no ref-qualifier or the ref-qualifier &, or
> - a friend of `C` having either two parameters of type `const C&` or two parameters of type `C`.

Suggested resolution:

1. Change in 9.5.2 [dcl.fct.def.default] paragraph 1 as follows:

   > A function definition whose function-body is of the form `= default ;` is called an explicitly-defaulted definition. A function that is explicitly defaulted shall
   > - be a special member function (11.4.4 [special]) or a comparison operator function (12.4.3 [over.binary], 11.10.1 [class.compare.default]), and
   > - not have default arguments (9.3.4.7 [dcl.fct.default]).

2. Change in 11.10.1 [class.compare.default] paragraph 1 as follows:

   > A defaulted comparison operator function (12.4.3 [over.binary]) for some class `C` shall be a non-template function that is
   > - a non-static const non-volatile member of some class `C` having one parameter of type `const C&` and either no ref-qualifier or the ref-qualifier &, or
   > - a friend of some class `C` having either two parameters of type `const C&` or two parameters of type `C`.

   > Such a comparison operator function is termed a comparison operator function for class `C`. A comparison operator function for class `C` that is defaulted on its first declaration ...

#### 2570. Clarify constexpr for defaulted functions

Section: 9.5.2 [dcl.fct.def.default] Status: open Submitter: Gabriel dos Reis Date: 2022-04-18

After the application of P2448R2, 9.5.2 [dcl.fct.def.default] paragraph 3 reads:

> A function explicitly defaulted on its first declaration is implicitly inline (9.2.8 [dcl.inline]), and is implicitly constexpr (9.2.6 [dcl.constexpr]) if it satisfies the requirements for a constexpr function.

It is unclear that no other such defaulted function is implicitly constexpr.

Suggested resolution:

> A function explicitly defaulted on its first declaration is implicitly inline (9.2.8 [dcl.inline]), and is implicitly constexpr (9.2.6 [dcl.constexpr]) if it satisfies the requirements for a constexpr function. [Note: Other defaulted functions are not implicitly constexpr. -- end note ]

#### 2562. Exceptions thrown during coroutine startup

Section: 9.5.4 [dcl.fct.def.coroutine] Status: open Submitter: Tomasz Kamiński Date: 2022-04-06

Subclause 9.5.4 [dcl.fct.def.coroutine] seems to miss specification about the behavior of coroutines when an exception is thrown during the early phases of a coroutine evaluation. It is unclear at what point the ownership of the coroutine frame is passed from the compiler to the user, who then needs to call `std::coroutine_handle::destroy` to destroy and free the coroutine frame, including the parameter copies. The following situations arise:

- Allocation fails, and `get_return_object_on_allocation_failure()` is returned.
- Allocation succeeds, but an exception is thrown before the return object is initialized (e.g. when initializing the promise or evaluating `get_return_object`). The coroutine state ought to be deallocated automatically and the exception propagated.
- The return object is successfully initialized, but an exception would leave the coroutine before it would first suspend, either because initial-suspend's `await_ready` or `await_suspend` exits via an exception, or because `unhandled_exception` rethrows. The coroutine ought to be suspended at its final suspend point, the return object destroyed, and the exception propagated; the return object's destructor is responsible for ensuring the coroutine is destroyed.
- The coroutine ever suspends, or the coroutine terminates normally without ever suspending. The return object is returned to the caller, carrying with it ownership of the coroutine state.

See also issue 2451.

#### 2563. Initialization of coroutine result object

Section: 9.5.4 [dcl.fct.def.coroutine] Status: open Submitter: Tomasz Kamiński Date: 2022-04-06 Liaison: EWG

Subclause 9.5.4 [dcl.fct.def.coroutine] paragraph 7 specifies:

> The expression `promise.get_return_object()` is used to initialize the returned reference or prvalue result object of a call to a coroutine. The call to `get_return_object` is sequenced before the call to initial-suspend and is invoked at most once.

It is unclear:

- whether `get_return_object()` is invoked inside or outside of the try-block shown in paragraph 5 (see issue 2562),
- whether the prvalue result object may be initialized later (e.g. before the first actual suspension), and
- if the initialization does occur later, by what mechanism the prvalue result of `get_return_object` is forwarded to that initialization.

There is implementation divergence.

Note that a user-defined conversion may be involved in the initialization of the coroutine's prvalue result object from `get_return_object()`. Note also that the return type of `get_return_object` might be non-copyable and non-movable. However, there are certain programming patterns that would benefit from a late-initialized return value.

See also compiler explorer.

Suggested resolution [SUPERSEDED]:

Change in 9.5.4 [dcl.fct.def.coroutine] paragraph 7 as follows:

> The expression `promise.get_return_object()` is used to initialize the The returned reference or prvalue result object of a call to a coroutine is copy-initialized with `promise.get_return_object()`. The call to `get_return_object` initialization is sequenced before the call to initial-suspend and is invoked at most once.

Additional notes (January, 2023)

See also clang bug report #56532.

Forwarded to EWG with paper issue 1414, by decision of the CWG chair.

EWG 2023-02-06

EWG agrees that `get_return_object` is invoked outside of the try-block and that, if a conversion is needed, the return value of `get_return_object` is considered an xvalue that is later converted to the result object.

#### 2340. Reference collapsing and structured bindings

Section: 9.6 [dcl.struct.bind] Status: open Submitter: Daveed Vandevoorde Date: 2017-03-29

According to 9.6 [dcl.struct.bind] paragraph 3,

> Given the type `T` i designated by `std::tuple_element<i, E>::type`, each `v` i is a variable of type “reference to `T` i” initialized with the initializer, where the reference is an lvalue reference if the initializer is an lvalue and an rvalue reference otherwise; the referenced type is `T` i.

If `T` i is already a reference type, should this do reference collapsing? Presumably yes, but reference collapsing is specified in terms of a typedef-name or decltype-specifier, which are not used in this description.

See also issue 2313.

#### 813. `typename` in a using-declaration with a non-dependent name

Section: 9.9 [namespace.udecl] Status: open Submitter: UK Date: 3 March, 2009
N2800 comment UK 101

9.9 [namespace.udecl] paragraph 20 says,

> If a using-declaration uses the keyword `typename` and specifies a dependent name (13.8.3 [temp.dep]), the name introduced by the using-declaration is treated as a typedef-name (9.2.4 [dcl.typedef]).

This wording does not address use of `typename` in a using-declaration with a non-dependent name; the primary specification of the `typename` keyword in 13.8 [temp.res] does not appear to describe this case, either.

Additional notes (March, 2022):

The relevant wording is now in 13.8.1 [temp.res.general] paragraph 5:

> A name that refers to a using-declarator whose terminal name is dependent is interpreted as a typedef-name if the using-declarator uses the keyword `typename`.

#### 2555. Ineffective redeclaration prevention for using-declarators

Section: 9.9 [namespace.udecl] Status: open Submitter: Christof Meerwald Date: 2022-03-23

Consider:

      template<int I>
      struct C { };

      struct B
      {
        C<1> foo();
        C<1> bar();
      };

      struct D : B
      {
        using B::foo;
        C<2> foo(this B &);

        using B::bar;
        C<2> bar(this D &);
      };

      struct DD : D
      {
        using D::foo;
        using D::bar;
      };

      void bar(D d, DD dd)
      {
        d.foo();
        dd.foo();

        d.bar();
        dd.bar();
      }

Which functions are called?

Subclause 9.9 [namespace.udecl] paragraph 11 specifies:

> The set of declarations named by a using-declarator that inhabits a class `C` does not include member functions and member function templates of a base class that correspond to (and thus would conflict with) a declaration of a function or function template in `C`.

The definition of "corresponds" considers the type of the implicit object parameter, which is a deviation from the status quo ante for a simple example like this one:

      struct B {
        void f();    // #1
      };
      struct D : B {
        void f();
        using B::f;  // should not name #1
      };

Suggested resolution:

Change in 9.9 [namespace.udecl] paragraph 11 as follows:

> The set of declarations named by a using-declarator that inhabits a class `C` does not include member functions and member function templates of a base class that, when considered as members of `C`, correspond to (and thus would conflict with) a declaration of a function or function template in `C`.

> [ Example:

>       struct B {
>         virtual void f(int);
>         virtual void f(char);
>         void g(int);
>         void h(int);
>         void i();
>         void j();
>       };

>       struct D : B {
>         using B::f;
>         void f(int);   // OK, D::f(int) overrides B::f(int)

>         using B::g;
>         void g(char);  // OK

>         using B::h;
>         void h(int);   // OK, D::h(int) hides B::h(int)

>         using B::i;
>         void i(this B &);  // OK

>         using B::j;
>         void j(this D &);  // OK, D::j() hides B::j()
>       };

>       void k(D* p)
>       {
>         p->f(1);        // calls D::f(int)
>         p->f('a');      // calls B::f(char)
>         p->g(1);        // calls B::g(int)
>         p->g('a');      // calls D::g(char)
>         p->i();         // calls B::i, because B::i as a member of D is a better match than D::i
>         p->j();         // calls D::j
>       }
>       ...

#### 1617. `alignas` and non-defining declarations

Section: 9.12.2 [dcl.align] Status: open Submitter: Richard Smith Date: 2012-02-02

According to 9.12.2 [dcl.align] paragraph 6,

> If the defining declaration of an entity has an alignment-specifier, any non-defining declaration of that entity shall either specify equivalent alignment or have no alignment-specifier. Conversely, if any declaration of an entity has an alignment-specifier, every defining declaration of that entity shall specify an equivalent alignment. No diagnostic is required if declarations of an entity have different alignment-specifiers in different translation units.

Because this is phrased in terms of the definition of an entity, an example like the following is presumably well-formed (even though there can be no definition of `n`):

       alignas(8) extern int n;
       alignas(16) extern int n;

Is this intentional?

#### 2541. Linkage specifications, module purview, and module attachment

Section: 10.1 [module.unit] Status: open Submitter: Nathan Sidwell Date: 2022-02-28

The interaction between linkage specifications (9.11 [dcl.link]) and named or global module purview and attachment (10.1 [module.unit]) is confusing. The addition of linkage declarations attaching their contents to the global module is not fully integrated into the wording and examples would also help.

- Module purview and attachment are not necessarily the same. This is never explicitly mentioned.
- 10.1 [module.unit] paragraph 7 explicitly calls out replaceable allocation and deallocation functions as being (implicitly) attached to the global module. This is unnecessary.
- 6.9.3.1 [basic.start.main] paragraph 1 specifies that there shall be a main function in the global namespace, but only in paragraph 3 do we specify that not attaching it to the global module is ill-formed.

Suggested resolution:

1. Change 6.9.3.1 [basic.start.main] paragraph 1 as follows:

   > A program shall contain exactly one function called `main` that belongs to the global scope and is attached to the global module. Executing a program starts a main thread of execution (6.9.2 [intro.multithread], 33.4 [thread.threads]) in which the `main` function is invoked. It is implementation-defined whether a program in a freestanding environment is required to define a `main` function.

2. Change 10.1 [module.unit] bullet 7.2 as follows:

   > - ...
   > - Otherwise, if the declaration
   >   - is a replaceable global allocation or deallocation function (17.6.3.2 [new.delete.single], 17.6.3.3 [new.delete.array]), or
   >   - is a namespace-definition with external linkage, or
   >   - appears within a linkage-specification,
   >   it is attached to the global module.
   > - Otherwise, ...

3. Add an example at the end of 10.1 [module.unit] paragraph 7:

   > [ Example:

   >       // Translation unit #1
   >       export module Foo;
   >       void f();              // module linkage, attached to named module Foo
   >       extern "C++" {
   >         export void g();     // nameable by importers
   >         void h();            // nameable in Foo's purview
   >       }

   > Both `g` and `h` have external linkage, are attached to the global module, and can thus also be redeclared in other translation units:

   >       // Legacy header "foo.h"
   >       extern "C++" void g();

   >       // Legacy header "foo-internal.h"
   >       extern "C++" void h();

   > -- end example ]

   > A module-declaration that contains neither...

4. Change in 10.2 [module.interface] paragraph 6 as follows:

   > A redeclaration of an entity X is implicitly exported if X was introduced by an exported declaration; otherwise it shall not be exported unless it has external linkage..

#### 2637. Injected-class-name as a simple-template-id

Section: 11.1 [class.pre] Status: open Submitter: Shafik Yaghmour Date: 2022-10-26

Issue 2237 sought to disallow simple-template-ids as constructor names, by referring to the injected-class-name. However, 11.1 [class.pre] paragraph 2 specifies:

> The class-name is also bound in the scope of the class (template) itself; this is known as the injected-class-name.

The grammar non-terminal class-name includes the option of a simple-template-id (for declaring a partial specialization).

Suggested resolution:

Change in 11.1 [class.pre] paragraph 2 as follows:

> The terminal name of the class-name is also bound in the scope of the class (template) itself; this is known as the injected-class-name. ...

#### 511. POD-structs with template assignment operators

Section: 11.2 [class.prop] Status: open Submitter: Alisdair Meredith Date: 19 Mar 2005

A POD-struct is not permitted to have a user-declared copy assignment operator (11.2 [class.prop] paragraph 1). However, a template assignment operator is not considered a copy assignment operator, even though its specializations can be selected by overload resolution for performing copy operations (11.4.6 [class.copy.assign] paragraph 12). Consequently, `X` in the following code is a POD, notwithstanding the fact that copy assignment (for a non-const operand) is a member function call rather than a bitwise copy:

        struct X {
          template<typename T> const X& operator=(T&);
        };
        void f() {
          X x1, x2;
          x1 = x2;  // calls X::operator=<X>(X&)
        }

Is this intentional?

#### 2463. Trivial copyability and unions with non-trivial members

Section: 11.2 [class.prop] Status: open Submitter: Daveed Vandevoorde Date: 2020-11-30 Liaison: EWG

According to 11.2 [class.prop] paragraph 1,

> A trivially copyable class is a class:

> - that has at least one eligible copy constructor, move constructor, copy assignment operator, or move assignment operator (11.4.4 [special], 11.4.5.3 [class.copy.ctor], 11.4.6 [class.copy.assign]),

> - where each eligible copy constructor, move constructor, copy assignment operator, and move assignment operator is trivial, and

> - that has a trivial, non-deleted destructor (11.4.7 [class.dtor]).

This definition has surprising effects in a union whose members are not trivial. For example:

      struct S {
        S& operator=(const S&);
      };
      union U {
        S s;
      };

In this case, `S` is not trivially copyable because its assignment operator is non-trivial, although its copy constructor is trivial. `U`, however, is trivially copyable because its assignment operator is not eligible (11.4.4 [special] paragraph 6) because it is deleted, but its copy constructor is trivial, thus satisfying the second bullet.

It is unclear why, for example, a complete object of type `S` cannot be `memcpy`ed but such an object can be `memcpy`ed when embedded in a union.

There is implementation divergence in the handling of this example.

CWG 2022-11-10

Traditionally, the rule for trivial copyability has been that each of the potentially user-written ways of copying a class (copy/move constructors, copy/move assignment operators) have to be trivial (or deleted). See C++17 subclause 12p6:

> A trivially copyable class is a class:
> - where each copy constructor, move constructor, copy assignment operator, and move assignment operator (15.8, 16.5.3) is either deleted or trivial,
> - that has at least one non-deleted copy constructor, move constructor, copy assignment operator, or move assignment operator, and
> - that has a trivial, non-deleted destructor (15.4).

That seems unhelpful. The rule should instead be that if there is any way of copying the class such that the compiler will generate a `memcpy` (because the corresponding operation is trivial), the user should be allowed to perform `memcpy`, too. In terms of wording, this amounts to striking the first bullet and adding "trivial" to the second bullet. (The wording in the current working draft considers eligibility, which complicates the treatment slightly in terms unrelated to the present issue.)

CWG is seeking EWG advice on this issue via cplusplus/papers#1363.

Additional notes (March, 2023)

`std::tuple` with trivially-copyable element types and with no elements (`std::tuple<>`) ought to be trivially copyable, but the recent addition of `const`-qualified assignment operators makes that not so under the status quo core language rules.

#### 2188. empty-declaration grammar ambiguity

Section: 11.4.1 [class.mem.general] Status: open Submitter: Jens Maurer Date: 2015-10-21

(The originally reported ambiguity between simple-declaration and empty-declaration does not seem to exist.)

There is a grammar ambiguity between

      empty-declaration:
         ;

and

      member-declaration :
          attribute-specifier-seqopt decl-specifier-seqopt member-declarator-listopt ;
          ...
          empty-declaration

#### 2661. Missing disambiguation rule for pure-specifier vs. brace-or-equal-initializer

Section: 11.4.1 [class.mem.general] Status: open Submitter: Richard Smith Date: 2022-10-07

Subclause 11.4.1 [class.mem.general] has this grammar:

        member-declarator:
            declarator virt-specifier-seq[opt] pure-specifier[opt]
            declarator brace-or-equal-initializer[opt]

        pure-specifier:
            = 0

The primary issue is that `foo = 0` matches both member-declarator productions. Secondarily, a declarator by itself is also ambiguous.

Code such as `virtual FunctionType f = 0;` can be valid, so disambiguation on the syntactic form of the declarator is not possible.

Suggested resolution:

Change and add before 11.4.1 [class.mem.general] paragraph 1 as follows:

>         member-declarator:
>             declarator virt-specifier-seqopt pure-specifieropt
>             declarator brace-or-equal-initializeropt

> The token sequence `= 0` is treated as a pure-specifier if the type of the declarator-id (9.3.4.1 [dcl.meaning.general]) is a function type, and is otherwise treated as a brace-or-equal-initializer. [ Note: If the member declaration acquires a function type through template instantiation, the program is ill-formed; see 13.9.1 [temp.spec.general]. --end note ]

#### 2595. "More constrained" for eligible special member functions

Section: 11.4.4 [special] Status: open Submitter: Barry Revzin Date: 2022-06-08

Consider:

      #include <type_traits>

      template<typename T>
      concept Int = std::is_same_v<T, int>;

      template<typename T>
      concept Float = std::is_same_v<T, float>;

      template<typename T>
      struct Foo {
        Foo() requires Int<T> = default; // #1
        Foo() requires Int<T> || Float<T> = default; // #2
      };

Per the wording, #1 is not eligible for `Foo<float>`, because the constraints are not satisfied. But #2 also is not eligible, because #1 is more constrained than #2. The intent is that #2 is eligible.

Suggested resolution:

Change in 11.4.4 [special] paragraph 6 as follows:

> An eligible special member function is a special member function for which:
> - the function is not deleted,
> - the associated constraints (13.5 [temp.constr]), if any, are satisfied, and
> - no special member function of the same kind whose associated constraints, if any, are satisfied is more constrained (13.5.5 [temp.constr.order]).

#### 2513. Ambiguity with requires-clause and operator-function-id

Section: 11.4.8.3 [class.conv.fct] Status: open Submitter: Richard Smith Date: 2021-12-09

An ambiguity can occur with a requires-clause that ends in an operator-function-id:

      template<typename T> requires T::operator int
      const // part of operator-type-id or return type?
      unsigned f();

      template<typename T> requires T::operator int
      [[attr]] // appertains to type int or to declaration of g?
      void g();

Such cases are always ill-formed, because they involve an atomic constraint of non-`bool` type.

#### 57. Empty unions

Section: 11.5 [class.union] Status: open Submitter: Steve Adamczyk Date: 13 Oct 1998

There doesn't seem to be a prohibition in 11.5 [class.union] against a declaration like

        union { int : 0; } x;

Should that be valid? If so, 9.4 [dcl.init] paragraph 5 third bullet, which deals with default-initialization of unions, should say that no initialization is done if there are no data members.

What about:

        union { } x;
        static union { };

If the first example is well-formed, should either or both of these cases be well-formed as well?

(See also the resolution for issue 151.)

Notes from 10/00 meeting: The resolution to issue 178, which was accepted as a DR, addresses the first point above (default initialization). The other questions have not yet been decided, however.

#### 2591. Implicit change of active union member for anonymous union in union

Section: 11.5.1 [class.union.general] Status: open Submitter: Richard Smith Date: 2022-05-29

Subclause 11.5.1 [class.union.general] paragraph 6 describes how union member subobjects are implicitly created by certain assignment operations that assign to union members. However, this description does not appear to properly handle the case of an anonymous union appearing within a union:

      union A {
        int x;
        union {
         int y;
        };
      };
      void f() {
        A a = {.x = 1};
        a.y = 2;
      }

Here, the expectation is that the assignment to `a.y` starts the lifetime of the anonymous union member subobject within `A` and also the int member subobject of the anonymous union member subobject. But the algorithm for computing S(`a.y`) determines that it is {`a.y`} and does not include the anonymous union member subobject.

Suggested resolution:

Change in 11.5.1 [class.union.general] paragraph 6 as follows:

> In an assignment expression of the form `E1 = E2` that uses either the built-in assignment operator (7.6.19 [expr.ass]) or a trivial assignment operator (11.4.6 [class.copy.assign]), for each element X of S(E1) and each anonymous union member X (11.5.2 [class.union.anon]) that is a member of a union and has such an element as an immediate subobject (recursively), if modification of X would have undefined behavior under 6.7.3 [basic.life], an object of the type of X is implicitly created in the nominated storage; no initialization is performed and the beginning of its lifetime is sequenced after the value computation of the left and right operands and before the assignment.

Editing note: Adding this rule into the definition of S would be more logical, but S(E) is a set of subexpressions of E and there is no form of expression that names an anonymous union member. Redefining S(E) to be a set of objects might be a better option.

#### 2675. `start_lifetime_as`, placement-new, and active union members

Section: 11.5.1 [class.union.general] Status: open Submitter: Nina Ranns Date: 2022-12-06

Consider:

      struct A { int i;};
      struct B { int j;};
      union U { A a; B b; };
      U u1{A{}};
      start_lifetime_as<B>(&u1.a); // #1
      start_lifetime_as<B>(&u1);   // #2
      U u2{B{}};
      start_lifetime_as<U>(&u2);   // #3
      start_lifetime_as<B>(&u1.b); // #4

It is unclear which of these constructs changes the active member of the union, and whether a union might have more than one active member. Subclause 11.5.1 [class.union.general] paragraph 2 appears contradictory if several members can be in-lifetime:

> In a union, a non-static data member is active if its name refers to an object whose lifetime has begun and has not ended (6.7.3 [basic.life]). At most one of the non-static data members of an object of union type can be active at any time, that is, the value of at most one of the non-static data members can be stored in a union at any time.

Similar questions arise for placement-new and for `construct_at`; for the latter also during constant evaluation.

#### 2554. Overriding virtual functions, also with explicit object parameters

Section: 11.7.3 [class.virtual] Status: open Submitter: Jens Maurer Date: 2021-12-10

Consider:

      struct B {
        virtual void f();   // #1
      };

      struct D : B {
        void f();           // #2
      };

Subclause 11.7.3 [class.virtual] paragraph 2 says:

> If a virtual member function F is declared in a class B, and, in a class D derived (directly or indirectly) from B, a declaration of a member function G corresponds (6.4.1 [basic.scope.scope]) to a declaration of F, ignoring trailing requires-clauses, then G overrides [ Footnote: ... ] F .

Subclause 6.4.1 [basic.scope.scope] paragraph 4 defines "corresponds" as follows:

> Two declarations correspond if they (re)introduce the same name, both declare constructors, or both declare destructors, unless
> - ...
> - each declares a function or function template, except when
>   - both declare functions with the same non-object-parameter-type-list, equivalent (13.7.7.2 [temp.over.link]) trailing requires-clauses (if any, except as specified in 13.7.5 [temp.friend]), and, if both are non-static members, they have corresponding object parameters, or
>   - both declare function templates with...

Subclause 6.4.1 [basic.scope.scope] paragraph 3 defines "corresponding object parameters" as follows:

> Two non-static member functions have corresponding object parameters if:
> - exactly one is an implicit object member function with no ref-qualifier and the types of their object parameters (9.3.4.6 [dcl.fct]), after removing top-level references, are the same, or
> - their object parameters have the same type.

In the example, `B::f` has an object parameter of type `B`, but `D::f` has an object parameter of type `D`. Thus, the two functions do not correspond, and thus `D::f` does not override `B::f`. That is an unintended alteration of the status quo ante.

See also issue 2553.

Suggested resolution:

Change in 11.7.3 [class.virtual] paragraph 2 as follows:

> If a virtual member function F is declared in a class B, and, in a class D derived (directly or indirectly) from B, a declaration of a member function G corresponds (6.4.1 [basic.scope.scope]) to a declaration of F considered as a member of D (12.2.2.1 [over.match.funcs.general]), ignoring trailing requires-clauses, and, if both are implicit object member functions, they have the same ref-qualifier (or absence thereof), and, if at least one is an explicit object member function, ignoring object parameters, then G overrides [ Footnote: ... ] F .

#### 718. Non-class, non-function friend declarations

Section: 11.8.4 [class.friend] Status: open Submitter: John Spicer Date: 18 September, 2008 Liaison: EWG

With the change from a scope-based to an entity-based definition of friendship (see issues 372 and 580), it could well make sense to grant friendship to enumerations and variables, for example:

        enum E: int;
        class C {
          static const int i = 5;  // Private
          friend E;
          friend int x;
        };
        enum E { e = C::i; };      // OK: E is a friend
        int x = C::i;              // OK: x is a friend

According to the current wording of 11.8.4 [class.friend] paragraph 3, the friend declaration of `E` is well-formed but ignored, while the friend declaration of `x` is ill-formed.

Additional notes (February, 2023)

Forwarding to EWG for review of this extension, per decision of the CWG chair. See cplusplus/papers#1466.

#### 2244. Base class access in aggregate initialization

Section: 11.8.5 [class.protected] Status: open Submitter: Richard Smith Date: 2016-03-08

The rules in 11.8.5 [class.protected] assume an object expression, perhaps implicit, that can be used to determine whether access to protected members is permitted or not. It is not clear how that applies to aggregates and constructors. For example:

      struct A {
      protected:
        A();
      };
      struct B : A {
        friend B f();
        friend B g();
        friend B h();
      };
      B f() { return {}; }     // ok?
      B g() { return {{}}; }   // ok?
      B h() { return {A{}}; }  // ok?

Notes from the December, 2016 teleconference:

The consensus favored accepting `f` and `g` while rejecting `h`.

Notes from the March, 2018 meeting:

CWG affirmed the earlier direction and felt that there should be an implicit object expression assumed for these cases.

#### 1915. Potentially-invoked destructors in non-throwing constructors

Section: 11.9.3 [class.base.init] Status: open Submitter: Aaron Ballman Date: 2014-04-15 Liaison: EWG

The requirement in 11.9.3 [class.base.init] paragraph 10,

> In a non-delegating constructor, the destructor for each potentially constructed subobject of class type is potentially invoked (11.4.7 [class.dtor]). [Note: This provision ensures that destructors can be called for fully-constructed sub-objects in case an exception is thrown (14.3 [except.ctor]). —end note]

is needlessly restrictive, preventing otherwise-reasonable code like

      class Base {
      protected:
        Base(int i) noexcept {}
        Base() = delete;
        ~Base() = delete;
      };

      class Derived : public Base {
      public:
        Derived() noexcept : Base(1) {}
        ~Derived() = delete;
      };

Since the derived class constructor is non-throwing, the deleted base class destructor need not be referenced.

Suggested resolution:

Change 11.9.3 [class.base.init] paragraph 10 as follows:

> In a non-delegating constructor without a non-throwing exception-specification (14.5 [except.spec]), the destructor for each potentially constructed subobject of class type is potentially invoked (11.4.7 [class.dtor]). [Note: This provision ensures that destructors can be called for fully-constructed sub-objects in case an exception is thrown (14.3 [except.ctor]) but does not prevent explicitly deleted destructors in the presence of a non-throwing constructor. —end note]

Rationale (June, 2014):

This request for a language change should be evaluated by EWG before any action is taken.

EWG 2022-11-11

This is a defect, but has ABI impact that should be explored in a paper to EWG. This is tracked in github issue cplusplus/papers#1371.

#### 2669. Lifetime extension for aggregate initialization

Section: 11.9.3 [class.base.init] Status: open Submitter: Ed Catmur Date: 2022-12-18

Issue 1815 presented the following example:

      struct A {};
      struct B { A&& a = A{}; };
      B b1;       // #1
      B b2{A{}};  // #2
      B b3{};     // #3

CWG intended to make #3 behave like #2, i.e. the temporary created in the brace-or-equal initializer is lifetime-extended. Issue 1696 was adopted, ostensibly resolving issue 1815, but making #3 (and #1) ill-formed per 11.9.3 [class.base.init] paragraph 11:

> A temporary expression bound to a reference member from a default member initializer is ill-formed. [ Example:

>       struct A {
>         A() = default;          // OK
>         A(int v) : v(v) { }     // OK
>         const int& v = 42;      // OK
>       };
>       A a1;     // error: ill-formed binding of temporary to reference
>       A a2(1);  // OK, unfortunately

> -- end example]

However, the example does not consider aggregate initialization, and the specified resolution for issue 1696 is contrary to the stated intent in issue 1815.

There is considerable implementation variance: #1 is rejected by clang, but accepted by gcc and EDG (with early destruction of the temporary); #2 is uniformly accepted and the lifetime of the temporary is extended; #3 is uniformly accepted, but only gcc, clang, and MSVC extend the lifetime of the temporary, whereas EDG does not.

Note that 9.4.1 [dcl.init.general] paragraph 7 specifies that default-initialization of aggregates is as-if the initializer `()` is used:

> To default-initialize an object of type T means:
> - If T is a (possibly cv-qualified) class type (Clause 11 [class]), constructors are considered. The applicable constructors are enumerated (12.2.2.4 [over.match.ctor]), and the best one for the initializer () is chosen through overload resolution (12.2 [over.match]). The constructor thus selected is called, with an empty argument list, to initialize the object.
> - If T is an array type, each element is default-initialized.
> - Otherwise, no initialization is performed.

Such treatment causes early destruction of temporaries per 6.7.7 [class.temporary] bullet 6.10:

> The exceptions to this lifetime rule are:
> - ...
> - A temporary object bound to a reference element of an aggregate of class type initialized from a parenthesized expression-list (9.4 [dcl.init]) persists until the completion of the full-expression containing the expression-list.

CWG 2023-01-06

CWG is soliciting the design guidance of EWG to resolve this issue:

1. Is #1 in the initial example intended to be ill-formed? If not, is the temporary created in the default member initializer for `a` intended to be lifetime-extended?
2. Does the specification treatment of #2 (well-formed; lifetime extension of the `A` temporary) reflect the design intent?
3. Is #3 intended to be ill-formed? If not, is the temporary created in the default member initializer for `a` intended to be lifetime-extended?

#### 6. Should the optimization that allows a class object to alias another object also allow the case of a parameter in an inline function to alias its argument?

Section: 11.9.6 [class.copy.elision] Status: open Submitter: unknown Date: unknown

[Picked up by evolution group at October 2002 meeting.]

(See also paper J16/99-0005 = WG21 N1182.)

At the London meeting, 11.4.5.3 [class.copy.ctor] paragraph 31 was changed to limit the optimization described to only the following cases:

- the source is a temporary object
- the return value optimization

One other case was deemed desirable as well:

- aliasing a parameter in an inline function call to the function call argument.

However, there are cases when this aliasing was deemed undesirable and, at the London meeting, the committee was not able to clearly delimit which cases should be allowed and which ones should be prohibited.

Can we find an appropriate description for the desired cases?

Rationale (04/99): The absence of this optimization does not constitute a defect in the Standard, although the proposed resolution in the paper should be considered when the Standard is revised.

Note (March, 2008):

The Evolution Working Group has accepted the intent of this issue and referred it to CWG for action (not for C++0x). See paper J16/07-0033 = WG21 N2173.

Notes from the June, 2008 meeting:

The CWG decided to take no action on this issue until an interested party produces a paper with analysis and a proposal.

#### 1049. Copy elision through reference parameters of inline functions

Section: 11.9.6 [class.copy.elision] Status: open Submitter: Jason Merrill Date: 2010-03-10

Consider the following example:

        int c;

        struct A {
           A() { ++c; }
           A(const A&) { ++c; }
        };

        struct B {
           A a;
           B(const A& a): a(a) { }
        };

        int main() {
           (B(A()));
           return c - 1;
        }

Here we would like to be able to avoid the copy and just construct the `A()` directly into the `A` subobject of `B`. But we can't, because it isn't allowed by 11.4.5.3 [class.copy.ctor] bullet 34.3:

- when a temporary class object that has not been bound to a reference (6.7.7 [class.temporary]) would be copied/moved to a class object with the same cv-unqualified type, the copy/move operation can be omitted by constructing the temporary object directly into the target of the omitted copy/move

The part about not being bound to a reference was added for an unrelated reason by issue 185. If that resolution were recast to require that the temporary object is not accessed after the copy, rather than banning the reference binding, this optimization could be applied.

The similar example using pass by value is also not one of the allowed cases, which could be considered part of issue 6.

#### 2568. Access checking during synthesis of defaulted comparison operator

Section: 11.10.1 [class.compare.default] Status: open Submitter: Nicolai Josuttis Date: 2022-04-11

Consider:

      struct Base {
      protected:
        bool operator==(const Base& other) const = default;
      };

      struct Child : Base {
        int i;
        bool operator==(const Child& other) const = default;
      };

Per 11.10.1 [class.compare.default] paragraph 6,

> Let xi be an lvalue denoting the i-th element in the expanded list of subobjects for an object x (of length n), where xi is formed by a sequence of derived-to-base conversions (12.2.4.2 [over.best.ics]), class member access expressions (7.6.1.5 [expr.ref]), and array subscript expressions (7.6.1.2 [expr.sub]) applied to x.

The derived-to-base conversion for `this` loses the context of access to the protected `Base::operator==`, violating 11.8.5 [class.protected] paragraph 1. The example is rejected by implementations, but ought to work.

For this related example, there is implementation divergence:

      struct B {
      protected:
        constexpr operator int() const { return 0; }
      };
      struct D : B {
        constexpr bool operator==(const D&) const = default;
      };
      template<typename T> constexpr auto comparable(T t) -> decltype(t == t) { return t == t; }
      constexpr bool comparable(...) { return false; }
      static_assert(comparable(D{}));

Is `D::operator==` deleted, because its defaulted definition violates the protected access rules? Is `D::operator==` not deleted, but synthesis fails on use because of the proctected access rules? Is the synthesis not in the immediate context, making the expression `comparable(D{})` ill-formed?

#### 2546. Defaulted secondary comparison operators defined as deleted

Section: 11.10.4 [class.compare.secondary] Status: open Submitter: Jim X Date: 2022-03-07

(See also editorial issues 5335 and 5336.)

Consider the example in 11.10.4 [class.compare.secondary] paragraph 3:

      struct HasNoLessThan { };
      struct C {
        friend HasNoLessThan operator<=>(const C&, const C&);
        bool operator<(const C&) const = default;  // OK, function is deleted
      };

While the comment may reflect the intent, it does not follow from the wording. 11.10.4 [class.compare.secondary] paragraph 2 specifies:

> The operator function with parameters `x` and `y` is defined as deleted if
> - overload resolution (12.2 [over.match]), as applied to `x @ y`, does not result in a usable candidate, or
> - the candidate selected by overload resolution is not a rewritten candidate.

> Otherwise, the operator function yields `x @ y`. The defaulted operator function is not considered as a candidate in the overload resolution for the `@` operator.

Overload resolution applied to `x < y` results in a usable candidate `operator<=>` (12.2.1 [over.match.general]) and that candidate is a rewritten candidate (12.2.2.3 [over.match.oper] bullet 3.4), thus `operator<` in the above example is not deleted. However, its definition is ill-formed, because the rewrite `(x <=> y) < 0` is ill-formed (12.2.2.3 [over.match.oper] paragraph 8).

There is implementation divergence.

Subclause 11.10.3 [class.spaceship] paragraph 1 seems to prefer an ill-formed program for similar synthesized situations:

> [Note 1: A synthesized three-way comparison is ill-formed if overload resolution finds usable candidates that do not otherwise meet the requirements implied by the defined expression. —end note]

Suggested resolution:

Change in 11.10.4 [class.compare.secondary] paragraph 2 as follows:

> The operator function with parameters `x` and `y` is defined as deleted if
> - a first overload resolution (12.2 [over.match]), as applied to `x @ y`,
>   - does not result in a usable candidate, or
>   - the selected candidate selected by overload resolution is not a rewritten candidate. , or
> - a second overload resolution for the expression resulting from the interpretation of `x @ y` using the selected rewritten candidate (12.2.2.3 [over.match.oper]) does not result in a usable candidate, or
> - a third overload resolution for a copy-initialization of an object of type `bool` from `x @ y` does not result in a usable candidate;

> in any overload resolution, the defaulted operator function is not considered as a candidate for the `@` operator. Otherwise, the operator function yields `x @ y`. The defaulted operator function is not considered as a candidate in the overload resolution for the `@` operator.

#### 2189. Surrogate call template

Section: 12.2.2.2.3 [over.call.object] Status: open Submitter: Jason Merrill Date: 2015-10-22

Consider:

      template <class T>
      using Fn = void (*)(T);

      struct A
      {
        template <class T>
        operator Fn<T>();
      };

      int main()
      {
        A()(42);
      }

12.2.2.2.3 [over.call.object] describes how conversion functions to pointer/reference to function work in overload resolution, but is silent about conversion function templates. Generalizing the wording there, in this case we could generate a surrogate conversion template

    template <class T>
      void /surrogate/ (Fn<T> f, T a) { return f(a); }

which would work as expected. But it seems that implementations don't actually do this currently.

#### 2564. Conversion to function pointer with an explicit object parameter

Section: 12.2.2.2.3 [over.call.object] Status: open Submitter: Christof Meerwald Date: 2022-04-11

Subclause 12.2.2.2.3 [over.call.object] paragraph 2 considers only those conversion funtions matching a particular grammar pattern. This unintendedly excludes conversion functions with an explicit object parameter (and, as a pre-existing defect, `noexcept` conversion functions):

> In addition, for each non-explicit conversion function declared in T of the form

>     operator conversion-type-id ( ) cv-qualifier-seqopt ref-qualifieropt noexcept-specifieropt attribute-specifier-seqopt ;

> where the optional cv-qualifier-seq is the same cv-qualification as, or a greater cv-qualification than, cv, and where conversion-type-id denotes the type “pointer to function of (P1 , . . . , Pn ) returning R”, or the type “reference to pointer to function of (P1 , . . . , Pn ) returning R”, or the type “reference to function of (P1 , . . . , Pn ) returning R”, a surrogate call function with the unique name call-function and having the form

>     R call-function ( conversion-type-id F, P1 a1 , ... , Pn an ) { return F (a1 , . . . , an ); }

> is also considered as a candidate function. Similarly, surrogate call functions are added to the set of candidate functions for each non-explicit conversion function declared in a base class of T provided the function is not hidden within T by another intervening declaration. [ Footnote: ...]

For example, there is implementation divergence in handling this example:

      using fn_t = void();
      struct C {
        operator fn_t * (this C const &);
      };

      void foo(C c) {
        c();
      }

#### 545. User-defined conversions and built-in operator overload resolution

Section: 12.2.2.3 [over.match.oper] Status: open Submitter: Steve Clamage Date: 31 October 2005

Consider the following example:

        class B1 {};
        typedef void (B1::*PB1) (); // memptr to B1

        class B2 {};
        typedef void (B2::*PB2) (); // memptr to B2

        class D1 : public B1, public B2 {};
        typedef void (D1::*PD) (); // memptr to D1

        struct S {
             operator PB1(); // can be converted to PD
        } s;
        struct T {
             operator PB2(); // can be converted to PD
        } t;

        void foo() {
             s == t; // Is this an error?
        }

According to 12.5 [over.built] paragraph 16, there is an `operator==` for `PD` (“For every pointer to member type...”), so why wouldn't it be used for this comparison?

Mike Miller: The problem, as I understand it, is that 12.2.2.3 [over.match.oper] paragraph 3, bullet 3, sub-bullet 3 is broader than it was intended to be. It says that candidate built-in operators must “accept operand types to which the given operand or operands can be converted according to 12.2.4.2 [over.best.ics].” 12.2.4.2.3 [over.ics.user] describes user-defined conversions as having a second standard conversion sequence, and there is nothing to restrict that second standard conversion sequence.

My initial thought on addressing this would be to say that user-defined conversion sequences whose second standard conversion sequence contains a pointer conversion or a pointer-to-member conversion are not considered when selecting built-in candidate operator functions. They would still be applicable after the hand-off to Clause 5 (e.g., in bringing the operands to their common type, 7.6.10 [expr.eq], or composite pointer type, 7.6.9 [expr.rel]), just not in constructing the list of built-in candidate operator functions.

I started to suggest restricting the second standard conversion sequence to conversions having Promotion or Exact Match rank, but that would exclude the Boolean conversions, which are needed for `!`, `&&`, and `||`. (It would have also restricted the floating-integral conversions, though, which might be a good idea. They can't be used implicitly, I think, because there would be an ambiguity among all the promoted integral types; however, none of the compilers I tested even tried those conversions because the errors I got were not ambiguities but things like “floating point operands not allowed for `%`”.)

Bill Gibbons: I recall seeing this problem before, though possibly not in committee discussions. As written this rule makes the set of candidate functions dependent on what classes have been defined, including classes not otherwise required to have been defined in order for "==" to be meaningful. For templates this implies that the set is dependent on what templates have been instantiated, e.g.

      template<class T> class U : public T { };
      U<B1> u;  // changes the set of candidate functions to include
                // operator==(U<B1>,U<B1>)?

There may be other places where the existence of a class definition, or worse, a template instantiation, changes the semantics of an otherwise valid program (e.g. pointer conversions?) but it seems like something to be avoided.

(See also issue 954.)

#### 1919. Overload resolution for `!` with explicit conversion operator

Section: 12.2.2.3 [over.match.oper] Status: open Submitter: Johannes Schaub Date: 2014-04-30

Although the intent is that the `!` operator should be usable with an operand that is a class object having an explicit conversion to `bool` (i.e., its operand is “contextually converted to `bool`”), the selection of the conversion operator is done via 12.2.2.3 [over.match.oper], 12.2.3 [over.match.viable], and 12.2.4 [over.match.best], which do not make specific allowance for this special characteristic of the `!` operator and thus will not select the explicit conversion function.

Notes from the June, 2014 meeting:

CWG noted that this same issue affects `&&` and `||`.

#### 2311. Missed case for guaranteed copy elision

Section: 12.2.2.8 [over.match.list] Status: open Submitter: Richard Smith Date: 2016-08-09

Consider:

      struct X {
        X();
      };
      X make();
      X x{make()};

We reach 9.4.5 [dcl.init.list] bullet 3.7:

> Otherwise, if `T` is a class type, constructors are considered. The applicable constructors are enumerated and the best one is chosen through overload resolution (12.2 [over.match], 12.2.2.8 [over.match.list]).

This means we perform a redundant copy. If `T` were an aggregate, 9.4.5 [dcl.init.list] bullet 3.2 would avoid the redundant copy:

> If `T` is an aggregate class and the initializer list has a single element of type cv `U`, where `U` is `T` or a class derived from `T`, the object is initialized from that element (by copy-initialization for copy-list-initialization, or by direct-initialization for direct-list-initialization).

See also issues 2137 and 2327.

#### 2425. Confusing wording for deduction from a type

Section: 12.2.2.9 [over.match.class.deduct] Status: open Submitter: Dawn Perchik Date: 2019-08-06

In 12.2.2.9 [over.match.class.deduct] paragraph 3 we read:

> The arguments of a template `A` are said to be deducible from a type `T` if, given a class template

>       template <typename> class AA;

> with a single partial specialization whose template parameter list is that of `A` and whose template argument list is a specialization of `A` with the template argument list of `A` (13.8.3.2 [temp.dep.type]), AA<T> matches the partial specialization.

The relationship between `A`, `AA` and its partial specialization, and the argument list of A is not clear. An example would be very helpful here. Also, using a different name than `A` would help, since `A` is used in close proximity to this wording to denote an alias template, while this wording applies to both class and alias templates. Finally, there should be a cross-reference to 13.7.6.2 [temp.spec.partial.match] for matching the partial specialization.

#### 2628. Implicit deduction guides should propagate constraints

Section: 12.2.2.9 [over.match.class.deduct] Status: open Submitter: Roy Jacobson Date: 2022-09-11

Consider:

    template<class T> concept True = true;

    template<class T> struct X {
      template<class U> requires True<T> X(T, U(&)[3]);
    };
    template<typename T, typename U> X(T, U(&)[3]) -> X<T>;
    int arr3[3];
    X z(3, arr3);     // #1

According to 12.2.2.9 [over.match.class.deduct] bullet 1.1, the requires-clause of the constructor is not propagated to the function template synthesized for the implicit deduction guide, making the implicit deduction guide ambiguous with the user-provided one. The example ought to be valid.

Possible resolution:

Change in 12.2.2.9 [over.match.class.deduct] bullet 1.1 as follows:

> - If `C` is defined, for each constructor of `C`, a function template with the following properties:
>   - The template parameters are the template parameters of `C` followed by the template parameters (including default template arguments) of the constructor, if any.
>   - The associated constraints (13.5.3 [temp.constr.decl]) are the conjunction of the associated constraints of `C` and the associated constraints of the constructor.
>   - The types of the function parameters are those of the constructor.
>   - The return type is the class template specialization designated by `C` and template arguments corresponding to the template parameters of `C`.

#### 2680. Class template argument deduction for aggregates with designated initializers

Section: 12.2.2.9 [over.match.class.deduct] Status: open Submitter: Christof Meerwald Date: 2023-01-09

Consider:

       template<typename T>
       struct X
       {
        int i{};
        T t;
       };

       X x{ .t = X<int>{ 1, 2 } }; // should deduce X<X<int>> (not X<int>)

and

       template<typename T>
       struct Y
       {
        int i;
        T t;
        int j;
       };

       template<typename T>
       Y(int, T) -> Y<T>;

       Y y{ .t = 'c', .j = 2 }; // should be ill-formed

Class template argument deduction does not limit deduction to the aggregate deduction candidate when a designated initializer list is used.

Suggested resolution:

1. Change in 12.2.2.9 [over.match.class.deduct] paragraph 1 as follows:

   > ... If the initializer has a designated-initializer-list, any guide that is not an aggregate deduction candidate is not viable; for an aggregate deduction candidate, the designated-initializer-list is replaced by an initializer-list with the same elements. In addition, if C is defined and inherits constructors (9.9 [namespace.udecl]) from a direct base class ...

2. Change in 12.2.2.9 [over.match.class.deduct] bullet 3.5 as follows:

   > - ...
   > - If f was generated from a deduction-guide (13.7.2.3 [temp.deduct.guide]), then f' is considered to be so as well.
   > - If f was generated from an aggregate deduction guide, then f' is considered to be so as well.

#### 2679. Implicit conversion sequence with a null pointer constant

Section: 12.2.4.2.1 [over.best.ics.general] Status: open Submitter: Lénárd Szolnoki Date: 2023-01-07

Subclause 12.2.4.2.1 [over.best.ics.general] paragraph 1, as modified by issue 2525, claims that only the type, cv-qualification, and value category of the argument are relevant in forming an implicit conversion sequence. This is inaccurate for the following situations:

- A literal `0` can be implicitly converted to a parameter of type `void*` (and in practice, an implicit conversion sequence does exist), yet the literal `0` has the same type, cv-qualification, and value category as the literal `1` despite no such implicit conversion being allowed for the latter.
- An initializer list does not have a type, cv-qualification, or value category at all.
- An overload set does not have a single (function) type.

#### 2169. Narrowing conversions and overload resolution

Section: 12.2.4.2.6 [over.ics.list] Status: open Submitter: David Krauss Date: 2015-08-26

Current implementations ignore narrowing conversions during overload resolution, emitting a diagnostic if calling the selected function would involve narrowing. For example:

      struct s { long m };
      struct ss { short m; };

      void f( ss );
      void f( s );
      void g() {
        f({ 1000000 }); // Ambiguous in spite of narrowing for f(ss)
      }

However, the current wording of 12.2.4.2.6 [over.ics.list] paragraph 7 says,

> Otherwise, if the parameter has an aggregate type which can be initialized from the initializer list according to the rules for aggregate initialization (9.4.2 [dcl.init.aggr]), the implicit conversion sequence is a user-defined conversion sequence with the second standard conversion sequence an identity conversion.

In the example above, `ss` cannot be initialized from `{ 1000000 }` because of the narrowing conversion, so presumably `f(ss)` should not be considered. If this is not the intended outcome, paragraph 7 should be restated in terms of having an implicit conversion sequence, as in, e.g., bullet 9.1, instead of a valid initialization.

Rationale (March, 2016):

This is a question of language design and thus more suited to consideration by EWG.

EWG (January, 2021):

Adjust the standard to follow existing implementations. See vote.

Additional notes (March, 2023)

A narrowing conversion is but one of the many circumstances that could make a conversion ill-formed despite the existence of an implicit conversion sequence. Other examples (with implementation divergence) are invoking a private or deleted function, binding a non-const reference to a bit-field, invoking an explicit constructor in list-initialization, invoking a consteval constructor with unsuitable arguments, or initializing an aggregate with incorrectly-ordered designated initializers. A resolution of this issue should thus not be focused on narrow conversions.

#### 1459. Reference-binding tiebreakers in overload resolution

Section: 12.2.4.3 [over.ics.rank] Status: open Submitter: Jason Merrill Date: 2012-02-07

Both paragraph 3 and paragraph 4 of 12.2.4.3 [over.ics.rank] have overload resolution tiebreakers for reference binding. It might be possible to merge those into a single treatment.

#### 1789. Array reference vs array decay in overload resolution

Section: 12.2.4.3 [over.ics.rank] Status: open Submitter: Faisal Vali Date: 2013-10-01

The current rules make an example like

      template<class T, size_t N> void foo(T (&)[N]);
      template<class T> void foo(T *t);

      int arr[3]{1, 2, 3};
      foo(arr);

ambiguous, even though the first is an identity match and the second requires an lvalue transformation. Is this desirable?

Proposed resolution (June, 2021):

Add the following as a new bullet following 12.2.4.3 [over.ics.rank] bullet 3.2.6:

> Two implicit conversion sequences of the same form are indistinguishable conversion sequences unless one of the following rules applies:

> - List-initialization sequence `L1` is a better conversion sequence...

> - Standard conversion sequence `S1` is a better conversion sequence than standard conversion sequence `S2` if

> - ...

> - `S1` and `S2` include reference bindings (9.4.4 [dcl.init.ref]), and the types to which the references refer are the same type except for top-level cv-qualifiers, and the type to which the reference initialized by `S2` refers is more cv-qualified than the type to which the reference initialized by `S1` refers. [Example 6:...

> - `S1` is a reference binding to an array and `S2` is an array-to-pointer conversion (7.3.3 [conv.array]). [Example 7:

>         template<class T, unsigned N> void f(T (&)[N]); // #1
>         template<class T> void f(T *t);                 // #2
>         int r[3]{1, 2, 3};
>         void g() {
>           f(r);    // OK: calls #1
>         }

>   —end example]

CWG 2023-02-06

The proposed resolution prefers the `T&` overload over the `T*` overload in the example below, which is undesirable. The wording needs to be amended to limit the tiebreaker to situations where the array declarator appears in the signature.

>     template<class T>
>     int f(T&);
>     template<class T>
>     int f(T*);
>     int x[5];
>     int z = f(x);

#### 2337. Incorrect implication of logic ladder for conversion sequence tiebreakers

Section: 12.2.4.3 [over.ics.rank] Status: open Submitter: Richard Smith Date: 2017-03-02

The bulleted list of 12.2.4.3 [over.ics.rank] paragraph 3 consists of a logic ladder of the form “A is better than B if [some predicate relating A to B], or, if not that, ...” For example, bullet 3.1 says,

- List-initialization sequence `L1` is a better conversion sequence than list-initialization sequence `L2` if

- `L1` converts to `std::initializer_list<X>` for some `X` and `L2` does not, or, if not that,

- `L1` converts to type “array of `N1 T`”, `L2` converts to type “array of `N2 T`”, and `N1` is smaller than `N2` ,

The intent is not to fall into the array case if `L2` converts to `std::initializer_list<X>` and `L1` does not — i.e., the inverse predicate holds — but that intent is not well reflected in the actual wording.

#### 2572. Address of overloaded function with no target

Section: 12.3 [over.over] Status: open Submitter: Jason Merrill Date: 2022-04-26

Consider:

      template <class T> T f(T);   // #1
      template <class T> T* f(T*); // #2
      auto p = &f<int>;

Accoring to 12.3 [over.over] paragraph 3 and 12.3 [over.over] paragraph 5:

> The specialization, if any, generated by template argument deduction (13.10.4 [temp.over], 13.10.3.3 [temp.deduct.funcaddr], 13.10.2 [temp.arg.explicit]) for each function template named is added to the set of selected functions considered.

> [...]

> Any given function template specialization F1 is eliminated if the set contains a second function template specialization whose function template is more specialized than the function template of F1 according to the partial ordering rules of 13.7.7.3 [temp.func.order]. After such eliminations, if any, there shall remain exactly one selected function.

Major implementations reject the example as ambiguous, yet the wording specifies to unambiguously choose #2.

Suggested resolution:

Change in 12.3 [over.over] paragraph 5 as follows:

> Any given function template specialization F1 is eliminated if the set contains a second function template specialization whose function template is more specialized better than the function template of F1. If there is no target, a function template is better than another if it is more constrained than the other; otherwise a function template is better than another if it is more specialized than the other according to the partial ordering rules of 13.7.7.3 [temp.func.order]. After such eliminations, if any, there shall remain exactly one selected function.

#### 1549. Overloaded comma operator with `void` operand

Section: 12.4.3 [over.binary] Status: open Submitter: Nikolay Ivchenkov Date: 2012-09-04

Even though a function cannot take a parameter of type `void`, the current rules for overload resolution require consideration of overloaded operators when one operand has a user-defined or enumeration type and the other has type `void`. This can result in side effects and possibly errors, for example:

      template <class T> struct A {
        T t;
        typedef T type;
      };

      struct X {
        typedef A<void> type;
      };

      template <class T> void operator ,(typename T::type::type, T) {}

      int main() {
        X(), void(); // OK
        void(), X(); // error: A<void> is instantiated with a field of
                     // type void
      }

#### 260. User-defined conversions and built-in `operator=`

Section: 12.5 [over.built] Status: open Submitter: Scott Douglas Date: 4 Nov 2000

According to the Standard (although not implemented this way in most implementations), the following code exhibits non-intuitive behavior:

      struct T {
        operator short() const;
        operator int() const;
      };

      short s;

      void f(const T& t) {
        s = t;  // surprisingly calls T::operator int() const
      }

The reason for this choice is 12.5 [over.built] paragraph 18:

> For every triple (L, VQ, R), where L is an arithmetic type, VQ is either `volatile` or empty, and R is a promoted arithmetic type, there exist candidate operator functions of the form

> VQ L`& operator=(`VQ L`&,` R`);`

Because R is a "promoted arithmetic type," the second argument to the built-in assignment operator is `int`, causing the unexpected choice of conversion function.

Suggested resolution: Provide built-in assignment operators for the unpromoted arithmetic types.

Related to the preceding, but not resolved by the suggested resolution, is the following problem. Given:

        struct T {
    	 operator int() const;
    	 operator double() const;
        };

I believe the standard requires the following assignment to be ambiguous (even though I expect that would surprise the user):

        double x;
        void f(const T& t) { x = t; }

The problem is that both of these built-in `operator=()`s exist (12.5 [over.built] paragraph 18):

        double& operator=(double&, int);
        double& operator=(double&, double);

Both are an exact match on the first argument and a user conversion on the second. There is no rule that says one is a better match than the other.

The compilers that I have tried (even in their strictest setting) do not give a peep. I think they are not following the standard. They pick `double& operator=(double&, double)` and use `T::operator double() const`.

I hesitate to suggest changes to overload resolution, but a possible resolution might be to introduce a rule that, for built-in `operator=` only, also considers the conversion sequence from the second to the first type. This would also resolve the earlier question.

It would still leave `x += t` etc. ambiguous -- which might be the desired behavior and is the current behavior of some compilers.

Notes from the 04/01 meeting:

The difference between initialization and assignment is disturbing. On the other hand, promotion is ubiquitous in the language, and this is the beginning of a very slippery slope (as the second report above demonstrates).

Additional note (August, 2010):

See issue 507 for a similar example involving comparison operators.

#### 954. Overload resolution of conversion operator templates with built-in types

Section: 12.5 [over.built] Status: open Submitter: Steve Clamage Date: 19 August, 2009

Consider the following example:

        struct NullClass {
            template<typename T> operator T () { return 0 ; }
        };

        int main() {
            NullClass n;
            n==5;        // #1
            return 0;
        }

The comparison at #1 is, according to the current Standard, ambiguous. According to 12.5 [over.built] paragraph 12, the candidates for `operator==(L, R)` include functions “for every pair of promoted arithmetic types,” so `L` could be either `int` or `long`, and the conversion operator template will provide an exact match for either.

Some implementations unambiguously choose the `int` candidate. Perhaps the overload resolution rules could be tweaked to prefer candidates in which `L` and `R` are the same type?

(See also issue 545.)

#### 1620. User-defined literals and extended integer types

Section: 12.6 [over.literal] Status: open Submitter: Jason Merrill Date: 2013-02-12

Although numeric literals can have extended integer types, user-defined literal operators cannot have a parameter of an extended integer type. This seems like an oversight.

#### 2617. Default template arguments for template members of non-template classes

Section: 13.2 [temp.param] Status: open Submitter: Mike Miller Date: 2022-08-22

Consider:

    struct S {
      template<typename> void f();
    };

    template<typename = int> void S::f() { }   // ok?

There is implementation divergence in the treatment of this example. The relevant wording appears to be 13.2 [temp.param] paragraph 12:

> A default template-argument shall not be specified in the template-parameter-lists of the definition of a member of a class template that appears outside of the member's class.

However, the example above deals with a member of an ordinary class, not a class template, but it is not clear why there should be a difference between a member template of a class template and a member template of a non-template class.

Alternatively, it is not clear why the example above should be treated differently from a non-member function template, e.g.,

    template<typename> void f();
    template<typename = int> void f() { }

which is explicitly permitted.

#### 579. What is a “nested” `>` or `>>`?

Section: 13.3 [temp.names] Status: open Submitter: Daveed Vandevoorde Date: 11 May 2006

The Standard does not normatively define which `>` and `>>` tokens are to be taken as closing a template-argument-list; instead, 13.3 [temp.names] paragraph 3 uses the undefined and imprecise term “non-nested:”

> When parsing a template-id, the first non-nested `>` is taken as the end of the template-argument-list rather than a greater-than operator. Similarly, the first non-nested `>>` is treated as two consecutive but distinct `>` tokens, the first of which is taken as the end of the template-argument-list and completes the template-id.

The (non-normative) footnote clarifies that

> A `>` that encloses the type-id of a `dynamic_cast`, `static_cast`, `reinterpret_cast` or `const_cast`, or which encloses the template-arguments of a subsequent template-id, is considered nested for the purpose of this description.

Aside from the questionable wording of this footnote (e.g., in what sense does a single terminating character “enclose” anything, and is a nested template-id “subsequent?”) and the fact that it is non-normative, it does not provide a complete definition of what “nesting” is intended to mean. For example, is the first `>` in this putative template-id “nested” or not?

        X<a ? b > c : d>

Additional note (January, 2014):

A similar problem exists for an `operator>` template:

      struct S;
      template<void (*)(S, S)> struct X {};
      void operator>(S, S);
      X<operator> > x;

Somehow the specification must be written to avoid taking the `>` token in the operator name as the end of the template argument list for `X`.

#### 440. Allow implicit pointer-to-member conversion on nontype template argument

Section: 13.4 [temp.arg] Status: open Submitter: David Abrahams Date: 13 Nov 2003

None of my compilers accept this, which surprised me a little. Is the base-to-derived member function conversion considered to be a runtime-only thing?

      template <class D>
      struct B
      {
          template <class X> void f(X) {}
          template <class X, void (D::*)(X) = &B<D>::f<X> >
          struct row {};
      };
      struct D : B<D>
      {
          void g(int);
          row<int,&D::g> r1;
          row<char*> r2;
      };

John Spicer: This is not among the permitted conversions listed in 14.3.

I'm not sure there is a terribly good reason for that. Some of the template argument rules for external entities were made conservatively because of concerns about issues of mangling template argument names.

David Abrahams: I'd really like to see that restriction loosened. It is a serious inconvenience because there appears to be no way to supply a usable default in this case. Zero would be an OK default if I could use the function pointer's equality to zero as a compile-time switch to choose an empty function implementation:

      template <bool x> struct tag {};

      template <class D>
      struct B
      {
          template <class X> void f(X) {}

          template <class X, void (D::*pmf)(X) = 0 >
          struct row {
              void h() { h(tag<(pmf == 0)>(), pmf); }
              void h(tag<1>, ...) {}
              void h(tag<0>, void (D::*q)(X)) { /*something*/}
          };
      };

      struct D : B<D>
      {
          void g(int);
          row<int,&D::g> r1;
          row<char*> r2;
      };

But there appears to be no way to get that effect either. The result is that you end up doing something like:

          template <class X, void (D::*pmf)(X) = 0 >
          struct row {
              void h() { if (pmf) /*something*/ }
          };

which invariably makes compilers warn that you're switching on a constant expression.

Additional notes (February, 2023)

Even the following simple case fails on all major implementations:

      struct B { void f(); };
      struct D : B {};

      template<void (D::*)()>
      int g();

      int x = f<&D::g>();

Subclause 7.7 [expr.const] paragraph 11 is unambiguous that the code is ill-formed, but that seems unfortunate.

#### 2105. When do the arguments for a parameter pack end?

Section: 13.4 [temp.arg] Status: open Submitter: Hubert Tong Date: 2015-03-17

There does not appear to be a clear statement in the Standard that the first template parameter pack in a template parameter list corresponds to all remaining arguments in the template argument list. For example:

      template <int> struct A;

      template <int ...N, typename T> void foo(A<N> *..., T);
      void bar() {
       foo<0>(0, 0);      // okay: N consists of one template parameter, 0. T is deduced to int
       foo<0, int>(0, 0); // error: int does not match the form of the corresponding parameter N
      }

See also issue 2055.

Notes from the February, 2016 meeting:

The comments in the example reflect the intent.

#### 2686. Pack expansion into a non-pack parameter of a concept

Section: 13.5.2 [temp.constr.constr] Status: open Submitter: Michał Dominiak Date: 2021-10-15

Consider:

      template<typename T>  concept A = true;
      template<typename... T> concept B = A<T...>;

Is this valid? MSVC accepts; gcc and clang reject. See also issue 1430 for the equivalent question for alias templates.

#### 2589. Context of access checks during constraint satisfaction checking

Section: 13.5.2.3 [temp.constr.atomic] Status: open Submitter: Jason Merrill Date: 2019-10-02

Consider:

      template<class T> concept ctible = requires { T(); };

      class A {
        template <class T> friend struct B;
        A();
      };

      template <class T> struct B;
      template <ctible T> struct B<T> { T t; };
      B<A> b;  // #1

      template <class T> struct C { };
      template <ctible T> struct C<T> { T t; };
      C<A> c;  // #2

Should the context of instantiation be considered for satisfaction checking? If satisfaction checking were always performed in an unrelated context, neither partial specialization is used, and #1 would be ill-formed (because `B` is incomplete), but #2 would be well-formed. If the satisfaction checking were performed in the context of the constrained declaration, #1 would be well-formed and #2 would be ill-formed, no diagnostic required, because the validity of `A()` is different in that context. That rule, however, could also consider the context, in which case #2 would also be well-formed.

The decision affects the amount of caching that an implementation can perform.

Subclause 13.5.2.3 [temp.constr.atomic] paragraph 3 should be clarified one way or another:

> To determine if an atomic constraint is satisfied, the parameter mapping and template arguments are first substituted into its expression. If substitution results in an invalid type or expression, the constraint is not satisfied. Otherwise, the lvalue-to-rvalue conversion (7.3.2 [conv.lval]) is performed if necessary, and E shall be a constant expression of type bool. The constraint is satisfied if and only if evaluation of E results in true. If, at different points in the program, the satisfaction result is different for identical atomic constraints and template arguments, the program is ill-formed, no diagnostic required.

#### 2697. Deduction guides using abbreviated function syntax

Section: 13.7.2.3 [temp.deduct.guide] Status: open Submitter: CWG Date: 2023-02-11 Liaison: EWG

It is unclear whether deduction guides can be expressed using abbreviated function syntax. Subclause 13.7.2.3 [temp.deduct.guide] paragraph 3 refers to the restrictions of a function's parameter-declaration-clause:

> The same restrictions apply to the parameter-declaration-clause of a deduction guide as in a function declaration (9.3.4.6 [dcl.fct]). ...

However, that subclause is silent on the meaning of abbreviated function syntax when used for deduction guides. Furthermore, 9.3.4.6 [dcl.fct] paragraph 22 explicitly restricts the definition to function templates, which deduction guides are not:

> An abbreviated function template is a function declaration that has one or more generic parameter type placeholders (9.2.9.6 [dcl.spec.auto]). ...

Arguably, the lack of template parameter names in abbreviated function syntax makes it less suitable to specifiy deduction guides.

CWG 2023-02-11

CWG solicits input from EWG whether abbreviated function syntax is intended to be used for deduction guides. See cplusplus/papers#1465.

#### 2707. Deduction guides cannot have a trailing requires-clause

Section: 13.7.2.3 [temp.deduct.guide] Status: open Submitter: Richard Smith Date: 2020-02-26

The grammar for deduction-guide does not, but should, allow a trailing requires-clause:

>     deduction-guide:
>        explicit-specifieropt template-name ( parameter-declaration-clause ) -> simple-template-id ;

Suggested resolution:

Change the grammar in 13.7.2.3 [temp.deduct.guide] paragraph 1 as follows:

>     deduction-guide:
>        explicit-specifieropt template-name ( parameter-declaration-clause ) requires-clauseopt -> simple-template-id ;

#### 1918. `friend` templates with dependent scopes

Section: 13.7.5 [temp.friend] Status: open Submitter: Richard Smith Date: 2014-04-27

It is not clear what should happen for an example like:

      template<typename T> struct A {
        class B {
          class C {};
        };
      };
      class X {
        static int x;
        template <typename T> friend class A<T>::B::C;
      };
      template<> struct A<int> {
        typedef struct Q B;
      };
      struct Q {
        class C {
          int f() { return X::x; }
        };
      };

It appears that the friend template matches `Q::C`, because that class is also `A<int>::B::C`, but neither GCC nor EDG allow this code (saying `X::x` is inaccessible). (Clang doesn't support friend template declarations with a dependent scope.)

A strict reading of 13.7.5 [temp.friend] paragraph 5 might suggest that the friend declaration itself is ill-formed, because it does not declare a member of a class template, but I can't find any compiler that implements template friends that way.

#### 1945. Friend declarations naming members of class templates in non-templates

Section: 13.7.5 [temp.friend] Status: open Submitter: Richard Smith Date: 2014-06-19

During the discussion of issue 1918, it was decided that the last part of the issue should be split off into a separate issue. According to 13.7.5 [temp.friend] paragraph 5,

> A member of a class template may be declared to be a friend of a non-template class.

Does this make the example from issue 1918,

      template<typename T> struct A {
        class B {
          class C {};
        };
      };
      class X {
        static int x;
        template <typename T> friend class A<T>::B::C;
      };
      template<> struct A<int> {
        typedef struct Q B;
      };
      struct Q {
        class C {
          int f() { return X::x; }
        };
      };

ill-formed because the friend declaration does not refer to a member of a class template? This does not appear to be the interpretation chosen by most implementations.

#### 2118. Stateful metaprogramming via friend injection

Section: 13.7.5 [temp.friend] Status: open Submitter: Richard Smith Date: 2015-04-27

Defining a friend function in a template, then referencing that function later provides a means of capturing and retrieving metaprogramming state. This technique is arcane and should be made ill-formed.

Notes from the May, 2015 meeting:

CWG agreed that such techniques should be ill-formed, although the mechanism for prohibiting them is as yet undetermined.

#### 708. Partial specialization of member templates of class templates

Section: 13.7.6 [temp.spec.partial] Status: open Submitter: James Widman Date: 8 Aug, 2008

The Standard does not appear to specify clearly the effect of a partial specialization of a member template of a class template. For example:

        template<class T> struct B {
             template<class U> struct A { // #1
                 void h() {}
             };
             template<class U> struct A<U*> {  // #2
                 void f() {}
             };
        };

        template<> template<class U> struct B<int>::A { // #3
             void g() {}
        };

        void q(B<int>::A<char*>& p) {
             p.f();  // #4
        }

The explicit specialization at #3 replaces the primary member template #1 of `B<int>`; however, it is not clear whether the partial specialization #2 should be considered to apply to the explicitly-specialized member template of `A<int>` (thus allowing the call to `p.f()` at #4) or whether the partial specialization will be used only for specializations of `B` that are implicitly instantiated (meaning that #4 could call `p.g()` but not `p.f()`).

#### 2173. Partial specialization with non-deduced contexts

Section: 13.7.6 [temp.spec.partial] Status: open Submitter: Mike Miller Date: 2015-09-14

During the discussion of issue 1315, it was observed that the example

      template <int I, int J> struct B {};
      template <int I> struct B<I, I*2> {};

is ill-formed because the deduction succeeds in both directions. This seems surprising. It was suggested that perhaps a non-deduced context should be considered more specialized than a deduced context.

#### 310. Can function templates differing only in parameter cv-qualifiers be overloaded?

Section: 13.7.7.2 [temp.over.link] Status: open Submitter: Andrei Iltchenko Date: 29 Aug 2001

I get the following error diagnostic [from the EDG front end]:

    line 8: error: function template "example<T>::foo<R,A>(A)" has
              already been declared
         R  foo(const A);
            ^

when compiling this piece of code:

    struct  example  {
       template<class R, class A>   // 1-st member template
       R  foo(A);
       template<class R, class A>   // 2-nd member template
       const R  foo(A&);
       template<class R, class A>   // 3-d  member template
       R  foo(const A);
    };

    /*template<> template<>
    int  example<char>::foo(int&);*/

    int  main()
    {
       int  (example<char>::* pf)(int&) =
          &example<char>::foo;
    }

The implementation complains that

       template<class R, class A>   // 1-st member template
       R  foo(A);
       template<class R, class A>   // 3-d  member template
       R  foo(const A);

cannot be overloaded and I don't see any reason for it since it is function template specializations that are treated like ordinary non-template functions, meaning that the transformation of a parameter-declaration-clause into the corresponding parameter-type-list is applied to specializations (when determining its type) and not to function templates.

What makes me think so is the contents of 13.7.7.2 [temp.over.link] and the following sentence from 13.10.3.2 [temp.deduct.call] "If P is a cv-qualified type, the top level cv-qualifiers of P are ignored for type deduction". If the transformation was to be applied to function templates, then there would be no reason for having that sentence in 13.10.3.2 [temp.deduct.call].

13.10.3.3 [temp.deduct.funcaddr], which my example is based upon, says nothing about ignoring the top level cv-qualifiers of the function parameters of the function template whose address is being taken.

As a result, I expect that template argument deduction will fail for the 2-nd and 3-d member templates and the 1-st one will be used for the instantiation of the specialization.

#### 2584. Equivalent types in function template declarations

Section: 13.7.7.2 [temp.over.link] Status: open Submitter: Jim X Date: 2022-04-08

According to 6.4.1 [basic.scope.scope] paragraph 4:

> Two declarations correspond if they (re)introduce the same name, both declare constructors, or both declare destructors, unless
> - ...
> - each declares a function or function template, except when
>   - ...
>   - both declare function templates with equivalent non-object-parameter-type-lists, return types (if any), template-heads, and trailing requires-clauses (if any), and, if both are non-static members, they have corresponding object parameters.

Assuming that two non-object-parameter-type-lists are equivalent if they have the same length and corresponding types are equivalent, the question remains when two (possibly dependent) types are equivalent. Subclause 13.7.7.2 [temp.over.link] should provide an answer, but only covers expressions appearing in such types (paragraph 5):

> Two expressions involving template parameters are considered equivalent if...

For example, the standard should specify whether these declarations correspond:

      template<class T> T   f();
      template<class T> T&& f();

      template<class T, class U> void g(decltype(T::foo));
      template<class T, class U> void g(decltype(U::foo));

A related issue is the determination whether two names are the same; for example:

      struct A {
        template<class T>
        operator T();

        template<class T>
        operator T&&();
      };

The latter issue could probably be fixed by amending 11.4.8.3 [class.conv.fct] to state that two conversion-function-ids are the same if their conversion-type-ids denote equivalent types, with a cross-reference to 13.7.7.2 [temp.over.link].

#### 402. More on partial ordering of function templates

Section: 13.7.7.3 [temp.func.order] Status: open Submitter: Nathan Sidwell Date: 7 Apr 2003

This was split off from issue 214 at the April 2003 meeting.

Nathan Sidwell: John Spicer's proposed resolution does not make the following well-formed.

      template <typename T> int Foo (T const *) {return 1;} //#1
      template <unsigned I> int Foo (char const (&)[I]) {return 2;} //#2

      int main ()
      {
        return Foo ("a") != 2;
      }

Both #1 and #2 can deduce the "a" argument, #1 deduces T as char and #2 deduces I as 2. However, neither is more specialized because the proposed rules do not have any array to pointer decay.

#1 is only deduceable because of the rules in 13.10.3.2 [temp.deduct.call] paragraph 2 that decay array and function type arguments when the template parameter is not a reference. Given that such behaviour happens in deduction, I believe there should be equivalent behaviour during partial ordering. #2 should be resolved as more specialized as #1. The following alteration to the proposed resolution of DR214 will do that.

Insert before,

- If A is a cv-qualified type, A is replaced by the cv-unqualified version of A.

the following

- If P was not originally a reference type,
  - If A is an array type, A is replaced by the pointer type produced by the array to pointer conversion
  - If A is a function type, A is replaced by the pointer type produced by the function to pointer conversion

For the example above, this change results in deducing 'T const *' against 'char const *' in one direction (which succeeds), and 'char [I]' against 'T const *' in the other (which fails).

John Spicer: I don't consider this a shortcoming of my proposed wording, as I don't think this is part of the current rules. In other words, the resolution of 214 might make it clearer how this case is handled (i.e., clearer that it is not allowed), but I don't believe it represents a change in the language.

I'm not necessarily opposed to such a change, but I think it should be reviewed by the core group as a related change and not a defect in the proposed resolution to 214.

Notes from the October 2003 meeting:

There was some sentiment that it would be desirable to have this case ordered, but we don't think it's worth spending the time to work on it now. If we look at some larger partial ordering changes at some point, we will consider this again.

#### 1157. Partial ordering of function templates is still underspecified

Section: 13.7.7.3 [temp.func.order] Status: open Submitter: CA Date: 2010-08-03
N3092 comment CA 7

13.7.7.3 [temp.func.order] paragraph 3 says,

> To produce the transformed template, for each type, non-type, or template template parameter (including template parameter packs (13.7.4 [temp.variadic]) thereof) synthesize a unique type, value, or class template respectively and substitute it for each occurrence of that parameter in the function type of the template.

The characteristics of the synthesized entities and how they are determined is not specified. For example, members of a dependent type referred to in non-deduced contexts are not specified to exist, even though the transformed function type would be invalid in their absence.

Example 1:

      template<typename T, typename U> struct A;
      template<typename T> void foo(A<T, typename T::u> *) { } // #1
        // synthetic T1 has member T1::u
      template <typename T> void foo(A<T, typename T::u::v> *) { } // #2
        // synthetic T2 has member T2::u and member T2::u::v
        // T in #1 deduces to synthetic T2 in partial ordering;
        // deduced A for the parameter is A<T2, T2::u> * --this is not necessarily compatible
        // with A<T2, T2::u::v> * and it does not need to be. See Note 1. The effect is that
        // (in the call below) the compatibility of B::u and B::u::v is respected.
        // T in #2 cannot be successfully deduced in partial ordering from A<T1, T1::u> *;
        // invalid type T1::u::v will be formed when T1 is substituted into non-deduced contexts.
      struct B {
        struct u { typedef u v; };
      };
      int main() {
        foo((A<B, B::u> *)0); // calls #2
      }

Note 1: Template argument deduction is an attempt to match a `P` and a deduced `A`; however, template argument deduction is not specified to fail if the `P` and the deduced `A` are incompatible. This may occur in the presence of non-deduced contexts. Notwithstanding the parenthetical statement in 13.10.3.5 [temp.deduct.partial] paragraph 9, template argument deduction may succeed in determining a template argument for every template parameter while producing a deduced `A` that is not compatible with the corresponding `P`.

Example 2:

      template <typename T, typename U, typename V> struct A;
      template <typename T>
        void foo(A<T, struct T::u, struct T::u::u> *); // #2.1
          // synthetic T1 has member non-union class T1::u
      template <typename T, typename U>
        void foo(A<T, U , U> *); // #2.2
          // synthetic T2 and U2 has no required properties
          // T in #2.1 cannot be deduced in partial ordering from A<T2, U2, U2> *;
          // invalid types T2::u and T2::u::u will be formed when T2 is substituted in nondeduced contexts.
          // T and U in #2.2 deduces to, respectively, T1 and T1::u from A<T1, T1::u, struct
    T1::u::u> * unless
          // struct T1::u::u does not refer to the injected-class-name of the class T1::u (if that is possible).
      struct B {
        struct u { };
      };
      int main() {
        foo((A<B, B::u, struct B::u::u> *)0); // calls #2.1
      }

It is, however, unclear to what extent an implementation will have to go to determine these minimal properties.

#### 2160. Issues with partial ordering

Section: 13.7.7.3 [temp.func.order] Status: open Submitter: Richard Smith Date: 2015-07-16

(From this editorial issue.)

Consistency of deduced values

      template <typename T> void foo(T, T); // (1)
      template <typename T, typename U> void foo(T, U); // (2)

13.10.3.6 [temp.deduct.type] paragraph 2 makes it clear that there must be exactly one set of deduced values for the `P`s. But there is no such statement in the partial ordering rule. The algorithm described only does pairwise `P`/`A` matching, so a synthesized call from (2) to (1) via `foo(U{}, V{})` could succeed in deduction. Both gcc and clang agree that (1) is more specialized.

Type Synthesis Template Instantiation

      template <typename T>
      struct identity { using type = T; };

      template<typename T> void bar(T, T ); // (1)
      template<typename T> void bar(T, typename identity<T>::type ); // (2)

Here, if synthesized for (2) `Unique2` and `typename identity<Unique2>::type == Unique2` , then type deduction would succeed in both directions and the call `bar(0,0)` would be ambiguous. However, it seems that both compilers instead simply treat `typename identity<Unique2>::type` as `Unique2_b`, thus making template deduction from (2) to (1) fail (based on the implied missing Consistency rule).

Non-deduced Context Omission

This is the same as the previous example, except now define

      template <typename T> struct identity;
      template <> struct identity<int> { using type = int; };

With no template instantiation during synthesis and consistency, the (2) ==> (1) deduction fails. But if we consider the (1) ==> (2) call, we'd match `T` against `Unique1` and then have the non-deduced context `typename identity<Unique1>::type` to match against `Unique1`, but that would be a substitution failure. It seems that the approach taken by gcc and clang (both of which prefer (1) here) is to ignore the non-deduced context argument, as long as that parameter type is deduced from a different template parameter type that did get matched.

Notes from the February, 2016 meeting:

None of these examples appears to reflect a defect in the current wording; in particular, the second and third examples involve a dependent type and there could be a later specialization of `identity`, so it's impossible to reason about those cases in the template definition context. The issue will be left open to allow for possible clarification of the intent of the wording.

#### 1430. Pack expansion into fixed alias template parameter list

Section: 13.7.8 [temp.alias] Status: open Submitter: Jason Merrill Date: 2011-12-13

Originally, a pack expansion could not expand into a fixed-length template parameter list, but this was changed in N2555. This works fine for most templates, but causes issues with alias templates.

In most cases, an alias template is transparent; when it's used in a template we can just substitute in the dependent template arguments. But this doesn't work if the template-id uses a pack expansion for non-variadic parameters. For example:

        template<class T, class U, class V>
        struct S {};

        template<class T, class V>
        using A = S<T, int, V>;

        template<class... Ts>
        void foo(A<Ts...>);

There is no way to express `A<Ts...>` in terms of `S`, so we need to hold onto the `A` until we have the `Ts` to substitute in, and therefore it needs to be handled in mangling.

Currently, EDG and Clang reject this testcase, complaining about too few template arguments for A. G++ did as well, but I thought that was a bug. However, on the ABI list John Spicer argued that it should be rejected.

(See also issue 1558.)

Notes from the October, 2012 meeting:

The consensus of CWG was that this usage should be prohibited, disallowing use of an alias template when a dependent argument can't simply be substituted directly into the type-id.

Additional note, April, 2013:

For another example, consider:

      template<class... x> class list{};
      template<class a, class... b> using tail=list<b...>;
      template <class...T> void f(tail<T...>);

      int main() {
        f<int,int>({});
      }

There is implementation variance in the handling of this example.

CWG 2022-11-11

There is no more implementation divergence; all known implementations reject the example.

#### 1257. Instantiation via non-dependent references in uninstantiated templates

Section: 13.8 [temp.res] Status: open Submitter: Johannes Schaub Date: 2011-03-09

The Standard does not appear to specify whether a non-dependent reference to a template specialization in a template definition that is never instantiated causes the implicit instantiation of the referenced specialization.

#### 2067. Generated variadic templates requiring empty pack

Section: 13.8 [temp.res] Status: open Submitter: Richard Smith Date: 2015-01-09

According to 13.8 [temp.res] paragraph 8,

> If every valid specialization of a variadic template requires an empty template parameter pack, the template is ill-formed, no diagnostic required.

I'm inclined to think that this rule should only apply to code the user wrote. That is, if every valid instantiation of an entity (that was not itself instantiated) requires at least one of the enclosing template argument lists to include an empty template argument pack, then the program is ill-formed (no diagnostic required).

#### 186. Name hiding and template template-parameters

Section: 13.8.2 [temp.local] Status: open Submitter: John Spicer Date: 11 Nov 1999

The standard prohibits a class template from having the same name as one of its template parameters (13.8.2 [temp.local] paragraph 4) . This prohibits

        template <class X> class X;

for the reason that the template name would hide the parameter, and such hiding is in general prohibited.

Presumably, we should also prohibit

        template <template <class T> class T> struct A;

for the same reason.

#### 459. Hiding of template parameters by base class members

Section: 13.8.2 [temp.local] Status: open Submitter: Daveed Vandevoorde Date: 2 Feb 2004

Currently, member of nondependent base classes hide references to template parameters in the definition of a derived class template.

Consider the following example:

       class B {
          typedef void *It;    // (1)
          // ...
        };

        class M: B {};

        template<typename> X {};

        template<typename It> struct S   // (2)
            : M, X<It> {   // (3)
          S(It, It);   // (4)
          // ...
        };

As the C++ language currently stands, the name "It" in line (3) refers to the template parameter declared in line (2), but the name "It" in line (4) refers to the typedef in the private base class (declared in line (1)).

This situation is both unintuitive and a hindrance to sound software engineering. (See also the Usenet discussion at http://tinyurl.com/32q8d .) Among other things, it implies that the private section of a base class may change the meaning of the derived class, and (unlike other cases where such things happen) there is no way for the writer of the derived class to defend the code against such intrusion (e.g., by using a qualified name).

Changing this can break code that is valid today. However, such code would have to:

1. name a template parameter and not use it after the opening brace, and
2. use that same name to access a base-class name within the braces.

I personally have no qualms breaking such a program.

It has been suggested to make situations like these ill-formed. That solution is unattractive however because it still leaves the writer of a derived class template without defense against accidental name conflicts with base members. (Although at least the problem would be guaranteed to be caught at compile time.) Instead, since just about everyone's intuition agrees, I would like to see the rules changed to make class template parameters hide members of the same name in a base class.

See also issue 458.

Notes from the March 2004 meeting:

We have some sympathy for a change, but the current rules fall straightforwardly out of the lookup rules, so they're not “wrong.” Making private members invisible also would solve this problem. We'd be willing to look at a paper proposing that.

Additional discussion (April, 2005):

John Spicer: Base class members are more-or-less treated as members of the class, [so] it is only natural that the base [member] would hide the template parameter.

Daveed Vandevoorde: Are base class members really “more or less” members of the class from a lookup perspective? After all, derived class members can hide base class members of the same name. So there is some pretty definite boundary between those two sets of names. IMO, the template parameters should either sit between those two sets, or they should (for lookup purposes) be treated as members of the class they parameterize (I cannot think of a practical difference between those two formulations).

John Spicer: How is [hiding template parameters] different from the fact that namespace members can be hidden by private parts of a base class? The addition of `int C` to `N::A` breaks the code in namespace `M` in this example:

        namespace N {
           class A {
        private:
             int C;
           };
        }

        namespace M {
           typedef int C;
           class B : public N::A {
             void f() {
                 C c;
             }
           };
        }

Daveed Vandevoorde: C++ has a mechanism in place to handle such situations: qualified names. There is no such mechanism in place for template parameters.

Nathan Myers: What I see as obviously incorrect ... is simply that a name defined right where I can see it, and directly attached to the textual scope of `B`'s class body, is ignored in favor of something found in some other file. I don't care that `C1` is defined in `A`, I have a `C1` right here that I have chosen to use. If I want `A::C1`, I can say so.

I doubt you'll find any regular C++ coder who doesn't find the standard behavior bizarre. If the meaning of any code is changed by fixing this behavior, the overwhelming majority of cases will be mysterious bugs magically fixed.

John Spicer: I have not heard complaints that this is actually a cause of problems in real user code. Where is the evidence that the status quo is actually causing problems?

In this example, the `T2` that is found is the one from the base class. I would argue that this is natural because base class members are found as part of the lookup in class `B`:

        struct A {
                 typedef int T2;
        };
        template <class T2> struct B : public A {
                 typedef int T1;
                 T1 t1;
                 T2 t2;
        };

This rule that base class members hide template parameters was formalized about a dozen years ago because it fell out of the principle that base class members should be found at the same stage of lookup as derived class members, and that to do otherwise would be surprising.

Gabriel Dos Reis: The bottom line is that:

1. the proposed change is a silent change of meaning;
2. the proposed change does not make the language any more regular; the current behavior is consistent with everything else, however “surprising” that might be;
3. the proposed change does have its own downsides.

Unless presented with real major programming problems the current rules exhibit, I do not think the simple rule “scopes nest” needs a change that silently mutates program meaning.

Mike Miller: The rationale for the current specification is really very simple:

1. “Unless redeclared in the derived class, members of a base class are also considered to be members of the derived class.” (11.7 [class.derived] paragraph 2)
2. In class scope, members hide nonmembers.

That's it. Because template parameters are not members, they are hidden by member names (whether inherited or not). I don't find that “bizarre,” or even particularly surprising.

I believe these rules are straightforward and consistent, so I would be opposed to changing them. However, I am not unsympathetic toward Daveed's concern about name hijacking from base classes. How about a rule that would make a program ill-formed if a direct or inherited member hides a template parameter?

Unless this problem is a lot more prevalent than I've heard so far, I would not want to change the lookup rules; making this kind of collision a diagnosable error, however, would prevent hijacking without changing the lookup rules.

Erwin Unruh: I have a different approach that is consistent and changes the interpretation of the questionable code. At present lookup is done in this sequence:

block scope
derived class scope
base class scope
template parameters
namespace scope

If we change this order to

template parameters
block scope
derived class scope
base class scope
namespace scope

it is still consistent in that no lookup is placed between the base class and the derived class. However, it introduces another inconsistency: now scopes do not nest the same way as curly braces nest — but base classes are already inconsistent this way.

Nathan Myers: This looks entirely satisfactory. If even this seems like too big a change, it would suffice to say that finding a different name by this search order makes the program ill-formed. Of course, a compiler might issue only a portability warning in that case and use the name found Erwin's way, anyhow.

Gabriel Dos Reis: It is a simple fact, even without templates, that a writer of a derived class cannot protect himself against declaration changes in the base class.

Richard Corden: If a change is to be made, then making it ill-formed is better than just changing the lookup rules.

        struct B
        {
          typedef int T;
          virtual void bar (T const & );
        };

        template <typename T>
        struct D : public B
        {
          virtual void bar (T const & );
        };

        template class D<float>;

I think changing the semantics of the above code silently would result in very difficult-to-find problems.

Mike Miller: Another case that may need to be considered in deciding on Erwin's suggestion or the “ill-formed” alternative is the treatment of `friend` declarations described in 6.5.3 [basic.lookup.unqual] paragraph 10:

        struct A {
            typedef int T;
            void f(T);
        };
        template<typename T> struct B {
            friend void A::f(T);  // Currently T is A::T
        };

Notes from the October, 2005 meeting:

The CWG decided not to consider a change to the existing rules at this time without a paper exploring the issue in more detail.

#### 1619. Definition of current instantiation

Section: 13.8.3.2 [temp.dep.type] Status: open Submitter: Johannes Schaub Date: 2013-02-04

The definition of the current instantiation, given in 13.8.3.2 [temp.dep.type] paragraph 1, is phrased in terms of the meaning of a name (“A name refers to the current instantiation if it is...”); it does not define when a type is the current instantiation. Thus the interpretation of `*this` and of phrases like “member of a class that is the current instantiation” is not formally specified.

#### 2250. Implicit instantiation, destruction, and TUs

Section: 13.8.4.1 [temp.point] Status: open Submitter: Dawn Perchik Date: 2016-03-21

Consider:

      template<class T> struct A {
        T* p;
        A();
        ~A() {
    	if (0 < sizeof (T))
    	  delete p;
        }
      };
      struct U;
      struct C {
        C();
        ~C();
        A<U> u;
      };
      int main() {
        C c;
        return 0;
      }

The intent is that `A<U>::~A` is instantiated where the special member functions of `C` are defined (at which point `U` is ostensibly complete). Preventing instantiation (and thus avoiding the completeness check) in the present translation unit is necessary for the `unique_ptr`-based pimpl idiom to work.

Notes from the December, 2016 teleconference:

The problem is that the current wording only connects name lookup with point of instantiation; other semantic checks, such as the requirement for completeness of a class, should also be performed at that point.

#### 1253. Generic non-template members

Section: 13.9 [temp.spec] Status: open Submitter: Nikolay Ivchenkov Date: 2011-03-06

Many statements in the Standard apply only to templates, for example, 13.8 [temp.res] paragraph 8:

> If no valid specialization can be generated for a template definition, and that template is not instantiated, the template definition is ill-formed, no diagnostic required.

This clearly should apply to non-template member functions of class templates, not just to templates per se. Terminology should be established to refer to these generic entities that are not actually templates.

Additional notes (August, 2012):

Among the generic entities that should be covered by such a term are default function arguments, as they can be instantiated independently. If issue 1330 is resolved as expected, exception-specifications should also be covered by the same term.

See also issues 1484 and 1626.

CWG 2022-11-11

With the introduction of the family of terms "templated entity", this has mostly been addressed. The remaining open issue is the timing details of the parsing of the complete-class contexts.

#### 2435. Alias template specializations

Section: 13.9 [temp.spec] Status: open Submitter: Krystian Stasiowski Date: 2019-09-28

According to 13.9 [temp.spec] paragraph 4,

> An instantiated template specialization can be either implicitly instantiated (13.9.2 [temp.inst]) for a given argument list or be explicitly instantiated (13.9.3 [temp.explicit]). A specialization is a class, variable, function, or class member that is either instantiated or explicitly specialized (13.9.4 [temp.expl.spec]).

The definition of “specialization” does not cover alias templates, although the terms “specialization of an alias template” and ”alias template specialization” are used in 13.7.8 [temp.alias]. (Note that there are differences between alias specializations and the specializations mentioned here; in particular, an alias template cannot be explicitly specialized, and it is not the result of instantiation (paragraph 1) but simply of substitution (13.7.8 [temp.alias] paragraph 2).)

#### 1378. When is an instantiation required?

Section: 13.9.2 [temp.inst] Status: open Submitter: Jason Merrill Date: 2011-08-18

A template instantiation can be “required” without there being a need for it at link time if it can appear in a constant expression:

        template <class T> struct A {
           static const T t;
        };
        template <class T> const T A<T>::t = 0;
        template <int I> struct B { };
        int a = sizeof(B<A<int>::t>);

        template <class T> constexpr T f(T t) { return t; }
        int b = sizeof(B<f(42)>);

It seems like it might be useful to define a term other than odr-used for this sort of use, which is like odr-used but doesn't depend on potentially evaluated context or lvalue-rvalue conversions.

Nikolay Ivchenkov:

Another possibility would be to introduce the extension described in the closed issue 1272 and then change 6.3 [basic.def.odr] paragraph 2 as follows:

> An expression `E` is potentially evaluated unless it is an unevaluated operand ( Clause 7 [expr]) or a subexpression thereof. if and only if

> - `E` is a full-expression, or

> - `E` appears in a context where a constant expression is required, or

> - `E` is a direct subexpression of a potentially-evaluated expression and `E` is not an unevaluated operand.

> An expression `S` is a direct subexpression of an expression `E` if and only if `S` and `E` are different expressions, `S` is a subexpression of `E`, and there is no expression `X` such that `X` differs from both `S` and `E`, `S` is a subexpression of `X`, and `X` is a subexpression of `E`. A variable whose name appears as a potentially-evaluated expression is odr-used unless it is an object that satisfies the requirements for appearing in a constant expression (7.7 [expr.const]) and the lvalue-to-rvalue conversion (4.1) is immediately applied...

> [Example:

>         template <class T> struct X {
>             static int const m = 1;
>             static int const n;
>         };
>         template <class T> int const X<T>::n = 2;

>         int main() {
>             // X<void>::m is odr-used,
>             // X<void>::m is defined implicitly
>             std::cout << X<void>::m << std::endl;

>             // X<void>::n is odr-used,
>             // X<void>::n is defined explicitly
>             std::cout << X<void>::n << std::endl;

>             // OK (issue 712 is not relevant here)
>             std::cout << (1 ? X<void>::m : X<void>::n) << std::endl;
>         }

(See also issues 712 and 1254.)

#### 1856. Indirect nested classes of class templates

Section: 13.9.2 [temp.inst] Status: open Submitter: Richard Smith Date: 2014-02-11

During the discussion of issue 1484, it was observed that the current rules do not adequately address indirect nested classes of class templates (i.e., member classes of member classes of class templates) in regard to their potential separate instantiation.

#### 2655. Instantiation of default arguments in lambda-expressions

Section: 13.9.2 [temp.inst] Status: open Submitter: Tom Honermann Date: 2022-08-16

Subclause 7.5.5.2 [expr.prim.lambda.closure] paragraph 2 specifies:

> The closure type is declared in the smallest block scope, class scope, or namespace scope that contains the corresponding lambda-expression.

This means the closure type is a local class if the lambda-expression appears at function scope. A note in 13.9.2 [temp.inst] paragraph 2 claims that default arguments inside local classes are not separately instantiated:

> [Note 3: Within a template declaration, a local class (11.6 [class.local]) or enumeration and the members of a local class are never considered to be entities that can be separately instantiated (this includes their default arguments, noexcept-specifier s, and non-static data member initializers, if any, but not their type-constraints or requires-clauses). As a result, the dependent names are looked up, the semantic constraints are checked, and any templates used are instantiated as part of the instantiation of the entity within which the local class or enumeration is declared. —end note]

However, 13.9.2 [temp.inst] paragraph 3 is not in harmony with the note:

> The implicit instantiation of a class template specialization does not cause the implicit instantiation of default arguments or noexcept-specifier s of the class member functions.

Example:

      template<typename T>
      void ft() {
       [](T p = T::value) {}; // error even though the lambda is never called
      }
      template void ft<int>();

Even for a lambda declared at namespace scope is an unused default argument instantiated by major implementations:

      template<typename T>
      using ta = decltype([](T p = T::value) { // error
    	     return p;
    	    });
      auto g = ta<int>{}(0);

#### 293. Syntax of explicit instantiation/specialization too permissive

Section: 13.9.3 [temp.explicit] Status: open Submitter: Mark Mitchell Date: 27 Jun 2001

13.9.3 [temp.explicit] defines an explicit instantiation as

explicit-instantiation:

`template` declaration

Syntactically, that allows things like:

        template int S<int>::i = 5, S<int>::j = 7;

which isn't what anyone actually expects. As far as I can tell, nothing in the standard explicitly forbids this, as written. Syntactically, this also allows:

        template namespace N { void f(); }

although perhaps the surrounding context is enough to suggest that this is invalid.

Suggested resolution:

I think we should say:

explicit-instantiation:

type-specifier-seqopt declaratoropt `;`

[Steve Adamczyk: presumably, this should have `template` at the beginning.]

and then say that:

- The declarator can be omitted only when the type-specifier-seq consists solely of an elaborated-type-specifier for a class type, in which case the instantiation is for the named class. If the declarator is present the instantiation is for the named entity.
- The type-specifier-seq can be omitted only when the declarator is for a constructor, destructor, or conversion operator.
- The type-specifier-seq cannot define any new types.

There are similar problems in 13.9.4 [temp.expl.spec]:

explicit-specialization:

`template` <> declaration

Here, I think we want:

explicit-specialization:

decl-specifier-seqopt init-declaratoropt `;`

with similar restrictions as above.

[Steve Adamczyk: This also needs to have `template <>` at the beginning, possibly repeated.]

#### 1046. What is a “use” of a class specialization?

Section: 13.9.3 [temp.explicit] Status: open Submitter: Michael Wong Date: 2010-03-08

According to 13.9.3 [temp.explicit] paragraph 10,

> An entity that is the subject of an explicit instantiation declaration and that is also used in the translation unit shall be the subject of an explicit instantiation definition somewhere in the program; otherwise the program is ill-formed, no diagnostic required.

The term “used” is too vague and needs to be defined. In particular, “use” of a class template specialization as an incomplete type — to form a pointer, for instance — should not require the presence of an explicit instantiation definition elsewhere in the program.

#### 264. Unusable template constructors and conversion functions

Section: 13.10.2 [temp.arg.explicit] Status: open Submitter: John Spicer Date: 17 Nov 2000

The note in paragraph 5 of 13.10.2 [temp.arg.explicit] makes clear that explicit template arguments cannot be supplied in invocations of constructors and conversion functions because they are called without using a name. However, there is nothing in the current wording of the Standard that makes declaring a constructor or conversion operator that is unusable because of nondeduced parameters (i.e., that would need to be specified explicitly) ill-formed. It would be a service to the programmer to diagnose this useless construct as early as possible.

#### 697. Deduction rules apply to more than functions

Section: 13.10.3 [temp.deduct] Status: open Submitter: Doug Gregor Date: 6 June, 2008

13.10.3 [temp.deduct] is all about function types, but these rules also apply, e.g., when matching a class template partial specialization. We should add a note stating that we could be doing substitution into the template-id for a class template partial specialization.

Additional note (August 2008):

According to 13.7.6.2 [temp.spec.partial.match] paragraph 2, argument deduction is used to determine whether a given partial specialization matches a given argument list. However, there is nothing in 13.7.6.2 [temp.spec.partial.match] nor in 13.10.3 [temp.deduct] and its subsections that describes exactly how argument deduction is to be performed in this case. It would seem that more than just a note is required to clarify this processing.

#### 1844. Defining “immediate context”

Section: 13.10.3 [temp.deduct] Status: open Submitter: Richard Smith Date: 2014-01-28

The handling of an example like

      template<typename T, std::size_t S = sizeof(T)> struct X {};
      template<typename T> X<T> foo(T*);
      void foo(...);

      void test() { struct S *s; foo(s); }

varies among implementations, presumably because the meaning of “immediate context” in determining whether an error is a substitution failure or a hard error is not clearly defined.

Notes from the February, 2016 meeting:

See also issue 1554; the resolution of this issue should also deal with alias templates.

#### 2296. Are default argument instantiation failures in the “immediate context”?

Section: 13.10.3 [temp.deduct] Status: open Submitter: Jason Merrill Date: 2016-06-25 Liaison: EWG

Consider the following example (taken from issue 3 of paper P0348R0):

      template <typename U> void fun(U u = U());

      struct X {
        X(int) {}
      };

      template <class T> decltype(fun<T>()) g(int) { }

      template<> void g(long) { }

      int main() { g<X>(0); }

When is the substitution into the return type done? The current specification makes this example ill-formed because the failure to instantiate the default argument in the `decltype` operand is not in the immediate context of the substitution, although a plausible argument for making this a SFINAE case can be made.

Notes from the June, 2016 meeting:

CWG decided to refer this question to EWG for their consideration.

EWG 2022-11-11

This is tracked in github issue cplusplus/papers#1377.

#### 2498. Partial specialization failure and the immediate context

Section: 13.10.3.1 [temp.deduct.general] Status: open Submitter: Daveed Vandevoorde Date: 2021-06-15

Consider the following example:

      template<typename T, typename U> struct S {};
      template<typename T> struct S<T, T> {};
      template<typename T, typename U> struct S<T*, U*> {};
      template<typename... Ts> using V = void;
      template<typename T, typename U = void> struct X {};
      template<typename T> struct X<T, V<typename S<T, T>::type>>;
      X<int*> xpi;

Determining whether the partial specialization of `X` matches `X<int*>` requires determining whether one of the partial specializations of `S` matches `S<int*,int*>`. The partial specializations of `S` are ambiguous for this case. The question is whether that ambiguity should be considered in the “immediate context” of the type (SFINAE) or whether it should result in a hard error. There is implementation divergence on the handling of this example.

Notes from the November, 2021 teleconference:

A similar example can be constructed involving overload resolution instead of partial specialization:

      template<typename T, typename U> struct S {};
      template<typename T> struct S<T, T> {};
      template<typename T, typename U> struct S<T*, U*> {};

      template<class T>
      bool f(T, typename S<T, T>::type = 0);
      bool f(...);

      int x;
      bool b = f(&x);  // hard error with gcc, ok with clang

#### 2672. Lambda body SFINAE is still required, contrary to intent and note

Section: 13.10.3.1 [temp.deduct.general] Status: open Submitter: Richard Smith Date: 2022-09-30

Subclause 13.10.3.1 [temp.deduct.general] paragraph 9 specifies:

> A lambda-expression appearing in a function type or a template parameter is not considered part of the immediate context for the purposes of template argument deduction. [Note 7: The intent is to avoid requiring implementations to deal with substitution failure involving arbitrary statements. ... -- end note ]

However, the intent of the note is not satisfied by the normative rule, because a lambda-expression appearing in a requires-expression has the same concerns as one in a function signature.

Suggested resolution: Change the rule to say that substitution into the body of a lambda is never in the immediate context of substitution into the lambda-expression and move the rule somewhere more general.

#### 503. Cv-qualified function types in template argument deduction

Section: 13.10.3.2 [temp.deduct.call] Status: open Submitter: Gabriel Dos Reis Date: 22 Feb 2005

Consider the following program:

        template <typename T> int ref (T&)                { return 0; }
        template <typename T> int ref (const T&)          { return 1; }
        template <typename T> int ref (const volatile T&) { return 2; }
        template <typename T> int ref (volatile T&)       { return 4; }

        template <typename T> int ptr (T*)                { return 0; }
        template <typename T> int ptr (const T*)          { return 8; }
        template <typename T> int ptr (const volatile T*) { return 16; }
        template <typename T> int ptr (volatile T*)       { return 32; }

        void foo() {}

        int main()
        {
            return ref(foo) + ptr(&foo);
        }

The Standard appears to specify that the value returned from `main` is 2. The reason for this result is that references and pointers are handled differently in template argument deduction.

For the reference case, 13.10.3.2 [temp.deduct.call] paragraph 3 says that “If `P` is a reference type, the type referred to by `P` is used for type deduction.” Because of issue 295, all four of the types for the `ref` function parameters are the same, with no cv-qualification; overload resolution does not find a best match among the parameters and thus the most-specialized function is selected.

For the pointer type, argument deduction does not get as far as forming a cv-qualified function type; instead, argument deduction fails in the cv-qualified cases because of the cv-qualification mismatch, and only the cv-unqualified version of `ptr` survives as a viable function.

I think the choice of ignoring cv-qualifiers in the reference case but not the pointer case is very troublesome. The reason is that when one considers function objects as function parameters, it introduces a semantic difference whether the function parameter is declared a reference or a pointer. In all other contexts, it does not matter: a function name decays to a pointer and the resulting semantics are the same.

(See also issue 1584.)

#### 1939. Argument conversions to nondeduced parameter types revisited

Section: 13.10.3.2 [temp.deduct.call] Status: open Submitter: Richard Smith Date: 2014-06-11

The intent of the resolution of issue 1184 appears not to have been completely realized. In particular, the phrase, “contains no template-parameters that participate in template argument deduction” in both the note in 13.10.3.2 [temp.deduct.call] paragraph 4 and the normative wording in 13.10.2 [temp.arg.explicit] paragraph 6 is potentially misleading and probably should say something like, “contains no template-parameters outside non-deduced contexts.” Also, the normative wording should be moved to 13.10.3.2 [temp.deduct.call] paragraph 4, since it applies when there are no explicitly-specified template arguments. For example,

      template<typename T>
      void f(T, typename identity<T>::type*);

Presumably the second parameter should allow pointer conversions, even though it does contain a template-parameter that participates in deduction (via the first function parameter).

Additional note, October, 2015:

See also issue 1391.

#### 1221. Partial ordering and reference collapsing

Section: 13.10.3.5 [temp.deduct.partial] Status: open Submitter: Michael Wong Date: 2010-11-08

The current partial ordering rules produce surprising results in the presence of reference collapsing.

Since partial ordering is currently based solely on the signature of the function templates, the lack of difference following substitution of the template type parameter in the following is not taken into account.

Especially unsettling is that the allegedly "more specialized" template (#2) is not a candidate in the first call where template argument deduction fails for it despite a lack of non-deduced contexts.

        template <typename T>
        void foo(T&&);  // #1

        template <typename T>
        void foo(volatile T&&);  // #2

        int main(void) {
          const int x = 0;
          foo(x);  // calls #1 with T='const int &'
          foo<const int &>(x);  // calls #2
        }

#### 1763. Length mismatch in template type deduction

Section: 13.10.3.6 [temp.deduct.type] Status: open Submitter: Canada Date: 2013-09-23
N3690 comment CA 4

It is not clear how an example like the following is to be handled:

      template <typename U>
      struct A {
        template <typename V> operator A<V>();
      };

      template <typename T>
      void foo(A<void (T)>);

      void foo();

      int main() {
        A<void (int, char)> a;
        foo<int>(a);
        foo(a); // deduces T to be int
      }

In sub13.10.3.6 [temp.deduct.type] paragraph 10, deduction from a function type considers P/A pairs from the parameter-type-list only where the "P" function type has a parameter. Deduction is not specified to fail if there are additional parameters in the corresponding "A" function type.

Notes from the September, 2013 meeting:

CWG agreed that this example should not be accepted. The existing rules seem to cover this case (deduction is not specified to “succeed,” so it's a reasonable conclusion that it fails), but it might be helpful to be clearer.

#### 2417. Explicit instantiation and exception specifications

Section: 14.5 [except.spec] Status: open Submitter: John Spicer Date: 2019-06-19

Consider the following example:

      template<class T>struct Y {
        typedef typename T::value_type blah;  // #1
        void swap(Y<T> &);
      };
      template<class T>
      void swap(Y<T>& Left, Y<T>& Right) noexcept(noexcept(Left.swap(Right))) { }

      template <class T> struct Z {
        void swap(Z<T> &);
      };
      template<class T>
      void swap(Z<T>& Left, Z<T>& Right) noexcept(noexcept(Left.swap(Right))) { }

      Z<int> x00, y00;
      constexpr bool b00 = noexcept(x00.swap(y00));
      template void swap<int>(Z<int>&, Z<int>&) noexcept(b00);  // #2

The question here is whether the explicit instantiation of

      swap<int>(Z<int>&, Z<int>&)

at #2 instantiates the exception specification of

      swap<int>(Y<int>&, Y<int>&)

which would instantiate `Y<int>`, resulting in an error on the declaration of

      typedef typename T::value_type blah;

at #1.

According to 13.9.2 [temp.inst] paragraph 14,

> The noexcept-specifier of a function template specialization is not instantiated along with the function declaration; it is instantiated when needed (14.5 [except.spec]).

According to 14.5 [except.spec] bullet 13.3, one of the reasons an exception specification is needed is:

> the exception specification is compared to that of another declaration (e.g., an explicit specialization or an overriding virtual function);

Such a comparison is presumably needed when determining which function template the explicit instantiation is referring to, making the program ill-formed. However, there is implementation variance on this point.

CWG 2022-11-10

There are related problems in this area; CWG is seeking input to form a holistic view.

#### 2002. White space within preprocessing directives

Section: Clause 15 [cpp] Status: open Submitter: Richard Smith Date: 2014-09-10 Liaison: WG14

According to Clause 15 [cpp] paragraphg 4,

> The only white-space characters that shall appear between preprocessing tokens within a preprocessing directive (from just after the introducing `#` preprocessing token through just before the terminating new-line character) are space and horizontal-tab (including spaces that have replaced comments or possibly other white-space characters in translation phase 3).

The effect of this restriction is unclear, however, since translation phase 3 is permitted to transform all white space characters and comments into spaces. The relationship between these two rules should be clarified.

#### 925. Type of character literals in preprocessor expressions

Section: 15.2 [cpp.cond] Status: open Submitter: Michael Wong Date: 29 June, 2009

According to 15.2 [cpp.cond] paragraph 4,

> The resulting tokens comprise the controlling constant expression which is evaluated according to the rules of 7.7 [expr.const] using arithmetic that has at least the ranges specified in 17.3 [support.limits], except that all signed and unsigned integer types act as if they have the same representation as, respectively, `intmax_t` or `uintmax_t` (_N3035_.18.4.2 [stdinth]). This includes interpreting character literals, which may involve converting escape sequences into execution character set members.

Ordinary character literals with a single c-char have the type `char`, which is neither a signed nor an unsigned integer type. Although 7.3.7 [conv.prom] paragraph 1 is clear that `char` values promote to `int`, regardless of whether the implementation treats `char` as having the values of `signed char` or `unsigned char`, 15.2 [cpp.cond] paragraph 4 isn't clear on whether character literals should be treated as signed or unsigned values. In C99, such literals have type `int`, so the question does not arise. If an implementation in which plain `char` has the values of `unsigned char` were to treat character literals as unsigned, an expression like `'0'-'1'` would thus have different values in C and C++, namely -1 in C and some large unsigned value in C++.

#### 1436. Interaction of constant expression changes with preprocessor expressions

Section: 15.2 [cpp.cond] Status: open Submitter: Richard Smith Date: 2012-01-02

It appears that some of the recent changes to the description of constant expressions have allowed constructs into preprocessor expressions that do not belong there. Some changes are required to restrict the current capabilities of constant expressions to what is intended to be allowed in preprocessor expressions.

Proposed resolution (February, 2012):

1. Change 15.2 [cpp.cond] paragraph 2 as follows:

> Each preprocessing token that remains (in the list of preprocessing tokens that will become the controlling expression) after all macro replacements have occurred shall be in the lexical form of a token (5.6 [lex.token]). Any such token that is a literal (5.13.1 [lex.literal.kinds]) shall be an integer-literal, a character-literal, or a boolean-literal.

2. Change 15.2 [cpp.cond] paragraph 4 as follows:

> ...using arithmetic that has at least the ranges specified in 17.3 [support.limits]. The only operators permitted in the controlling constant expression are `?:`, `||`, `&&`, `|`, `^`, `&`, `==`, `!=`, `<`, `<=`, `>`, `>=`, `<<`, `>>`, `-`, `+`, `*`, `/`, `%`, `!`, and `~`. For the purposes of this token conversion...

Additional note (March, 2022):

The proposed resolution shown above allows lambda-expressions, which is undesirable.

CWG 2022-11-11

A possible approach, in addition to the proposed resolution above, is to prohibit the `[` and `{` pp-tokens in the controlling expression of a conditional inclusion. C++ also permits using the alternative tokens, e.g. `not`, which might need clarification in the wording.

#### 2190. Insufficient specification of `__has_include`

Section: 15.2 [cpp.cond] Status: open Submitter: Hubert Tong Date: 2015-10-24

The following example ought to be well-formed, but the standard is insufficiently clear:

      #define X <ciso646> )

      #if __has_include( X && ( defined(X) )
      #endif

Also, there is insufficient prohibition for the formation of `__has_include` by token pasting when performing macro replacement for the controlling expression of a conditional inclusion.

#### 2575. Undefined behavior when macro-replacing "defined" operator

Section: 15.2 [cpp.cond] Status: open Submitter: US Date: 2019-10-23 Liaison: SG12

(From National Body comment US 130 on the C++20 DIS.)

Subclause 15.2 [cpp.cond] paragraph 10 specifies:

> If the token `defined` is generated as a result of this replacement process or use of the `defined` unary operator does not match one of the two specified forms prior to macro replacement, the behavior is undefined.

Undefined behavior during lexing is not acceptable. The behavior ought to be well-defined, ill-formed, or conditionally-supported.

Additional notes (January, 2023):

Forwarded to SG12 with paper issue 1407, by decision of the CWG and SG12 chairs.

#### 2576. Undefined behavior with macro-expanded `#include` directives

Section: 15.3 [cpp.include] Status: open Submitter: US Date: 2019-10-23 Liaison: SG12

(From National Body comment US 131 on the C++20 DIS.)

Subclause 15.3 [cpp.include] paragraph 4 specifies:

> If the directive resulting after all replacements does not match one of the two previous forms, the behavior is undefined.

Undefined behavior during lexing is not acceptable. This ought to be ill-formed.

Additional notes (January, 2023):

Forwarded to SG12 with paper issue 1408, by decision of the CWG and SG12 chairs.

#### 2577. Undefined behavior for preprocessing directives in macro arguments

Section: 15.6.1 [cpp.replace.general] Status: open Submitter: US Date: 2019-10-23 Liaison: SG12

(From National Body comment US 141 on the C++20 DIS.)

Subclause 15.6.1 [cpp.replace.general] paragraph 13 specifies:

> If there are sequences of preprocessing tokens within the list of arguments that would otherwise act as preprocessing directives, [ Footnote: ...] the behavior is undefined.

Undefined behavior during lexing is not acceptable. The behavior ought to be ill-formed or conditionally-supported. Some implementation support an `#if` section, others reject that.

Additional notes (January, 2023):

Forwarded to SG12 with paper issue 1413, by decision of the CWG and SG12 chairs.

#### 1625. Adding spaces between tokens in stringizing

Section: 15.6.3 [cpp.stringize] Status: open Submitter: Chandler Carruth Date: 2013-02-18 Liaison: WG14

Given the following input,

      #define F(A, B, C) A ## x.B ## y.C ## z
      #define STRINGIFY(x) #x
      #define EXPAND_AND_STRINGIFY(x) STRINGIFY(x)
      char v[] = EXPAND_AND_STRINGIFY(F(a, b, c))

there is implementation variance in the value of `v`: some produce the string `"ax.by.cz"` and others produce the string `"ax. by. cz"`. Although 15.6.3 [cpp.stringize] paragraph 2 is explicit in its treatment of leading and trailing white space, it is not clear whether there is latitude for inserting spaces between tokens, as some implementations do, since the description otherwise is written solely in terms of preprocessing tokens. There may be cases in which such spaces would be needed to preserve the original tokenization, but it is not clear whether the result of stringization needs to produce something that would lex to the same tokens.

See also issue 2522.

Notes from the April, 2013 meeting:

Because the preprocessor specification is primarily copied directly from the C Standard, this issue has been referred to the C liaison for consultation with WG14.

#### 2578. Undefined behavior when creating an invalid string literal via stringizing

Section: 15.6.3 [cpp.stringize] Status: open Submitter: US Date: 2019-10-23 Liaison: SG12

(From National Body comment US 142 on the C++20 DIS.)

Subclause 15.6.3 [cpp.stringize] paragraph 2 specifies:

> If the replacement that results is not a valid character string literal, the behavior is undefined.

Undefined behavior during lexing is not acceptable. This ought to be ill-formed.

Additional notes (January, 2023):

Forwarded to SG12 with paper issue 1409, by decision of the CWG and SG12 chairs.

#### 2522. Removing placemarker tokens and retention of whitespace

Section: 15.6.4 [cpp.concat] Status: open Submitter: Hubert Tong Date: 2018-04-08 Liaison: WG14

It appears that the replacement of a function-like macro following further rescanning and replacement may begin or end with whitespace when the leftmost or rightmost tokens were placemarker tokens. GCC and MSVC preserve the whitespace in both the starting and ending positions. Clang and ICC preserve only the trailing whitespace. For example:

      #define F( X ) X##X/**/>
      #define G( X ) !/**/X##X
      #define F0( X ) F(X)
      #define G0( X ) G(X)
      #define STR2( X ) #X
      #define STR( X ) STR2(X)
      #define Q
      static_assert(STR(>F0(Q))[1] == '\u0020', STR(>F0(Q)));
        // ^ Works with GCC and MSVC; fails with Clang and ICC.
      static_assert(STR(G0(Q)=)[1] == '\u0020', STR(G0(Q)=));

This appears to be an issue in C as well; see ISO C 6.10.3.

See also issue 1625.

#### 2579. Undefined behavior when token pasting does not create a preprocessing token

Section: 15.6.4 [cpp.concat] Status: open Submitter: US Date: 2019-10-23 Liaison: SG12

(From National Body comment US 143 on the C++20 DIS.)

Subclause 15.6.4 [cpp.concat] paragraph 3 specifies:

> If the result begins with a sequence matching the syntax of universal-character-name, the behavior is undefined. [Note: ... --end note] If the result is not a valid preprocessing token, the behavior is undefined.

Undefined behavior during lexing is not acceptable. The behavior ought to be well-defined, ill-formed, or conditionally-supported.

Additional notes (January, 2023):

Forwarded to SG12 with paper issue 1410, by decision of the CWG and SG12 chairs.

#### 268. Macro name suppression in rescanned replacement text

Section: 15.6.5 [cpp.rescan] Status: open Submitter: Bjarne Stroustrup Date: 18 Jan 2001 Liaison: WG14

It is not clear from the Standard what the result of the following example should be:

>     #define NIL(xxx) xxx
>     #define G_0(arg) NIL(G_1)(arg)
>     #define G_1(arg) NIL(arg)
>     G_0(42)

The relevant text from the Standard is found in 15.6.5 [cpp.rescan] paragraph 2:

> If the name of the macro being replaced is found during this scan of the replacement list (not including the rest of the source file's preprocessing tokens), it is not replaced. Further, if any nested replacements encounter the name of the macro being replaced, it is not replaced. These nonreplaced macro name preprocessing tokens are no longer available for further replacement even if they are later (re)examined in contexts in which that macro name preprocessing token would otherwise have been replaced.

The sequence of expansion of `G0(42)` is as follows:

>     G0(42)
>     NIL(G_1)(42)
>     G_1(42)
>     NIL(42)

The question is whether the use of `NIL` in the last line of this sequence qualifies for non-replacement under the cited text. If it does, the result will be `NIL(42)`. If it does not, the result will be simply `42`.

The original intent of the J11 committee in this text was that the result should be `42`, as demonstrated by the original pseudo-code description of the replacement algorithm provided by Dave Prosser, its author. The English description, however, omits some of the subtleties of the pseudo-code and thus arguably gives an incorrect answer for this case.

Suggested resolution (Mike Miller): Replace the cited paragraph with the following:

> As long as the scan involves only preprocessing tokens from a given macro's replacement list, or tokens resulting from a replacement of those tokens, an occurrence of the macro's name will not result in further replacement, even if it is later (re)examined in contexts in which that macro name preprocessing token would otherwise have been replaced.

> Once the scan reaches the preprocessing token following a macro's replacement list — including as part of the argument list for that or another macro — the macro's name is once again available for replacement. [Example:

>         #define NIL(xxx) xxx
>         #define G_0(arg) NIL(G_1)(arg)
>         #define G_1(arg) NIL(arg)
>         G_0(42)                         // result is 42, not NIL(42)

> The reason that `NIL(42)` is replaced is that `(42)` comes from outside the replacement list of `NIL(G_1)`, hence the occurrence of `NIL` within the replacement list for `NIL(G_1)` (via the replacement of `G_1(42)`) is not marked as nonreplaceable. —end example]

(Note: The resolution of this issue must be coordinated with J11/WG14.)

Notes (via Tom Plum) from April, 2004 WG14 Meeting:

Back in the 1980's it was understood by several WG14 people that there were tiny differences between the "non-replacement" verbiage and the attempts to produce pseudo-code. The committee's decision was that no realistic programs "in the wild" would venture into this area, and trying to reduce the uncertainties is not worth the risk of changing conformance status of implementations or programs.

Additional notes (June, 2022):

It is unclear whether the phrasing "following the macro's replacement list" in the suggested resolution covers this case:

      #define f(x) b
      #define b(x) x(1)

      f(1)(f) // the result is 1

#### 2580. Undefined behavior with `#line`

Section: 15.7 [cpp.line] Status: open Submitter: US Date: 2019-10-23 Liaison: SG12

(From National Body comments US 144 and US 145 on the C++20 DIS.)

Subclause 15.7 [cpp.line] paragraph 3 specifies:

> If the digit sequence specifies zero or a number greater than 2147483647, the behavior is undefined.

Subclause 15.7 [cpp.line] paragraph 5 specifies:

> If the directive resulting after all replacements does not match one of the two previous forms, the behavior is undefined; otherwise, the result is processed as appropriate.

Undefined behavior during lexing is not acceptable. This ought to be ill-formed.

Additional notes (January, 2023):

Forwarded to SG12 with paper issue 1411, by decision of the CWG and SG12 chairs.

#### 2693. Escape sequences for the string-literal of `#line`

Section: 15.7 [cpp.line] Status: open Submitter: CWG Date: 2023-02-07 Liaison: WG14

The grammar for `#line` is inconsistent between 15.7 [cpp.line] paragraph 4:

>     # line digit-sequence " s-char-sequenceopt " new-line

and 15.7 [cpp.line] paragraph 1:

> The string-literal of a `#line` directive, if present, shall be a character string literal.

The characters comprising the string-literal are not necessarily interpreted for e.g. escape sequences or universal-character-names, so the use of string-literal here is questionable. This directive names a source file, thus it might be better to use header-name (5.8 [lex.header]) instead.

#### 2581. Undefined behavior for predefined macros

Section: 15.11 [cpp.predefined] Status: open Submitter: US Date: 2019-10-23 Liaison: SG12

(From National Body comments US 148 and US 149 on the C++20 DIS.)

Subclause 15.11 [cpp.predefined] paragraph 4 specifies:

> If any of the pre-defined macro names in this subclause, or the identifier `defined`, is the subject of a `#define` or a `#undef` preprocessing directive, the behavior is undefined.

Undefined behavior during lexing is not acceptable. This ought to be ill-formed.

Additional notes (January, 2023):

Forwarded to SG12 with paper issue 1412, by decision of the CWG and SG12 chairs.

#### 897. `_Pragma` and extended string-literals

Section: 15.12 [cpp.pragma.op] Status: open Submitter: Daniel Krügler Date: 9 May, 2009

The specification of how the string-literal in a `_Pragma` operator is handled does not deal with the new kinds of string literals. 15.12 [cpp.pragma.op] says,

> The string literal is destringized by deleting the `L` prefix, if present, deleting the leading and trailing double-quotes, replacing each escape sequence...

The various other prefixes should either be handled or prohibited.

Additional note (October, 2013):

If raw string literals are supported, the question of how to handle line splicing is relevant. The wording says that “the characters are processed through translation phase 3,” which is a bit ambiguous as to whether that includes phases 1 and 2 or not. It would be better to be explicit and say that the processing of phase 3 or of phases 1 through 3 is applied.

#### 2694. string-literals of the `_Pragma` operator

Section: 15.12 [cpp.pragma.op] Status: open Submitter: CWG Date: 2023-02-07 Liaison: WG14

Subclause 15.12 [cpp.pragma.op] paragraph 1 specifies:

> A unary operator expression of the form:

>           _Pragma ( string-literal )

> is processed as follows: The string-literal is destringized by deleting the `L` prefix, if present, deleting the leading and trailing double-quotes, replacing each escape sequence `\"` by a double-quote, and replacing each escape sequence `\\` by a single backslash. The resulting sequence of characters is processed through translation phase 3 to produce preprocessing tokens that are executed as if they were the pp-tokens in a pragma directive. The original four preprocessing tokens in the unary operator expression are removed.

In contrast, C23 section 6.10.6 specifies:

> A unary operator expression of the form:

>         _Pragma ( string-literal )

> is processed as follows: The string literal is destringized by deleting any encoding prefix, ...

While in C, any encoding prefix is deleted, C++ only deletes the L prefix and does not consider UTF-8, UTF-16, and UTF-32 string literals. This was probably an oversight when C++ obtained UTF-x string literals.

However, a string-literal entails lexing of escape sequences and universal-character-names, which seems not useful given that the string literal is de-stringized immediately afterwards. It might be more appropriate to employ a simpler lexical structure such as a q-char-sequence as used in a header-name instead. See also issue 2693.

#### 2361. Unclear description of `longjmp` undefined behavior

Section: 17.13.3 [csetjmp.syn] Status: open Submitter: Zhihao Yuan Date: 2017-10-20

According to 17.13.3 [csetjmp.syn] paragraph 2,

> A `setjmp`/`longjmp` call pair has undefined behavior if replacing the `setjmp` and `longjmp` by `catch` and `throw` would invoke any non-trivial destructors for any automatic objects.

The intent is clear, that transferring control from point A to point B via `longjmp` has undefined behavior if throwing an exception at point A and catching it at point B would invoke non-trivial destructors. The wording could be more precise.

See also the corresponding editorial issue for additional discussion.

Notes from the October, 2018 teleconference:

There are a number of unanswered questions in the current wording, including the impact on the current exception (whether it still exists) after a `longjmp` out of a handler, the impact on the initialization of locks if jumping from the initialization of a local static data member, etc. One thought was to restrict use of `longjmp` to “plain C functions”. Another was to say if the program would have different behavior via the use of exceptions the behavior is undefined. There was no consensus on how to proceed.

Additional note, January, 2022:

See also library issues 1265 and 3652.

#### 1944. New C incompatibilities

Section: Clause Annex C [diff] Status: open Submitter: Mike Miller Date: 2014-06-18

Some new features of C++ not only introduce incompatibilities with previous versions of C++ but also with C; however, the organization of Annex Clause Annex C [diff] makes it difficult to specify that a given feature is incompatible with both languages, and the practice has been only to document the C++ incompatibilities. Some means of specifying both sets of incompatibilities should be found, hopefully without excessive duplication between the C and C++ sections.

#### 1248. Updating Annex C to C99

Section: C.6 [diff.iso] Status: open Submitter: Jonathan Wakely Date: 2011-02-28

The description of incompatibilities with C in Annex C.6 [diff.iso] refers to C89, but there are a number of new features in C99 that should be covered.