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### 1177. Improve "diagnostic required" wording

Section: 27.5 [time.duration] Status: C++11 Submitter: Howard Hinnant Opened: 2009-07-18 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [time.duration].

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Discussion:

"diagnostic required" has been used (by me) for code words meaning "use `enable_if` to constrain templated functions. This needs to be improved by referring to the function signature as not participating in the overload set, and moving this wording to a Remarks paragraph.

[ 2009-10 Santa Cruz: ]

> Moved to Ready.

[ 2009-11-19 Pete opens: ]

> Oh, and speaking of 1177, most of the changes result in rather convoluted prose. Instead of saying

> > A shall be B, else C

> it should be

> > C if A is not B

> That is:

> > `Rep2` shall be implicitly convertible to `CR(Rep1, Rep2)`, else this signature shall not participate in overload resolution.

> should be

> > This signature shall not participate in overload resolution if `Rep2` is not implicitly convertible to `CR(Rep1, Rep2)`.

> That is clearer, and eliminates the false requirement that `Rep2` "shall be" convertible.

[ 2009-11-19 Howard adds: ]

> I've updated the wording to match Pete's suggestion and included bullet 16 from 1195.

[ 2009-11-19 Jens adds: ]

> Further wording suggestion using "unless":

> > This signature shall not participate in overload resolution unless `Rep2` is implicitly convertible to `CR(Rep1, Rep2)`.

[ 2009-11-20 Howard adds: ]

> I've updated the wording to match Jens' suggestion.

[ 2009-11-22 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

[ This proposed resolution addresses 947 and 974. ]

1. Change 27.5.2 [time.duration.cons] (and reorder the Remarks paragraphs per 16.3.2.4 [structure.specifications]):

   >     template <class Rep2>
   >       explicit duration(const Rep2& r);

   > > Requires: Remarks: This constructor shall not participate in overload resolution unless `Rep2` shall be is implicitly convertible to `rep` and

   > > - `treat_as_floating_point<rep>::value` shall be is `true`, or
   > > - `treat_as_floating_point<Rep2>::value` shall be is `false`.

   > > Diagnostic required [Example:

   > > >     duration<int, milli> d(3); // OK
   > > >     duration<int, milli> d(3.5); // error

   > > — end example]

   > > Effects: Constructs an object of type `duration`.

   > > Postcondition: `count() == static_cast<rep>(r)`.

   >     template <class Rep2, class Period2>
   >       duration(const duration<Rep2, Period2>& d);

   > > Requires: Remarks: This constructor shall not participate in overload resolution unless `treat_as_floating_point<rep>::value` shall be is `true` or `ratio_divide<Period2, period>::type::den` shall be is 1. Diagnostic required. [Note: This requirement prevents implicit truncation error when converting between integral-based duration types. Such a construction could easily lead to confusion about the value of the duration. — end note] [Example:

   > > >     duration<int, milli> ms(3);
   > > >     duration<int, micro> us = ms; // OK
   > > >     duration<int, milli> ms2 = us; // error

   > > — end example]

   > > Effects: Constructs an object of type `duration`, constructing `rep_` from `duration_cast<duration>(d).count()`.

2. Change the following paragraphs in 27.5.6 [time.duration.nonmember]:

   >     template <class Rep1, class Period, class Rep2>
   >       duration<typename common_type<Rep1, Rep2>::type, Period>
   >       operator*(const duration<Rep1, Period>& d, const Rep2& s);

   > > Requires Remarks: This operator shall not participate in overload resolution unless `Rep2` shall be is implicitly convertible to `CR(Rep1, Rep2)`. Diagnostic required.

   >     template <class Rep1, class Period, class Rep2>
   >       duration<typename common_type<Rep1, Rep2>::type, Period>
   >       operator*(const Rep1& s, const duration<Rep2, Period>& d);

   > > Requires Remarks: This operator shall not participate in overload resolution unless `Rep1` shall be is implicitly convertible to `CR(Rep1, Rep2)`. Diagnostic required.

   >     template <class Rep1, class Period, class Rep2>
   >       duration<typename common_type<Rep1, Rep2>::type, Period>
   >       operator/(const duration<Rep1, Period>& d, const Rep2& s);

   > > Requires Remarks: This operator shall not participate in overload resolution unless `Rep2` shall be is implicitly convertible to `CR(Rep1, Rep2)` and `Rep2` shall not be is not an instantiation of `duration`. Diagnostic required.

   >     template <class Rep1, class Period, class Rep2>
   >       duration<typename common_type<Rep1, Rep2>::type, Period>
   >       operator%(const duration<Rep1, Period>& d, const Rep2& s);

   > > Requires Remarks: This operator shall not participate in overload resolution unless `Rep2` shall be is implicitly convertible to `CR(Rep1, Rep2)` and `Rep2` shall not be is not an instantiation of `duration`. Diagnostic required.

3. Change the following paragraphs in 27.5.8 [time.duration.cast]:

   >     template <class ToDuration, class Rep, class Period>
   >       ToDuration duration_cast(const duration<Rep, Period>& d);

   > > Requires Remarks: This function shall not participate in overload resolution unless `ToDuration` shall be is an instantiation of `duration`. Diagnostic required.

4. Change 27.6.2 [time.point.cons]/3 as indicated:

   > Requires: `Duration2` shall be implicitly convertible to `duration`. Diagnostic required.

   > Remarks: This constructor shall not participate in overload resolution unless `Duration2` is implicitly convertible to `duration`.

5. Change the following paragraphs in 27.6.8 [time.point.cast]:

   >     template <class ToDuration, class Clock, class Duration>
   >       time_point<Clock, ToDuration> time_point_cast(const time_point<Clock, Duration>& t);

   > > Requires Remarks: This function shall not participate in overload resolution unless `ToDuration` shall be is an instantiation of `duration`. Diagnostic required.