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### 3383. §[time.zone.leap.nonmembers] `sys_seconds` should be replaced with `seconds`

Section: 27.11.8.3 [time.zone.leap.nonmembers] Status: C++20 Submitter: Jiang An Opened: 2020-01-30 Last modified: 2021-02-25

Priority: 1

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Discussion:

In N4849 27.11.8.3 [time.zone.leap.nonmembers]/12, the type template parameter `Duration` is constrained by `three_way_comparable_with<sys_seconds>`. However, since `std::chrono::sys_seconds` is a time point type and `Duration` must be a duration type, they can never be compared directly via `operator<=>`.

I guess that the actual intent is comparing `Duration` with the duration type of `std::chrono::sys_seconds`, i.e. `std::chrono::seconds`. And thus `sys_seconds` should be replaced with `seconds` here.

[2020-02 Prioritized as P1 Monday morning in Prague]

Previous resolution [SUPERSEDED]:

> This wording is relative to N4849.

> 1. Modify 27.2 [time.syn], header `<chrono>` synopsis, as indicated:

>    >     namespace std {
>    >       […]
>    >       namespace chrono {
>    >         […]
>    >         template<three_way_comparable_with<sys_seconds> Duration>
>    >           auto operator<=>(const leap& x, const sys_time<Duration>& y);
>    >         […]
>    >       }
>    >       […]
>    >     }

> 2. Modify 27.11.8.3 [time.zone.leap.nonmembers] as indicated:

>    >     template<three_way_comparable_with<sys_seconds> Duration>
>    >       constexpr auto operator<=>(const leap& x, const sys_time<Duration>& y) noexcept;

>    > > -12- Returns: `x.date() <=> y`.

[2020-02-10, Prague; Howard suggests alternative wording]

The below shown alternative wording does more directly describe the constrained code (by comparing `time_point`s), even though in the very end the code specifying the effects finally goes down to actually return `x.date().time_since_epoch() <=> y.time_since_epoch()` (thus comparing `duration`s).

Previous resolution [SUPERSEDED]:

> This wording is relative to N4849.

> 1. Modify 27.2 [time.syn], header `<chrono>` synopsis, as indicated:

>    > The synopsis does provide an additional drive-by fix to eliminate the mismatch of the `constexpr` and `noexcept` in declaration and prototype specification.

>    >     namespace std {
>    >       […]
>    >       namespace chrono {
>    >         […]
>    >         template<three_way_comparable_with<sys_seconds>class Duration>
>    >           requires three_way_comparable_with<sys_seconds, sys_time<Duration>>
>    >             constexpr auto operator<=>(const leap& x, const sys_time<Duration>& y) noexcept;
>    >         […]
>    >       }
>    >       […]
>    >     }

> 2. Modify 27.11.8.3 [time.zone.leap.nonmembers] as indicated:

>    >     template<three_way_comparable_with<sys_seconds>class Duration>
>    >       requires three_way_comparable_with<sys_seconds, sys_time<Duration>>
>    >         constexpr auto operator<=>(const leap& x, const sys_time<Duration>& y) noexcept;

>    > > -12- Returns: `x.date() <=> y`.

[2020-02-11, Prague; Daniel suggests alternative wording]

During today's LWG discussion of this issue the observation was made that there also exists a mismatch regarding the `noexcept` specifier for both declarations, but for this second deviation a corresponding change does not seem to be a good drive-by fix candidate, because we have a function template here that allows supporting user-defined types, whose comparison may throw (Note that the corresponding `operator<=>` or other comparison function declarations of the `duration` and `time_point` templates are not specified as `noexcept` function templates). The revised wording below does therefore intentionally not change the currently existing `noexcept`-specifier mismatch, but a separate issue should instead be opened for the general `noexcept`-specifier mismatches for all comparison function templates of `std::chrono::leap`. Daniel has volunteered to take care for this issue.

[2020-02 Moved to Immediate on Tuesday in Prague.]

Proposed resolution:

This wording is relative to N4849.

1. Modify 27.2 [time.syn], header `<chrono>` synopsis, as indicated:

   > [Drafting note: The synopsis does provide an additional drive-by fix to eliminate the mismatch of the `constexpr` in declaration and prototype specification, but does not so for a similar mismatch of the exception-specifications of both declarations.]

   >     namespace std {
   >       […]
   >       namespace chrono {
   >         […]
   >         template<three_way_comparable_with<sys_seconds>class Duration>
   >           requires three_way_comparable_with<sys_seconds, sys_time<Duration>>
   >             constexpr auto operator<=>(const leap& x, const sys_time<Duration>& y);
   >         […]
   >       }
   >       […]
   >     }

2. Modify 27.11.8.3 [time.zone.leap.nonmembers] as indicated:

   >     template<three_way_comparable_with<sys_seconds>class Duration>
   >       requires three_way_comparable_with<sys_seconds, sys_time<Duration>>
   >         constexpr auto operator<=>(const leap& x, const sys_time<Duration>& y) noexcept;

   > > -12- Returns: `x.date() <=> y`.