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

Section: 30.11.8.3 [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 30.11.8.3 [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Previous resolution [SUPERSEDED]: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 30.2 [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 30.11.8.3 [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_points), 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 durations).

Previous resolution [SUPERSEDED]:

This wording is relative to N4849.

  1. Modify 30.2 [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 30.11.8.3 [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 30.2 [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 30.11.8.3 [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.