[Bug libstdc++/114279] utc_clock does not support leap seconds

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114279

--- Comment #3 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:f4a52c184d608325c14338b57f464f7d0bbc7526

commit r14-9405-gf4a52c184d608325c14338b57f464f7d0bbc7526
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Mar 8 14:41:47 2024 +0000

    libstdc++: Fix parsing of leap seconds as chrono::utc_time [PR114279]

    Implementing all chrono::from_stream overloads in terms of
    chrono::sys_time meant that a leap second time like 23:59:60.001 cannot
    be parsed, because that cannot be represented in a sys_time.

    The fix to support parsing leap seconds as utc_time is to convert the
    parsed date to utc_time<days> and then add the parsed time to that,
    which allows the result to land in a leap second, rather than doing all
    the arithmetic with sys_time which doesn't have leap seconds.

    For local_time we also allow %S to parse a 60s value, because doing
    otherwise might disallow some valid uses. We can't know all use cases
    users have for treating times as local_time.

    For all other clocks, we can reject times that have 60 or 60.nnn as the
    seconds part, because that cannot occur in a valid UNIX, GPS, or TAI
    time. Since our chrono::file_clock uses sys_time, it can't occur for
    that clock either.

    In order to support this a new _M_is_leap_second member is needed in the
    _Parser type. This can be added at the end, where most targets currently
    have padding bytes. Similar to what I did recently for formatter _Spec
    structs, we can also reserve additional padding bits for future
    expansion.

    This also fixes bugs in the from_stream overloads for utc_time,
    tai_time, gps_time, and file_time, which were not using time_point_cast
    to explicitly convert to the result type. That's needed because the
    result type might have lower precision than the value returned from
    from_sys or from_utc, which has a precision no lower than seconds.

    libstdc++-v3/ChangeLog:

            PR libstdc++/114279
            * include/bits/chrono_io.h (_Parser::_M_is_leap_second): New
            data member.
            (_Parser::_M_reserved): Reserve padding bits for future use.
            (_Parser::operator()): Set _M_is_leap_second if %S reads 60s.
            (from_stream): Only allow _M_is_leap_second for utc_time and
            local_time. Adjust arithmetic for utc_time so that leap seconds
            are preserved. Use time_point_cast to convert to a possibly
            lower-precision result type.
            * testsuite/std/time/parse.cc: Move to ...
            * testsuite/std/time/parse/parse.cc: ... here.
            * testsuite/std/time/parse/114279.cc: New test.