[Bug libstdc++/114244] Need to use round when parsing fractional seconds

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114244

--- Comment #6 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:5f9d7a5b6cf64639274e63051caf70fbc8418ea2

commit r14-9376-g5f9d7a5b6cf64639274e63051caf70fbc8418ea2
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Mar 7 13:15:41 2024 +0000

    libstdc++: Fix parsing of fractional seconds [PR114244]

    When converting a chrono::duration<long double> to a result type with an
    integer representation we should use chrono::round<_Duration> so that we
    don't truncate towards zero. Rounding ensures that e.g. 0.001999s
    becomes 2ms not 1ms.

    We can also remove some redundant uses of chrono::duration_cast to
    convert from seconds to _Duration, because the _Parser class template
    requires _Duration type to be able to represent seconds without loss of
    precision.

    This also fixes a bug where no fractional part would be parsed for
    chrono::duration<long double> because its period is ratio<1>. We should
    also consider treat_as_floating_point<rep> when deciding whether to skip
    reading a fractional part.

    libstdc++-v3/ChangeLog:

            PR libstdc++/114244
            * include/bits/chrono_io.h (_Parser::operator()): Remove
            redundant uses of duration_cast. Use chrono::round to convert
            long double value to durations with integer representations.
            Check represenation type when deciding whether to skip parsing
            fractional seconds.
            * testsuite/20_util/duration/114244.cc: New test.
            * testsuite/20_util/duration/io.cc: Check that a floating-point
            duration with ratio<1> precision can be parsed.