[Bug libstdc++/89102] 'common_type' of single abominable function should not have a nested typename

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89102

--- Comment #10 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:7dea2f84f26d454331d34b8536c261b6e96205d7

commit r10-9608-g7dea2f84f26d454331d34b8536c261b6e96205d7
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Aug 27 22:36:03 2020 +0100

    libstdc++: Make std::chrono::duration use reduced ratio for period

    This implements the changes from P0548 "common_type and duration". That
    was a change for C++17, but as it corrects some issues introduced by DRs
    I'm also treating it as a DR and changing it for all modes from C++11
    up.

    The main change is that duration<R,P>::period no longer denotes P, but
    rather P::type, the reduced ratio. The unary operator+ and operator-
    members of duration should now return a duration using that reduced
    ratio.

    The requirement that common_type<T>::type is the same type as
    common_type<T, T>::type (rather than simply T) was already implemented
    for PR 89102.

    The standard says that duration::operator+() and duration::operator-()
    should return common_type_t<duration>, but that seems unnecessarily
    expensive to compute. This uses duration<common_type_t<rep>, period>
    which is the same type, so we avoid a common_type specialization.

    As an optimization, this also adds partial specializations of
    common_type for two durations of the same type, a single duration, two
    time_points of the same type, and a single time_point. These
    specializations avoid instantiating other specializations of common_type
    and one or both of __duration_common_type or __timepoint_common_type for
    the cases where the answer is trivial to obtain.

    libstdc++-v3/ChangeLog:

            * include/std/chrono (__duration_common_type): Ensure the
            reduced ratio is used. Remove unused partial specialization
            using __failure_type.
            (common_type): Pass reduced ratios to __duration_common_type.
            Add partial specializations for simple cases involving a single
            duration or time_point type.
            (duration::period): Use reduced ratio.
            (duration::operator+(), duration::operator-()): Return duration
            type using the reduced ratio.
            * testsuite/20_util/duration/requirements/typedefs_neg2.cc:
            Adjust expected errors.
            * testsuite/20_util/duration/requirements/reduced_period.cc: New
test.

    (cherry picked from commit 82030d51017323c5706d58d8c8626324ece007e4)