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 <r...@gcc.gnu.org>: https://gcc.gnu.org/g:7dea2f84f26d454331d34b8536c261b6e96205d7 commit r10-9608-g7dea2f84f26d454331d34b8536c261b6e96205d7 Author: Jonathan Wakely <jwak...@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)
