This change ensures that std::common_type<> is a complete type (LWG
2408), and that std::common_type<T>, std::common_type<cv T1, cv T2>, and
std::common_type<T1, T2, R...> will use program-defined specializations
for std::common_type<T1, T2> (LWG 2465).
The implementation of common_type<T1, T2, R...> is changed to use
void_t, and the specializations for duration and time_point are modified
to also use void_t instead of depending on implementation details of
common_type.
PR libstdc++/89102
* doc/xml/manual/intro.xml: Document DR 2408 and 2465 changes.
* include/std/chrono (__duration_common_type_wrapper): Replace with ...
(__duration_common_type): New helper.
(common_type<chrono::duration<R1, P2>, chrono::duration<R2, P2>>): Use
__duration_common_type.
(__timepoint_common_type_wrapper): Replace with ...
(__timepoint_common_type): New helper.
(common_type<chrono::time_point<C, D2>, chrono::time_point<C, D2>>):
Use __time_point_common_type.
* include/std/type_traits (common_type<>): Define, as per LWG 2408.
(__common_type_impl): If either argument is transformed by decay,
use the common_type of the decayed types.
(__common_type_impl<_Tp, _Up, _Tp, _Up>): If the types are already
decayed, use __do_common_type_impl to get the common_type.
(common_type<_Tp>): Use common_type<_Tp, _Tp>.
(__do_member_type_wrapper, __member_type_wrapper)
(__expanded_common_type_wrapper): Remove.
(__common_type_pack, __common_type_fold): New helpers.
(common_type<_Tp, _Up, _Vp...>): Use new helpers instead of
__member_type_wrapper and __expanded_common_type_wrapper.
* testsuite/20_util/common_type/requirements/explicit_instantiation.cc:
Test zero-length template argument list.
* testsuite/20_util/common_type/requirements/sfinae_friendly_1.cc:
Test single argument cases and argument types that should decay.
* testsuite/20_util/common_type/requirements/sfinae_friendly_2.cc:
Adjust expected error.
* testsuite/20_util/duration/literals/range_neg.cc: Use zero for
dg-error lineno.
* testsuite/20_util/duration/requirements/typedefs_neg1.cc: Likewise.
* testsuite/20_util/duration/requirements/typedefs_neg2.cc: Likewise.
* testsuite/20_util/duration/requirements/typedefs_neg3.cc: Likewise.
From-SVN: r270987
