This defines operator<=> as a non-member function template and does not
alter operator==. This contradicts the changes made by P1614R2, which
specify both as hidden friends, but that specification of operator<=> is
broken and the subject of a soon-to-be-published LWG issue.
* include/bits/stl_pair.h [__cpp_lib_three_way_comparison]
(operator<=>): Define for C++20.
* libsupc++/compare (__cmp2way_res_t): Rename to __cmp3way_res_t,
move into __detail namespace. Do not turn argument types into lvalues.
(__cmp3way_helper): Rename to __cmp3way_res_impl, move into __detail
namespace. Constrain with concepts instead of using void_t.
(compare_three_way_result): Adjust name of base class.
(compare_three_way_result_t): Use __cmp3way_res_impl directly.
(__detail::__3way_cmp_with): Add workaround for PR 91073.
(compare_three_way): Use workaround.
(__detail::__synth3way, __detail::__synth3way_t): Define new helpers
implementing synth-three-way and synth-three-way-result semantics.
* testsuite/20_util/pair/comparison_operators/constexpr_c++20.cc: New
test.
From-SVN: r278951
