My "simplification" of std::compare_three_way's constraints in commit f214ffb336d582a66149068a2a96b7fcf395b5de was incorrect, because std::three_way_comparable_with imposes additional restrictions beyond the <=> expression being valid.
* libsupc++/compare (compare_three_way): Fix constraint so that BUILTIN-PTR-THREE-WAY does not require three_way_comparable_with. * testsuite/18_support/comparisons/object/builtin-ptr-three-way.cc: New test. Tested powerpc64l-linux, committed to master.
commit e1e9e8d7aa7bf1b0d3de34345b6c649e07da7a1e Author: Jonathan Wakely <jwak...@redhat.com> Date: Tue Apr 14 21:58:03 2020 +0100 libstdc++: Fix constraints on std::compare_three_way My "simplification" of std::compare_three_way's constraints in commit f214ffb336d582a66149068a2a96b7fcf395b5de was incorrect, because std::three_way_comparable_with imposes additional restrictions beyond the <=> expression being valid. * libsupc++/compare (compare_three_way): Fix constraint so that BUILTIN-PTR-THREE-WAY does not require three_way_comparable_with. * testsuite/18_support/comparisons/object/builtin-ptr-three-way.cc: New test. diff --git a/libstdc++-v3/libsupc++/compare b/libstdc++-v3/libsupc++/compare index 5f1df19e445..e5fb322ed9e 100644 --- a/libstdc++-v3/libsupc++/compare +++ b/libstdc++-v3/libsupc++/compare @@ -481,13 +481,14 @@ namespace std // BUILTIN-PTR-THREE-WAY(T, U) template<typename _Tp, typename _Up> concept __3way_builtin_ptr_cmp - = three_way_comparable_with<_Tp, _Up> + = requires(_Tp&& __t, _Up&& __u) + { static_cast<_Tp&&>(__t) <=> static_cast<_Up&&>(__u); } && convertible_to<_Tp, const volatile void*> && convertible_to<_Up, const volatile void*> && ! requires(_Tp&& __t, _Up&& __u) - { operator<=>(static_cast<_Tp&&>(__t), static_cast<_Up&&>(__u)); } + { operator<=>(static_cast<_Tp&&>(__t), static_cast<_Up&&>(__u)); } && ! requires(_Tp&& __t, _Up&& __u) - { static_cast<_Tp&&>(__t).operator<=>(static_cast<_Up&&>(__u)); }; + { static_cast<_Tp&&>(__t).operator<=>(static_cast<_Up&&>(__u)); }; } // namespace __detail // [cmp.object], typename compare_three_way @@ -495,6 +496,7 @@ namespace std { template<typename _Tp, typename _Up> requires three_way_comparable_with<_Tp, _Up> + || __detail::__3way_builtin_ptr_cmp<_Tp, _Up> constexpr auto operator()(_Tp&& __t, _Up&& __u) const noexcept(noexcept(std::declval<_Tp>() <=> std::declval<_Up>())) diff --git a/libstdc++-v3/testsuite/18_support/comparisons/object/builtin-ptr-three-way.cc b/libstdc++-v3/testsuite/18_support/comparisons/object/builtin-ptr-three-way.cc new file mode 100644 index 00000000000..38b3c6e3211 --- /dev/null +++ b/libstdc++-v3/testsuite/18_support/comparisons/object/builtin-ptr-three-way.cc @@ -0,0 +1,45 @@ +// Copyright (C) 2020 Free Software Foundation, Inc. +// +// This file is part of the GNU ISO C++ Library. This library is free +// software; you can redistribute it and/or modify it under the +// terms of the GNU General Public License as published by the +// Free Software Foundation; either version 3, or (at your option) +// any later version. + +// This library is distributed in the hope that it will be useful, +// but WITHOUT ANY WARRANTY; without even the implied warranty of +// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the +// GNU General Public License for more details. + +// You should have received a copy of the GNU General Public License along +// with this library; see the file COPYING3. If not see +// <http://www.gnu.org/licenses/>. + +// { dg-options "-std=gnu++2a" } +// { dg-do compile { target c++2a } } + +#include <compare> + +void +test01() +{ + struct X + { + operator int() const { return 0; } + operator long*() const { return nullptr; } + } x; + + // Not three-way-comparable because of ambiguous conversion to int or long*: + static_assert( ! std::three_way_comparable<X> ); + + // And therefore X is not three-way-comparable-with anything else + // (because std::three_way_comparable_with<X, Y> requires that both + // three_way_comparable<X> and three_way_comparable<Y> are true). + static_assert( ! std::three_way_comparable_with<X, long*> ); + + long l; + // But <=> is valid and resolves to a builtin operator comparing pointers: + auto c = &l <=> x; + // So std::compare_three_way should be usable: + auto c2 = std::compare_three_way()(&l, x); +}