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libstdc++: Fix constraints on std::compare_three_way 

My "simplification" of std::compare_three_way's constraints in commit
f214ffb336 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.
This commit is contained in:
Jonathan Wakely 2020-04-14 21:58:03 +01:00
parent f5fa62ed19
commit e1e9e8d7aa
3 changed files with 55 additions and 3 deletions
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5
libstdc++-v3/ChangeLog
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@ -1,5 +1,10 @@
2020-04-14 Jonathan Wakely <jwakely@redhat.com>
* 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.
PR libstdc++/94562
* include/bits/shared_ptr.h (operator<=>): Define for C++20.
* include/bits/shared_ptr_base.h (operator<=>): Likewise.

8
libstdc++-v3/libsupc++/compare
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@ -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>()))

45
libstdc++-v3/testsuite/18_support/comparisons/object/builtin-ptr-three-way.cc Normal file
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@ -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);
}