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From: Jonathan Wakely <redi@gcc.gnu.org> To: gcc-cvs@gcc.gnu.org, libstdc++-cvs@gcc.gnu.org Subject: [gcc r11-9924] libstdc++: Fix ambiguous comparisons for iterators in C++20 Date: Thu, 21 Apr 2022 15:33:45 +0000 (GMT) [thread overview] Message-ID: <20220421153345.585813856DF4@sourceware.org> (raw) https://gcc.gnu.org/g:84ff93fcda2675cb3937de61c2f322759e77eb71 commit r11-9924-g84ff93fcda2675cb3937de61c2f322759e77eb71 Author: Jonathan Wakely <jwakely@redhat.com> Date: Thu Dec 9 22:22:42 2021 +0000 libstdc++: Fix ambiguous comparisons for iterators in C++20 Since r11-1571 (c++: Refinements to "more constrained") was changed in the front end, the following comment from stl_iterator.h stopped being true: // These extra overloads are not needed in C++20, because the ones above // are constrained with a requires-clause and so overload resolution will // prefer them to greedy unconstrained function templates. The requires-clause is no longer considered when comparing unrelated function templates. That means that the constrained operator== specified in the standard is no longer more constrained than the pathological comparison operators defined in the testsuite_greedy_ops.h header. This was causing several tests to FAIL in C++20 mode: FAIL: 23_containers/deque/types/1.cc (test for excess errors) FAIL: 23_containers/vector/types/1.cc (test for excess errors) FAIL: 24_iterators/move_iterator/greedy_ops.cc (test for excess errors) FAIL: 24_iterators/normal_iterator/greedy_ops.cc (test for excess errors) FAIL: 24_iterators/reverse_iterator/greedy_ops.cc (test for excess errors) The solution is to restore some of the non-standard comparison operators that are more specialized than the greedy operators in the testsuite. libstdc++-v3/ChangeLog: * include/bits/stl_iterator.h (operator==, operator<=>): Define overloads for homogeneous specializations of reverse_iterator, __normal_iterator and move_iterator. (cherry picked from commit 2c7fb16b5283cf90c4a7f7470e91e1010ee80fcc) Diff: --- libstdc++-v3/include/bits/stl_iterator.h | 45 ++++++++++++++++++++++++++++---- 1 file changed, 40 insertions(+), 5 deletions(-) diff --git a/libstdc++-v3/include/bits/stl_iterator.h b/libstdc++-v3/include/bits/stl_iterator.h index beeb4ef4c8a..4247bb116c3 100644 --- a/libstdc++-v3/include/bits/stl_iterator.h +++ b/libstdc++-v3/include/bits/stl_iterator.h @@ -537,6 +537,22 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION operator<=>(const reverse_iterator<_IteratorL>& __x, const reverse_iterator<_IteratorR>& __y) { return __y.base() <=> __x.base(); } + + // Additional, non-standard overloads to avoid ambiguities with greedy, + // unconstrained overloads in associated namespaces. + + template<typename _Iterator> + constexpr bool + operator==(const reverse_iterator<_Iterator>& __x, + const reverse_iterator<_Iterator>& __y) + requires requires { { __x.base() == __y.base() } -> convertible_to<bool>; } + { return __x.base() == __y.base(); } + + template<three_way_comparable _Iterator> + constexpr compare_three_way_result_t<_Iterator, _Iterator> + operator<=>(const reverse_iterator<_Iterator>& __x, + const reverse_iterator<_Iterator>& __y) + { return __y.base() <=> __x.base(); } #endif // C++20 ///@} @@ -1113,6 +1129,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION const __normal_iterator<_IteratorR, _Container>& __rhs) noexcept(noexcept(std::__detail::__synth3way(__lhs.base(), __rhs.base()))) { return std::__detail::__synth3way(__lhs.base(), __rhs.base()); } + + template<typename _Iterator, typename _Container> + constexpr bool + operator==(const __normal_iterator<_Iterator, _Container>& __lhs, + const __normal_iterator<_Iterator, _Container>& __rhs) + noexcept(noexcept(__lhs.base() == __rhs.base())) + requires requires { + { __lhs.base() == __rhs.base() } -> std::convertible_to<bool>; + } + { return __lhs.base() == __rhs.base(); } + + template<typename _Iterator, typename _Container> + constexpr std::__detail::__synth3way_t<_Iterator> + operator<=>(const __normal_iterator<_Iterator, _Container>& __lhs, + const __normal_iterator<_Iterator, _Container>& __rhs) + noexcept(noexcept(std::__detail::__synth3way(__lhs.base(), __rhs.base()))) + { return std::__detail::__synth3way(__lhs.base(), __rhs.base()); } #else // Forward iterator requirements template<typename _IteratorL, typename _IteratorR, typename _Container> @@ -1588,20 +1621,22 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION #endif { return !(__x < __y); } -#if ! (__cplusplus > 201703L && __cpp_lib_concepts) // Note: See __normal_iterator operators note from Gaby to understand // why we have these extra overloads for some move_iterator operators. - // These extra overloads are not needed in C++20, because the ones above - // are constrained with a requires-clause and so overload resolution will - // prefer them to greedy unconstrained function templates. - template<typename _Iterator> inline _GLIBCXX17_CONSTEXPR bool operator==(const move_iterator<_Iterator>& __x, const move_iterator<_Iterator>& __y) { return __x.base() == __y.base(); } +#if __cpp_lib_three_way_comparison + template<three_way_comparable _Iterator> + constexpr compare_three_way_result_t<_Iterator> + operator<=>(const move_iterator<_Iterator>& __x, + const move_iterator<_Iterator>& __y) + { return __x.base() <=> __y.base(); } +#else template<typename _Iterator> inline _GLIBCXX17_CONSTEXPR bool operator!=(const move_iterator<_Iterator>& __x,
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