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libstdc++: Fix ambiguous comparisons for iterators in C++20

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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.
This commit is contained in:
Jonathan Wakely 2021-12-09 22:22:42 +00:00
parent 9042302ef0
commit 2c7fb16b52
1 changed files with 45 additions and 5 deletions
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50
libstdc++-v3/include/bits/stl_iterator.h
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@ -574,6 +574,24 @@ _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>
[[nodiscard]]
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>
[[nodiscard]]
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
///@}
@ -1161,6 +1179,25 @@ _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>
[[nodiscard]]
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>
[[nodiscard]]
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>
@ -1689,14 +1726,9 @@ _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>
[[__nodiscard__]]
inline _GLIBCXX17_CONSTEXPR bool
@ -1704,6 +1736,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
const move_iterator<_Iterator>& __y)
{ return __x.base() == __y.base(); }
#if __cpp_lib_three_way_comparison
template<three_way_comparable _Iterator>
[[__nodiscard__]]
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>
[[__nodiscard__]]
inline _GLIBCXX17_CONSTEXPR bool