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.

1 files changed, 45 insertions, 5 deletions

diff --git a/libstdc++-v3/include/bits/stl_iterator.h b/libstdc++-v3/include/bits/stl_iterator.h
index f6504ec..6bd860b 100644
--- a/libstdc++-v3/include/bits/stl_iterator.h
+++ b/libstdc++-v3/include/bits/stl_iterator.h
@@ -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