libstdc++: Fix std::indirectly_readable ambiguity [LWG 3446]

This implements the proposed resolution of LWG 3446. I'm also adding
another new constrained specialization which isn't proposed by 3446, to
resolve the ambiguity when a type has both value_type and element_type
but denoting different types.

libstdc++-v3/ChangeLog:

	* include/bits/iterator_concepts.h (indirectly_readable): Add
	partial specializations to resolve ambiguities (LWG 3446).
	* testsuite/24_iterators/associated_types/readable.traits.cc:
	Check types with both value_type and element_type.

libstdc++-v3/include/bits/iterator_concepts.h

@@ -220,6 +220,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     template<typename _Tp> requires is_object_v<_Tp>
       struct __cond_value_type<_Tp>
       { using value_type = remove_cv_t<_Tp>; };
+
+    template<typename _Tp>
+      concept __has_member_value_type
+	= requires { typename _Tp::value_type; };
+
+    template<typename _Tp>
+      concept __has_member_element_type
+	= requires { typename _Tp::element_type; };
+
   } // namespace __detail
 
   template<typename> struct indirectly_readable_traits { };
@@ -238,16 +247,33 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     : indirectly_readable_traits<_Iter>
     { };
 
-  template<typename _Tp> requires requires { typename _Tp::value_type; }
+  template<__detail::__has_member_value_type _Tp>
     struct indirectly_readable_traits<_Tp>
     : __detail::__cond_value_type<typename _Tp::value_type>
     { };
 
-  template<typename _Tp> requires requires { typename _Tp::element_type; }
+  template<__detail::__has_member_element_type _Tp>
     struct indirectly_readable_traits<_Tp>
     : __detail::__cond_value_type<typename _Tp::element_type>
     { };
 
+  // _GLIBCXX_RESOLVE_LIB_DEFECTS
+  // 3446. indirectly_readable_traits ambiguity for types with both [...]
+  template<__detail::__has_member_value_type _Tp>
+    requires __detail::__has_member_element_type<_Tp>
+    && same_as<remove_cv_t<typename _Tp::element_type>,
+	       remove_cv_t<typename _Tp::value_type>>
+    struct indirectly_readable_traits<_Tp>
+    : __detail::__cond_value_type<typename _Tp::value_type>
+    { };
+
+  // LWG 3446 doesn't add this, but it's needed for the case where
+  // value_type and element_type are both present, but not the same type.
+  template<__detail::__has_member_value_type _Tp>
+    requires __detail::__has_member_element_type<_Tp>
+    struct indirectly_readable_traits<_Tp>
+    { };
+
   namespace __detail
   {
     template<typename _Tp>

libstdc++-v3/testsuite/24_iterators/associated_types/readable.traits.cc

@@ -141,3 +141,29 @@ struct J
 // iterator_traits<J> matches constrained specialization in the library,
 // so use its value_type.
 static_assert( check_alias<J, int> );
+
+struct I2
+{
+  using element_type = int;
+};
+// iterator_traits<I2> is not specialized, and no standard specialization
+// matches, so use indirectly_readable_traits.
+static_assert( check_alias<I2, std::indirectly_readable_traits<I2>::value_type> );
+
+// LWG 3446
+struct I3
+{
+  using value_type = long;
+  using element_type = const long;
+};
+// iterator_traits<I3> is not specialized, and no standard specialization
+// matches, so use indirectly_readable_traits.
+static_assert( check_alias<I3, std::indirectly_readable_traits<I3>::value_type> );
+
+// Correction to LWG 3446
+struct I4
+{
+  using value_type = int;
+  using element_type = long;
+};
+static_assert( ! has_alias<I4> );