LWG3446 indirectly_readable_traits ambiguity for types with both value_type and element_type

Author: burblebee

source/iterators.tex

@@ -745,6 +745,12 @@
   using value_type = remove_cv_t<T>;
 };
 
+template<class T>
+  concept @\defexposconcept{has-member-value-type}@ = requires { typename T::value_type; };         // \expos
+
+template<class T>
+  concept @\defexposconcept{has-member-element-type}@ = requires { typename T::element_type; };     // \expos
+
 template<class> struct indirectly_readable_traits { };
 
 template<class T>
@@ -761,16 +767,20 @@
 struct indirectly_readable_traits<const I>
   : indirectly_readable_traits<I> { };
 
-template<class T>
-  requires requires { typename T::value_type; }
+template<@\exposconcept{has-member-value-type}@ T>
 struct indirectly_readable_traits<T>
   : @\placeholder{cond-value-type}@<typename T::value_type> { };
 
-template<class T>
-  requires requires { typename T::element_type; }
+template<@\exposconcept{has-member-element-type}@ T>
 struct indirectly_readable_traits<T>
   : @\placeholder{cond-value-type}@<typename T::element_type> { };
 
+template<@\exposconcept{has-member-value-type}@ T>
+  requires @\exposconcept{has-member-element-type}@<T> &&
+           @\libconcept{same_as}@<remove_cv_t<typename T::element_type>, remove_cv_t<typename T::value_type>>
+struct indirectly_readable_traits<T>
+  : @\placeholder{cond-value-type}@<typename T::value_type> { };
+
 template<class T> using iter_value_t = @\seebelow@;
 \end{codeblock}