LWG3862 basic_const_iterator's common_type specialization is underconstrained

Author: burblebee

source/iterators.tex

@@ -350,14 +350,17 @@
     class basic_const_iterator;                                                     // freestanding
 
   template<class T, @\libconcept{common_with}@<T> U>
+    requires @\libconcept{input_iterator}@<common_type_t<T, U>>
   struct common_type<basic_const_iterator<T>, U> {                                  // freestanding
     using type = basic_const_iterator<common_type_t<T, U>>;
   };
   template<class T, @\libconcept{common_with}@<T> U>
+    requires @\libconcept{input_iterator}@<common_type_t<T, U>>
   struct common_type<U, basic_const_iterator<T>> {                                  // freestanding
     using type = basic_const_iterator<common_type_t<T, U>>;
   };
   template<class T, @\libconcept{common_with}@<T> U>
+    requires @\libconcept{input_iterator}@<common_type_t<T, U>>
   struct common_type<basic_const_iterator<T>, basic_const_iterator<U>> {            // freestanding
     using type = basic_const_iterator<common_type_t<T, U>>;
   };