Implement LWG-4253 basic_const_iterator should provide iterator_type (#5868)

Author: frederick-vs-ja

stl/inc/xutility

@@ -2377,6 +2377,7 @@ private:
     }
 
 public:
+    using iterator_type    = _Iter;
     using iterator_concept = decltype(_Get_iter_concept());
     using value_type       = iter_value_t<_Iter>;
     using difference_type  = iter_difference_t<_Iter>;

tests/std/tests/P2278R4_basic_const_iterator/test.cpp

@@ -38,6 +38,7 @@ constexpr void test_one(It iter) {
     static_assert(contiguous_iterator<ConstIt> == contiguous_iterator<It>);
 
     // Validate nested types
+    static_assert(same_as<typename ConstIt::iterator_type, It>);
     static_assert(same_as<typename ConstIt::value_type, iter_value_t<It>>);
     static_assert(same_as<typename ConstIt::difference_type, iter_difference_t<It>>);
     if constexpr (forward_iterator<It>) {