Proposed resolution:

This wording is relative to N5008.

1. Modify [const.iterators.iterator] as indicated:

   namespace std {
     […]
     template<input_iterator Iterator>
     class basic_const_iterator {
       Iterator current_ = Iterator();                             // exposition only
       using reference = iter_const_reference_t<Iterator>;         // exposition only
       using rvalue-reference =                                    // exposition only
         iter-const-rvalue-reference-t<Iterator>;
             
       public:
         using iterator_type = I;
         using iterator_concept = see below;
         using iterator_category = see below;  // not always present
         using value_type = iter_value_t<Iterator>;
         using difference_type = iter_difference_t<Iterator>;
         […]
     };
   }
   

Currently, iterator adaptors in <iterator> that wrap a single iterator such as `reverse_iterator`, `move_iterator`, and `counted_iterator` all provide a public `iterator_type` member for users to access the underlying iterator type, except for `basic_const_iterator` (demo):

#include <iterator>

using I  = int*;
using RI = std::reverse_iterator<I>;
using MI = std::move_iterator<I>;
using CI = std::counted_iterator<I>;
using BI = std::basic_const_iterator<I>;

static_assert(std::same_as<RI::iterator_type, I>);
static_assert(std::same_as<MI::iterator_type, I>);
static_assert(std::same_as<CI::iterator_type, I>);
static_assert(std::same_as<BI::iterator_type, I>); // error

It seems reasonable to add one for `basic_const_iterator` for consistency.