Proposed resolution:

This wording is relative to N4892.

1. Modify [incrementable.traits] as indicated:

   namespace std {
     […]
     template<class T>
       requires (!requires { typename T::difference_type; } &&
                 requires(const T& a, const T& b) { { a - b } -> integral; })
     struct incrementable_traits<T> {
       using difference_type = make_signed_t<decltype(declval<const T&>() - declval<const T&>())>;
     };
     […]
   }
   

[ 2022-01-29; Reflector poll ]

Set priority to 3 after reflector poll that failed to reach consensus. Some suggested NAD: "Implicit expression variations ([concepts.equality]/6) apply here."

The last specialization of incrementable_traits requires a - b to be well-formed, where the types of both operands are const lvalue reference of type T. However inside the struct, it uses decltype(declval<T>() - declval<T>()) to define the difference_type, that is, non-const rvalue reference of type T.