Proposed resolution:

Resolved by P1878.

[ 2019-11; Resolved by the adoption of P1878 in Belfast ]

[ 2019-11 This should be resolved by P1878 ]

Previous resolution [SUPERSEDED]:

This wording is relative to N4830.

1. Modify [iterator.concept.readable], concept readable synopsis, as indicated:

   template<class In>
     concept readable =
       requires {
         typename iter_value_t<remove_reference_t<In>>;
         typename iter_reference_t<In>;
         typename iter_rvalue_reference_t<In>;
       } &&
       common_reference_with<iter_reference_t<In>&&, iter_value_t<remove_reference_t<In>>&> &&
       common_reference_with<iter_reference_t<In>&&, iter_rvalue_reference_t<In>&&> &&
       common_reference_with<iter_rvalue_reference_t<In>&&, const iter_value_t<remove_reference_t<In>>&>;
   

[ 2019-10 Priority set to 1 after reflector discussion ]

In the current spec, shared_ptr<int> is readable, but shared_ptr<int>& is not. That is because readable_traits is not stripping top-level references before testing for nested typedefs.

Either readable_traits should see through cv- and ref-qualifiers, or else the readable concept should strip top-level references when building the iter_value_t associated type (e.g., iter_value_t<remove_reference_t<In>>).

Suggest priority P1 because it effects the definition of a concept which cannot change after C++20.