Proposed resolution:

This wording is relative to N4830.

1. Modify [iterator.traits] as indicated:

   -2- The definitions in this subclause make use of the following exposition-only concepts:

   template<class I>
   concept cpp17-iterator =
     copyable<I> && requires(I i) {
       {   *i } -> can-reference;
       {  ++i } -> same_as<I&>;
       { *i++ } -> can-reference;
     };
   
   template<class I>
   concept cpp17-input-iterator =
     cpp17-iterator<I> && equality_comparable<I> && requires(I i) {
       typename incrementable_traits<I>::difference_type;
       typename readable_traits<I>::value_type;
       typename common_reference_t<iter_reference_t<I>&&,
                                   typename readable_traits<I>::value_type&>;
       typename common_reference_t<decltype(*i++)&&,
                                   typename readable_traits<I>::value_type&>;
       { *i } -> convertible_to<const typename readable_traits<I>::value_type&>;
       { *i++ } -> convertible_to<const typename readable_traits<I>::value_type&>;
       requires signed_integral<typename incrementable_traits<I>::difference_type>;
     };
   
   […]
   

[ 2019-10-14 Issue Prioritization ]

Priority to 3 after reflector discussion.

The new C++20 iterator concepts use common_reference to constrain the value, reference, and rvalue_reference associated types in order to support proxy references (see [iterator.concept.readable]).

However, the C++17 iterators did not support proxy references, so the use of common_reference in [iterator.traits]/p2 is needlessly complex. The common_reference constraints can be replaced with simple convertibility requirements to a const lvalue reference to the value type.

This fix has been implemented in range-v3.