Proposed resolution:

This wording is relative to N4885.

1. Modify [concept.convertible] as indicated:

   template<class From, class To>
     concept convertible_to =
       is_convertible_v<From, To> &&
       requires(add_rvalue_reference_t<From> (&f)()) {
         static_cast<To>(fdeclval<From>());
       };