Proposed resolution:

This wording is relative to N4835.

1. Modify [basic.string] p3, class template basic_string synopsis, as indicated:

   template<class T>
   constexpr basic_string& operator=(charT c);
   

2. Modify [string.cons] as indicated:

   template<class T>
   constexpr basic_string& operator=(charT c);
   

   -?- Constraints: is_same_v<T, charT> is true.

   -30- Effects: Equivalent to:

   return *this = basic_string_view<charT, traits>(addressof(c), 1);