Proposed resolution:

This wording is relative to N4741.

1. Edit [istreambuf.iterator.proxy] as indicated:

   namespace std {
     template<class charT, class traits = char_traits<charT>>
     class istreambuf_iterator<charT, traits>::proxy { // exposition only
       charT keep_;
       basic_streambuf<charT,traits>* sbuf_;
       proxy(charT c, basic_streambuf<charT,traits>* sbuf)
         : keep_(c), sbuf_(sbuf) { }
       public:
         charT operator*() const { return keep_; }
     };
   }