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_; }
     };
   }
   

[ 2018-06-18 after reflector discussion ]

Priority set to 3

operator* on iterators is usually intended to be const; see [input.iterators] Table 87, *a, where a is of type X or const X. (Technically, proxy is implementing the *r++ requirement in this table, and r doesn't imply a const iterator, but there's no reason for the iterator's operator* to differ from the proxy)