Proposed resolution:

This wording is relative to N4741.

1. Edit [istreambuf.iterator] as indicated:

   namespace std {
     template<class charT, class traits = char_traits<charT>>
     class istreambuf_iterator {
     public:
       […]
       using istream_type = basic_istream<charT,traits>;
     
       class proxy; // exposition only
     
       constexpr istreambuf_iterator() noexcept;
       […]
     private:
       class proxy; // exposition only
       streambuf_type* sbuf_; // exposition only
     };
     […]
   }
   

[ 2018-06-18 after reflector discussion ]

Priority set to 4

LWG has said recently that we don't want public exposition-only things, as that may encourage users to try to use those names (and some implementers to actually use those names).