Proposed resolution:

This wording is relative to N4750.

1. Change [syncstream.osyncstream.overview], class template basic_osyncstream synopsis, as indicated:

   namespace std {
     template<class charT, class traits, class Allocator>
     class basic_osyncstream : public basic_ostream<charT, traits> {
     public:
       […]
   
       // [syncstream.osyncstream.members], member functions
       void emit();
       streambuf_type* get_wrapped() const noexcept;
       syncbuf_type* rdbuf() const noexcept { return const_cast<syncbuf_type*>(&sb); }
       […]
     };
   }
   

[ 2018-11, Adopted in San Diego ]

[ 2018-07-20 Status set to Tentatively Ready after five positive votes on the reflector. ]

The current specification of basic_osyncstream::rdbuf() is

    syncbuf_type* rdbuf() const noexcept { return &sb; }

This is ill-formed because the exposition-only member sb is const inside this const member function, but the return type is a pointer to non-const syncbuf_type. It needs to cast away the constness, consistent with the other streams with embedded stream buffers (such as string and file streams).