Proposed resolution:

This wording is relative to N4830.

1. Modify [syncstream.syncbuf.overview], class template basic_syncbuf synopsis, as indicated:

   template<class charT, class traits, class Allocator>
   class basic_syncbuf : public basic_streambuf<charT, traits> {
   public:
     […]
     // [syncstream.syncbuf.cons], construction and destruction
     basic_syncbuf()
       : basic_syncbuf(nullptr) {}
     explicit basic_syncbuf(streambuf_type* obuf = nullptr)
       : basic_syncbuf(obuf, Allocator()) {}
     […]
   };
   

[ 2019-09-02 Issue Prioritization ]

Status to Tentatively Ready and priority to 0 after seven positive votes on the reflector.

When P0935 "Eradicating unnecessarily explicit default constructors from the standard library" was applied, basic_syncbuf was not in the working paper. basic_syncbuf should not have an explicit default constructor.