Proposed resolution:

This wording is relative to N4885.

1. Modify [ostream.manip] as indicated:

   template<class charT, class traits>
     basic_ostream<charT, traits>& flush_emit(basic_ostream<charT, traits>& os);
   

   -12- Effects: Calls os.flush(). Then, if os.rdbuf() is a basic_syncbuf<charT, traits, Allocator>*, called buf for the purpose of exposition, behaves as an unformatted output function ([ostream.unformatted]) of os. After constructing a sentry object, calls buf->emit(). If that call returns false, calls os.setstate(ios_base::badbit).

[ 2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP. ]

[ 2021-06-23; Reflector poll ]

Set status to Tentatively Ready after six votes in favour during reflector poll.

basic_osyncstream::emit is specified to set badbit if it fails ([syncstream.osyncstream.members] p1), but the emit part of the flush_emit manipulator isn't, even though the flush part does set badbit if it fails.

More generally, given an osyncstream s, s << flush_emit; should probably have the same behavior as s.flush(); s.emit().

The reference implementation linked in P0753R2 does set badbit on failure, so at least this part appears to be an oversight. As discussed in LWG 3570, basic_osyncstream::emit should probably be an unformatted output function, so the emit part of flush_emit should do so too.