[syncstream.syncbuf.special]
31 Input/output library [input.output]
31.11 Synchronized output streams [syncstream]
31.11.2 Class template basic_syncbuf [syncstream.syncbuf]
31.11.2.6 Specialized algorithms [syncstream.syncbuf.special]
template<class charT, class traits, class Allocator>
void swap(basic_syncbuf<charT, traits, Allocator>& a,
basic_syncbuf<charT, traits, Allocator>& b);