template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>>
class basic_syncbuf;
template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT>>
class basic_osyncstream;
Also file an LWG issue to make clear which headers make <ios> available. Once that's done, remove the default template arguments, because they're implied and redundant from <iosfwd>.
The default template arguments are missing.
Proposed change: