[Bug libstdc++/60711] basic_ostringstream,basic_ostream,u16string,char16_t do not work together

[glisse at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60711

--- Comment #4 from Marc Glisse <glisse at gcc dot gnu.org> ---
  uostream ucout;

Where did you see in the standard that basic_ostream is default constructible?
The only constructor I can find is the explicit one from basic_streambuf*.