[Bug libstdc++/53683] cout doesn't support std::u16string

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53683

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Because your program is ill-formed, and not supposed to compile, so what you
reported is not a bug.