[Bug libstdc++/60594] std::function of a type with a declared (but not defined) return type fails to compile

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60594

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Mitsuru Kariya from comment #5)
> FYI, the BUG2 above is compiled successfully by gcc 4.7.3.

Yes, but std::function in 4.7.3 fails other, more important tests.