[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 #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Are you sure this is valid? Instantiating std::function (like most std::lib
templates) with an incomplete type is undefined behaviour. Is the function type
bar() complete if bar is not complete?