[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 #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
> It looks like, somehow, instantiating the declaration of the std::function
> copy constructor relies on the completeness of 'bar'.

I suspect it's not the copy constructor, but that checking for copy
constructibility performs overload resolution, which instantiates the SFINAE
constraint on the function(F&&) constructor, which probably relies on
completeness of the result_type