[Bug libstdc++/68515] std::result_of<F(X)> doesn't work when F is abstract (with pure virtual functions)

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68515

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I think it's a front-end problem, it (rightly) says that F(X) is not a valid
function type, because functions can't return abstract types. The problem is 
that we don't really have (or want) a function of that type, it's just the
notational trick used by result_of.