[Bug c++/58569] Compilation error when a class contains multiple std::function

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58569

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Daniel Krügler from comment #3)
> I don't see a good
> reasons why std::function<foo(...)> should check std::is_convertible<foo,
> foo> when instantiating the class template.

It's done to check the constraint imposed by the resolution of
http://cplusplus.github.io/LWG/lwg-active.html#2132