[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 #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
We can fix it in the library by adding this to std::function

      template<typename _CallRes>
    struct _CheckResult<_CallRes, _CallRes>
    : true_type { };