[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 #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This might be a front-end bug, I'm struggling to reduce it though.

If I produce prepreocessed code then clang++ compiles it fine.

Adding this to the global namespace (so that version gets found by unqualified
lookup instead of std::__callable_function) fixes the error:

template<typename _Functor>
  inline _Functor&
  __callable_functor(_Functor& __f)
  { return __f; }

So there seems to be some interaction with the std::__callable_function
overloads.