Some improvements for conditional compilation

[frederick-vs-ja]

• Handle rvalue-invocable function objects in pre-std::invoke modes.
• Handle deprecation/removal of std::remove_of (in C++17/20 respectively).

Explanation for using enable_if in __do_invoke:

• if the argument for __func is a class (including union) rvalue, the first overload is selected, and operator() correctly works,
• otherwise, the argument is either an lvalue, or a scalar rvalue. The semantics doesn't change if we treat the scalar rvalue as an lvalue here.

[lhmouse]

That file needs to be completely rewritten in a 'satisfying' way. For example, ideally these __wait_* functions and _INVOKE belong in another file..

[lhmouse]

A more conforming solution would be:

template<class _Member, class _Class, class... _Args>
__MCF_CXX14(constexpr)
void  // for simplicity
__invoke(_Member _Class::* __memp, _Args&&... __args)
  {
    ::std::mem_fn(__memp) (::std::forward<_Args>(__args)...);
  }

template<class _Callable, class... _Args>
__MCF_CXX14(constexpr)
void  // for simplicity
__invoke(_Callable&& __callable, _Args&&... __args)
  {
    ::std::forward<_Callable>(__callable) (::std::forward<_Args>(__args)...);
  }

[lhmouse]

This has been obsoleted.