<future>: Divorce packaged_task from function

[frederick-vs-ja]

Currently, the internal state class _Packaged_state used by packaged_task stores a function.

STL/stl/inc/future

Lines 481 to 486 in 37120ed

	template <class _Ret, class... _ArgTypes>
	class _Packaged_state<_Ret(_ArgTypes...)> : public _Associated_state<typename _P_arg_type<_Ret>::type> {
	public:
	using _Mybase = _Associated_state<typename _P_arg_type<_Ret>::type>;
	using _Mydel = typename _Mybase::_Mydel;
	using _Function_type = function<_Ret(_ArgTypes...)>; // TRANSITION, ABI, should not use std::function

STL/stl/inc/future

Lines 550 to 551 in 37120ed

	private:
	_Function_type _Fn;

However, function requires the stored target object to be copy constructible while packaged_task doesn't, which rendered our implementation strategy non-conforming (reject-valid) for a long while, see #321.

#4946 partially fixed the non-conformance. However, the fix was imperfect:

• it introduced handling for move-only functors which function isn't supposed to support,
• it was broken when one specializes function for program-defined types, although I don't think anyone should do this, and
• when the functor's copy constructor is eligible but ill-formed, there's still hard error from it, which isn't supposed to happen for packaged_task.

In vNext where ABI breakage is available, we need to change the internal storage type. It seems that the implementation details of that type can be shared with move_only_function, but it doesn't seem possible to directly use move_only_function because move_only_function is only available since C++23.

vNext note: Resolving this issue will require breaking binary compatibility. We won't be able to accept pull requests for this issue until the vNext branch is available. See #169 for more information.