PR libstdc++/77998
	* include/std/future (__future_base::_Deferred_state)
	(__future_base::_Async_state_impl): Use decltype to deduce return
	type, instead of _Invoker::result_type.
	* include/std/thread (thread::_Invoker::operator()): Likewise.
	(thread::_Invoker::result_type): Remove.

From-SVN: r241236