PR libstdc++/80316
	* include/std/future (_State_baseV2::_Setter::operator()): Remove
	_S_check calls that are done after the pointer to the shared state is
	already dereferenced.
	(_State_baseV2::_Setter<_Res, void>): Define specialization for void
	as partial specialization so it can be defined within the definition
	of _State_baseV2.
	(_State_baseV2::__setter): Call _S_check.
	(_State_baseV2::__setter(promise<void>*)): Add overload for use by
	promise<void>::set_value and promise<void>::set_value_at_thread_exit.
	(promise<T>, promise<T&>, promise<void>): Make _State a friend.
	(_State_baseV2::_Setter<void, void>): Remove explicit specialization.
	(promise<void>::set_value, promise<void>::set_value_at_thread_exit):
	Use new __setter overload.
	* testsuite/30_threads/promise/members/at_thread_exit2.cc: New test.
	* testsuite/30_threads/promise/members/set_exception.cc: Test
	promise<T&> and promise<void> specializations.
	* testsuite/30_threads/promise/members/set_exception2.cc: Likewise.
	Test for no_state error condition.
	* testsuite/30_threads/promise/members/set_value2.cc: Likewise.

From-SVN: r247064