PR libstdc++/55861
	* include/std/future (_State_base::_S_check(const shared_ptr<T>&)):
	Fix return type.
	(__basic_future::_M_get_result()): Const qualify.
	(shared_future::get()): Likewise.
	* testsuite/30_threads/shared_future/members/get.cc: Use const
	objects.

From-SVN: r195314