libstdc++: Use type_identity_t for non-deducible std::atomic_xxx args

This is LWG 3220 which is about to become Tentatively Ready.

libstdc++-v3/ChangeLog:

	* include/std/atomic (__atomic_val_t): Use __type_identity_t
	instead of atomic<T>::value_type, as per LWG 3220.
	* testsuite/29_atomics/atomic/lwg3220.cc: New test.

libstdc++-v3/include/std/atomic

@@ -1244,8 +1244,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   { atomic_flag_clear_explicit(__a, memory_order_seq_cst); }
 
   /// @cond undocumented
+  // _GLIBCXX_RESOLVE_LIB_DEFECTS
+  // 3220. P0558 broke conforming C++14 uses of atomic shared_ptr
   template<typename _Tp>
-    using __atomic_val_t = typename atomic<_Tp>::value_type;
+    using __atomic_val_t = __type_identity_t<_Tp>;
   template<typename _Tp>
     using __atomic_diff_t = typename atomic<_Tp>::difference_type;
   /// @endcond

libstdc++-v3/testsuite/29_atomics/atomic/lwg3220.cc

@@ -0,0 +1,13 @@
+// { dg-do compile { target c++11 } }
+// DR 3220. P0558 broke conforming C++14 uses of atomic shared_ptr
+
+#include <atomic>
+#include <memory>
+
+struct Abstract { virtual void test() = 0; };
+struct Concrete : Abstract { virtual void test() override {} };
+
+int main() {
+  std::shared_ptr<Abstract> ptr;
+  std::atomic_store<Abstract>(&ptr, std::make_shared<Concrete>());
+}