Implement LWG-3893 LWG-3661 broke atomic<shared_ptr<T>> a; a = nullptr; (#3782)

Author: frederick-vs-ja

stl/inc/memory

@@ -4003,6 +4003,10 @@ public:
         store(_STD move(_Value));
     }
 
+    void operator=(nullptr_t) noexcept {
+        store(nullptr);
+    }
+
     ~atomic() {
         const auto _Rep = this->_Repptr._Unsafe_load_relaxed();
         if (_Rep) {

tests/std/tests/P0718R2_atomic_smart_ptrs/test.cpp

@@ -2,9 +2,11 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 #include <cassert>
+#include <cstddef>
 #include <cstdint>
 #include <memory>
 #include <thread>
+#include <type_traits>
 #ifdef _DEBUG
 #include <crtdbg.h>
 #endif // _DEBUG
@@ -632,6 +634,13 @@ int main() {
     ensure_member_calls_compile<atomic<shared_ptr<int[2][2]>>>();
     ensure_member_calls_compile<atomic<weak_ptr<int[2][2]>>>();
 
+    // LWG-3893: LWG 3661 broke atomic<shared_ptr<T>> a; a = nullptr;
+    static_assert(is_nothrow_assignable_v<atomic<shared_ptr<bool>>, nullptr_t>);
+    static_assert(is_nothrow_assignable_v<atomic<shared_ptr<int>>, nullptr_t>);
+    static_assert(is_nothrow_assignable_v<atomic<shared_ptr<int[]>>, nullptr_t>);
+    static_assert(is_nothrow_assignable_v<atomic<shared_ptr<int[][2]>>, nullptr_t>);
+    static_assert(is_nothrow_assignable_v<atomic<shared_ptr<int[2][2]>>, nullptr_t>);
+
 #ifdef _DEBUG
     sptr0 = {};
     sptr1 = {};