From 0a000873392308fd0f4b4286b16814c5f3d8f41e Mon Sep 17 00:00:00 2001
From: "A. Jiang" <de34@live.cn>
Date: Sun, 18 Jun 2023 00:48:22 +0800
Subject: [PATCH] Implement LWG-3893

---
 stl/inc/memory                                     | 4 ++++
 tests/std/tests/P0718R2_atomic_smart_ptrs/test.cpp | 9 +++++++++
 2 files changed, 13 insertions(+)

diff --git a/stl/inc/memory b/stl/inc/memory
index 00a358493d1..5ed5de5528f 100644
--- a/stl/inc/memory
+++ b/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) {
diff --git a/tests/std/tests/P0718R2_atomic_smart_ptrs/test.cpp b/tests/std/tests/P0718R2_atomic_smart_ptrs/test.cpp
index 710a09a4545..0150911878d 100644
--- a/tests/std/tests/P0718R2_atomic_smart_ptrs/test.cpp
+++ b/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 = {};