Addressed review comments

Author: H-G-Hristov

libcxx/include/__atomic/atomic_ref.h

@@ -221,7 +221,7 @@ struct __atomic_ref_base {
   _LIBCPP_HIDE_FROM_ABI void notify_one() const noexcept { std::__atomic_notify_one(*this); }
   _LIBCPP_HIDE_FROM_ABI void notify_all() const noexcept { std::__atomic_notify_all(*this); }
 #  if _LIBCPP_STD_VER >= 26
-  _LIBCPP_HIDE_FROM_ABI constexpr _Tp* address() const noexcept { return __ptr_; }
+  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr _Tp* address() const noexcept { return __ptr_; }
 #  endif
 
 protected:

libcxx/test/libcxx/diagnostics/atomic.nodiscard.verify.cpp

@@ -0,0 +1,37 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03
+
+// check that <atomic> functions are marked [[nodiscard]]
+
+// clang-format off
+
+#include <atomic>
+
+#include "atomic_helpers.h"
+#include "test_macros.h"
+
+template <typename T>
+struct TestAtomicRef {
+  void operator()() const {
+    T x(T(1));
+    const std::atomic_ref<T> a(x);
+
+#if TEST_STD_VER >= 26
+  a.address(); // expected-warning 4 {{ignoring return value of function declared with 'nodiscard' attribute}}
+#endif
+  }
+};
+
+void test() {
+  TestAtomicRef<UserAtomicType>()();
+  TestAtomicRef<int>()();
+  TestAtomicRef<float>()();
+  TestAtomicRef<char*>()();
+}

libcxx/test/std/atomics/atomics.ref/address.pass.cpp

@@ -15,6 +15,7 @@
 #include <atomic>
 #include <cassert>
 #include <concepts>
+#include <memory>
 
 #include "atomic_helpers.h"
 #include "test_macros.h"