[libc++][atomic] P2835R7: Expose std::atomic_ref's object address

Implements https://wg21.link/P2835R7

Closes #118377

# References

- https://wg21.link/atomics.ref.generic.general
- https://wg21.link/atomics.ref.int
- https://wg21.link/atomics.ref.float
- https://wg21.link/atomics.ref.pointer

Author: H-G-Hristov

libcxx/docs/ReleaseNotes/22.rst

@@ -43,6 +43,7 @@ Implemented Papers
 - P3044R2: sub-``string_view`` from ``string`` (`Github <https://llvm.org/PR148140>`__)
 - P3223R2: Making ``std::istream::ignore`` less surprising (`Github <https://llvm.org/PR148178>`__)
 - P3060R3: Add ``std::views::indices(n)`` (`Github <https://llvm.org/PR148175>`__)
+- P2835R7: Expose ``std::atomic_ref``'s object address (`Github <https://llvm.org/PR118377>`__)
 - P3168R2: Give ``std::optional`` Range Support (`Github <https://llvm.org/PR105430>`__)
 
 Improvements and New Features

libcxx/docs/Status/Cxx2cPapers.csv

@@ -85,7 +85,7 @@
 "`P3222R0 <https://wg21.link/P3222R0>`__","Fix C++26 by adding transposed special cases for P2642 layouts","2024-11 (Wrocław)","","","`#118374 <https://github.com/llvm/llvm-project/issues/118374>`__",""
 "`P3050R2 <https://wg21.link/P3050R2>`__","Fix C++26 by optimizing ``linalg::conjugated`` for noncomplex value types","2024-11 (Wrocław)","","","`#118375 <https://github.com/llvm/llvm-project/issues/118375>`__",""
 "`P3396R1 <https://wg21.link/P3396R1>`__","``std::execution`` wording fixes","2024-11 (Wrocław)","","","`#118376 <https://github.com/llvm/llvm-project/issues/118376>`__",""
-"`P2835R7 <https://wg21.link/P2835R7>`__","Expose ``std::atomic_ref``'s object address","2024-11 (Wrocław)","","","`#118377 <https://github.com/llvm/llvm-project/issues/118377>`__",""
+"`P2835R7 <https://wg21.link/P2835R7>`__","Expose ``std::atomic_ref``'s object address","2024-11 (Wrocław)","|Complete|","22","`#118377 <https://github.com/llvm/llvm-project/issues/118377>`__",""
 "`P3323R1 <https://wg21.link/P3323R1>`__","cv-qualified types in ``atomic`` and ``atomic_ref``","2024-11 (Wrocław)","","","`#118378 <https://github.com/llvm/llvm-project/issues/118378>`__",""
 "`P3508R0 <https://wg21.link/P3508R0>`__","Wording for ""constexpr for specialized memory algorithms""","2024-11 (Wrocław)","","","`#118379 <https://github.com/llvm/llvm-project/issues/118379>`__",""
 "`P3369R0 <https://wg21.link/P3369R0>`__","constexpr for ``uninitialized_default_construct``","2024-11 (Wrocław)","","","`#118380 <https://github.com/llvm/llvm-project/issues/118380>`__",""

libcxx/include/__atomic/atomic_ref.h

@@ -220,6 +220,9 @@ 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_; }
+#endif
 
 protected:
   using _Aligned_Tp [[__gnu__::__aligned__(required_alignment), __gnu__::__nodebug__]] = _Tp;

libcxx/test/std/atomics/atomics.ref/address.pass.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
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: std-at-least-c++26
+
+// XFAIL: !has-64-bit-atomics
+// XFAIL: !has-1024-bit-atomics
+
+// constexpr T* address() const noexcept;
+
+#include <atomic>
+#include <cassert>
+
+#include "atomic_helpers.h"
+#include "test_macros.h"
+
+template <typename T>
+struct TestAddress {
+  void operator()() const {
+    T x(T(1));
+    std::atomic_ref<T> const a(x);
+
+    assert(a.address() == std::addressof(x));
+
+    static_assert(noexcept((a.address())));
+  }
+};
+
+int main(int, char**) {
+  TestEachAtomicType<TestAddress>()();
+
+  return 0;
+}