From 6e81ed68c59e4af13bbe53fbf666435204f9df58 Mon Sep 17 00:00:00 2001
From: "Stephan T. Lavavej" <stl@nuwen.net>
Date: Sun, 16 Jun 2024 07:10:11 -0700
Subject: [PATCH 1/3] Fix GH 4728.

For `!_ATOMIC_HAS_DCAS`:
If we're `is_always_lock_free` (power of two, at most 1 pointer), return true.
Otherwise, we have to be `_Is_potentially_lock_free` (power of two, at most 2 pointers) and have cmpxchg16b.
---
 stl/inc/atomic | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/stl/inc/atomic b/stl/inc/atomic
index 5f7e7300499..d093032d371 100644
--- a/stl/inc/atomic
+++ b/stl/inc/atomic
@@ -2371,7 +2371,7 @@ public:
         if constexpr (is_always_lock_free) {
             return true;
         } else {
-            return __std_atomic_has_cmpxchg16b() != 0;
+            return _Is_potentially_lock_free && __std_atomic_has_cmpxchg16b() != 0;
         }
 #endif // ^^^ !_ATOMIC_HAS_DCAS ^^^
     }

From 5e03fc6bf728b2ccad3f38faa6335647a53f95c8 Mon Sep 17 00:00:00 2001
From: "Stephan T. Lavavej" <stl@nuwen.net>
Date: Sun, 16 Jun 2024 08:39:24 -0700
Subject: [PATCH 2/3] Test GH 4728.

---
 tests/std/tests/P0019R8_atomic_ref/test.cpp | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/tests/std/tests/P0019R8_atomic_ref/test.cpp b/tests/std/tests/P0019R8_atomic_ref/test.cpp
index 61c25f68eb3..6cdfd4518ad 100644
--- a/tests/std/tests/P0019R8_atomic_ref/test.cpp
+++ b/tests/std/tests/P0019R8_atomic_ref/test.cpp
@@ -447,6 +447,22 @@ void test_gh_4472() {
     assert(ar.is_lock_free());
 }
 
+// GH-4728 "<atomic>: On x64, atomic_ref::is_lock_free() incorrectly returns true when it shouldn't"
+void test_gh_4728() {
+    struct Large {
+        char str[100]{};
+    };
+
+    alignas(std::atomic_ref<Large>::required_alignment) Large lg{};
+
+    static_assert(std::atomic_ref<Large>::required_alignment == alignof(Large));
+
+    static_assert(!std::atomic_ref<Large>::is_always_lock_free);
+
+    std::atomic_ref<Large> ar{lg};
+    assert(!ar.is_lock_free());
+}
+
 int main() {
     test_ops<false, char>();
     test_ops<false, signed char>();
@@ -500,6 +516,7 @@ int main() {
 
     test_gh_1497();
     test_gh_4472();
+    test_gh_4728();
 }
 
 #endif // ^^^ run test ^^^

From a76faa5f917c9c95329d828bb58bfb09661e9a69 Mon Sep 17 00:00:00 2001
From: "Stephan T. Lavavej" <stl@nuwen.net>
Date: Mon, 17 Jun 2024 07:23:13 -0700
Subject: [PATCH 3/3] Use `if constexpr (_Is_potentially_lock_free)`.

Co-authored-by: Alex Guteniev <gutenev@gmail.com>
---
 stl/inc/atomic | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/stl/inc/atomic b/stl/inc/atomic
index d093032d371..307168b01e2 100644
--- a/stl/inc/atomic
+++ b/stl/inc/atomic
@@ -2370,8 +2370,10 @@ public:
 #else // ^^^ _ATOMIC_HAS_DCAS / !_ATOMIC_HAS_DCAS vvv
         if constexpr (is_always_lock_free) {
             return true;
+        } else if constexpr (_Is_potentially_lock_free) {
+            return __std_atomic_has_cmpxchg16b() != 0;
         } else {
-            return _Is_potentially_lock_free && __std_atomic_has_cmpxchg16b() != 0;
+            return false;
         }
 #endif // ^^^ !_ATOMIC_HAS_DCAS ^^^
     }