[libcxx-commits] [libcxx] [libc++] cv-qualified types in atomic and atomic_ref (P3323R1) (PR #121414)

[A. Jiang via libcxx-commits]

================
@@ -317,56 +414,112 @@ struct atomic_ref<_Tp> : public __atomic_ref_base<_Tp> {
 
   _LIBCPP_HIDE_FROM_ABI atomic_ref(const atomic_ref&) noexcept = default;
 
-  _LIBCPP_HIDE_FROM_ABI _Tp operator=(_Tp __desired) const noexcept { return __base::operator=(__desired); }
+  _LIBCPP_HIDE_FROM_ABI value_type operator=(value_type __desired) const noexcept
+    requires(!is_const_v<_Tp>)
+  {
+    return __base::operator=(__desired);
+  }
 
   atomic_ref& operator=(const atomic_ref&) = delete;
 
-  _LIBCPP_HIDE_FROM_ABI _Tp fetch_add(_Tp __arg, memory_order __order = memory_order_seq_cst) const noexcept {
-    _Tp __old = this->load(memory_order_relaxed);
-    _Tp __new = __old + __arg;
+  _LIBCPP_HIDE_FROM_ABI value_type
+  fetch_add(value_type __arg, memory_order __order = memory_order_seq_cst) const noexcept
+    requires(!is_const_v<_Tp>)
+  {
+    value_type __old = this->load(memory_order_relaxed);
+    value_type __new = __old + __arg;
     while (!this->compare_exchange_weak(__old, __new, __order, memory_order_relaxed)) {
       __new = __old + __arg;
     }
     return __old;
   }
-  _LIBCPP_HIDE_FROM_ABI _Tp fetch_sub(_Tp __arg, memory_order __order = memory_order_seq_cst) const noexcept {
-    _Tp __old = this->load(memory_order_relaxed);
-    _Tp __new = __old - __arg;
+  _LIBCPP_HIDE_FROM_ABI value_type
+  fetch_sub(value_type __arg, memory_order __order = memory_order_seq_cst) const noexcept
+    requires(!is_const_v<_Tp>)
+  {
+    value_type __old = this->load(memory_order_relaxed);
+    value_type __new = __old - __arg;
     while (!this->compare_exchange_weak(__old, __new, __order, memory_order_relaxed)) {
       __new = __old - __arg;
     }
     return __old;
   }
 
-  _LIBCPP_HIDE_FROM_ABI _Tp operator+=(_Tp __arg) const noexcept { return fetch_add(__arg) + __arg; }
-  _LIBCPP_HIDE_FROM_ABI _Tp operator-=(_Tp __arg) const noexcept { return fetch_sub(__arg) - __arg; }
+  _LIBCPP_HIDE_FROM_ABI value_type operator+=(value_type __arg) const noexcept
+    requires(!is_const_v<_Tp>)
+  {
+    return fetch_add(__arg) + __arg;
+  }
+  _LIBCPP_HIDE_FROM_ABI value_type operator-=(value_type __arg) const noexcept
+    requires(!is_const_v<_Tp>)
+  {
+    return fetch_sub(__arg) - __arg;
+  }
 };
 
 template <class _Tp>
-struct atomic_ref<_Tp*> : public __atomic_ref_base<_Tp*> {
-  using __base = __atomic_ref_base<_Tp*>;
+  requires(std::is_pointer_v<_Tp>)
----------------
frederick-vs-ja wrote:

Per [[atomics.ref.pointer]/1](https://eel.is/c++draft/atomics.ref.pointer#1), only pointer-to-object types are specially handled.

```suggestion
  requires(is_pointer_v<_Tp> && is_object_v<remove_pointer_t<_Tp>>)
```

Also, `<__type_traits/is_object.h>` should be included, and `std/atomics/atomics.ref/member_types.compile.pass.cpp` needs to be modified.

BTW, we don't need to add `std::` for non-function things unless there's conflict.

https://github.com/llvm/llvm-project/pull/121414