Finish Fixing Atomic Initialization (P0883R2) (#390)

Co-authored-by: AdamBucior https://github.com/AdamBucior
Co-authored-by: Casey Carter <cartec69@gmail.com>
Co-authored-by: Billy O'Neal <billy.oneal@gmail.com>

stl/inc/atomic

@@ -1443,11 +1443,7 @@ public:
 
     using _Base::_Base;
 
-#ifdef __clang__ // TRANSITION, VSO-406237
     constexpr atomic() noexcept(is_nothrow_default_constructible_v<_Ty>) : _Base() {}
-#else // ^^^ no workaround / workaround vvv
-    atomic()                                  = default;
-#endif // TRANSITION, VSO-406237
 
     atomic(const atomic&) = delete;
     atomic& operator=(const atomic&) = delete;
@@ -1609,7 +1605,8 @@ _NODISCARD bool atomic_is_lock_free(const atomic<_Ty>* _Mem) noexcept {
 }
 
 template <class _Ty>
-void atomic_init(atomic<_Ty>* const _Mem, const typename atomic<_Ty>::value_type _Value) noexcept {
+_CXX20_DEPRECATE_ATOMIC_INIT void atomic_init(
+    atomic<_Ty>* const _Mem, const typename atomic<_Ty>::value_type _Value) noexcept {
 #if 1 // TRANSITION, ABI
     _CSTD memcpy(_STD addressof(_Mem->_Storage), _STD addressof(_Value), sizeof(_Ty));
 #else // ^^^ don't break ABI / break ABI vvv
@@ -1619,7 +1616,8 @@ void atomic_init(atomic<_Ty>* const _Mem, const typename atomic<_Ty>::value_type
 }
 
 template <class _Ty>
-void atomic_init(volatile atomic<_Ty>* const _Mem, const typename atomic<_Ty>::value_type _Value) noexcept {
+_CXX20_DEPRECATE_ATOMIC_INIT void atomic_init(
+    volatile atomic<_Ty>* const _Mem, const typename atomic<_Ty>::value_type _Value) noexcept {
     // NB: respecting volatility here appears unimplementable
     _STD atomic_init(const_cast<atomic<_Ty>*>(_Mem), _Value);
 }
@@ -1947,11 +1945,7 @@ struct atomic_flag { // flag with test-and-set semantics
         _Storage.store(false, _Order);
     }
 
-#ifdef __clang__ // TRANSITION, VSO-406237
     constexpr atomic_flag() noexcept = default;
-#else // ^^^ no workaround / workaround vvv
-    atomic_flag() noexcept = default;
-#endif // TRANSITION, VSO-406237
 
 #if 1 // TRANSITION, ABI
     atomic<long> _Storage;

stl/inc/yvals_core.h

@@ -54,6 +54,7 @@
 // P0771R1 noexcept For std::function's Move Constructor
 // P0777R1 Avoiding Unnecessary decay
 // P0809R0 Comparing Unordered Containers
+// P0883R2 Fixing Atomic Initialization
 // P0941R2 Feature-Test Macros
 // P0972R0 noexcept For <chrono> zero(), min(), max()
 // P1164R1 Making create_directory() Intuitive
@@ -886,7 +887,19 @@
 #define _CXX20_DEPRECATE_REL_OPS
 #endif // ^^^ warning disabled ^^^
 
-// next warning number: STL4028
+#if _HAS_CXX20 && !defined(_SILENCE_CXX20_ATOMIC_INIT_DEPRECATION_WARNING) \
+    && !defined(_SILENCE_ALL_CXX20_DEPRECATION_WARNINGS)
+#define _CXX20_DEPRECATE_ATOMIC_INIT                                                  \
+    [[deprecated("warning STL4028: "                                                  \
+                 "std::atomic_init() overloads are deprecated in C++20. "             \
+                 "The constructors of std::atomic provide equivalent functionality. " \
+                 "You can define _SILENCE_CXX20_ATOMIC_INIT_DEPRECATION_WARNING "     \
+                 "or _SILENCE_ALL_CXX20_DEPRECATION_WARNINGS to acknowledge that you have received this warning.")]]
+#else // ^^^ warning enabled / warning disabled vvv
+#define _CXX20_DEPRECATE_ATOMIC_INIT
+#endif // ^^^ warning disabled ^^^
+
+// next warning number: STL4029
 
 
 // LIBRARY FEATURE-TEST MACROS
@@ -977,6 +990,8 @@
 #endif // _HAS_CXX17
 
 // C++20
+#define __cpp_lib_atomic_value_initialization 201911L
+
 #if _HAS_CXX20
 #define __cpp_lib_atomic_float 201711L
 #define __cpp_lib_bind_front   201907L