Implement LWG-4169 std::atomic<T>'s default constructor should be constrained (#5128)

Co-authored-by: Casey Carter <[email protected]>

Author: frederick-vs-ja

stl/inc/atomic

@@ -586,8 +586,7 @@ struct _Atomic_storage {
 
     _Atomic_storage() = default;
 
-    /* implicit */ constexpr _Atomic_storage(conditional_t<is_reference_v<_Ty>, _Ty, const _TVal> _Value) noexcept
-        : _Storage(_Value) {
+    /* implicit */ constexpr _Atomic_storage(const _Ty& _Value) noexcept : _Storage(_Value) {
         // non-atomically initialize this atomic
     }
 
@@ -714,13 +713,11 @@ public:
 
 template <class _Ty>
 struct _Atomic_storage<_Ty, 1> { // lock-free using 1-byte intrinsics
-
     using _TVal = remove_reference_t<_Ty>;
 
     _Atomic_storage() = default;
 
-    /* implicit */ constexpr _Atomic_storage(conditional_t<is_reference_v<_Ty>, _Ty, const _TVal> _Value) noexcept
-        : _Storage{_Value} {
+    /* implicit */ constexpr _Atomic_storage(const _Ty& _Value) noexcept : _Storage{_Value} {
         // non-atomically initialize this atomic
     }
 
@@ -817,13 +814,11 @@ struct _Atomic_storage<_Ty, 1> { // lock-free using 1-byte intrinsics
 
 template <class _Ty>
 struct _Atomic_storage<_Ty, 2> { // lock-free using 2-byte intrinsics
-
     using _TVal = remove_reference_t<_Ty>;
 
     _Atomic_storage() = default;
 
-    /* implicit */ constexpr _Atomic_storage(conditional_t<is_reference_v<_Ty>, _Ty, const _TVal> _Value) noexcept
-        : _Storage{_Value} {
+    /* implicit */ constexpr _Atomic_storage(const _Ty& _Value) noexcept : _Storage{_Value} {
         // non-atomically initialize this atomic
     }
 
@@ -919,13 +914,11 @@ struct _Atomic_storage<_Ty, 2> { // lock-free using 2-byte intrinsics
 
 template <class _Ty>
 struct _Atomic_storage<_Ty, 4> { // lock-free using 4-byte intrinsics
-
     using _TVal = remove_reference_t<_Ty>;
 
     _Atomic_storage() = default;
 
-    /* implicit */ constexpr _Atomic_storage(conditional_t<is_reference_v<_Ty>, _Ty, const _TVal> _Value) noexcept
-        : _Storage{_Value} {
+    /* implicit */ constexpr _Atomic_storage(const _Ty& _Value) noexcept : _Storage{_Value} {
         // non-atomically initialize this atomic
     }
 
@@ -1021,13 +1014,11 @@ struct _Atomic_storage<_Ty, 4> { // lock-free using 4-byte intrinsics
 
 template <class _Ty>
 struct _Atomic_storage<_Ty, 8> { // lock-free using 8-byte intrinsics
-
     using _TVal = remove_reference_t<_Ty>;
 
     _Atomic_storage() = default;
 
-    /* implicit */ constexpr _Atomic_storage(conditional_t<is_reference_v<_Ty>, _Ty, const _TVal> _Value) noexcept
-        : _Storage{_Value} {
+    /* implicit */ constexpr _Atomic_storage(const _Ty& _Value) noexcept : _Storage{_Value} {
         // non-atomically initialize this atomic
     }
 
@@ -1148,7 +1139,8 @@ struct _Atomic_storage<_Ty&, 16> { // lock-free using 16-byte intrinsics
 
     _Atomic_storage() = default;
 
-    /* implicit */ constexpr _Atomic_storage(conditional_t<is_reference_v<_Ty&>, _Ty&, const _TVal> _Value) noexcept
+    // TRANSITION, ABI: replace _this_ occurrence of '_Ty&' with 'const _Ty&'
+    /* implicit */ constexpr _Atomic_storage(_Ty& _Value) noexcept
         : _Storage{_Value} {} // non-atomically initialize this atomic
 
     void store(const _TVal _Value) noexcept { // store with sequential consistency
@@ -2121,10 +2113,11 @@ public:
 
     using value_type = _Ty;
 
-    using _Base::_Base;
-
+    template <class _Uty = _Ty, enable_if_t<is_default_constructible_v<_Uty>, int> = 0>
     constexpr atomic() noexcept(is_nothrow_default_constructible_v<_Ty>) : _Base() {}
 
+    /* implicit */ constexpr atomic(const _Ty _Value) noexcept : _Base(_Value) {}
+
     atomic(const atomic&)                     = delete;
     atomic& operator=(const atomic&)          = delete;
     atomic& operator=(const atomic&) volatile = delete;

tests/std/tests/Dev11_0863628_atomic_compare_exchange/test.cpp

@@ -12,11 +12,11 @@
 #include <cstdint>
 #include <cstdlib>
 #include <cstring>
+#include <functional>
 #include <limits>
 #include <new>
 #include <type_traits>
 
-
 using namespace std;
 
 #define STATIC_ASSERT(...) static_assert(__VA_ARGS__, #__VA_ARGS__)
@@ -438,6 +438,17 @@ STATIC_ASSERT(atomic<void*>::is_always_lock_free);
 STATIC_ASSERT(atomic<int (*)(int)>::is_always_lock_free);
 #endif // _HAS_CXX17
 
+// Also test LWG-4169 std::atomic<T>'s default constructor should be constrained
+// (backported to C++14/17 modes as we backported P0883R2)
+STATIC_ASSERT(is_default_constructible_v<atomic<int>>);
+STATIC_ASSERT(is_default_constructible_v<atomic<bool>>);
+STATIC_ASSERT(is_default_constructible_v<atomic<void*>>);
+STATIC_ASSERT(is_default_constructible_v<atomic<X>>);
+STATIC_ASSERT(is_default_constructible_v<atomic<Y>>);
+STATIC_ASSERT(!is_default_constructible_v<atomic<reference_wrapper<int>>>);
+STATIC_ASSERT(!is_default_constructible_v<atomic<reference_wrapper<const int>>>);
+STATIC_ASSERT(!is_default_constructible_v<atomic<reference_wrapper<int()>>>);
+
 
 // Also test P0418R2 atomic compare_exchange memory_order Requirements
 void test_compare_exchange_relaxed_memory_orders() {