Implement LWG-4169

Author: frederick-vs-ja

stl/inc/atomic

@@ -2123,6 +2123,7 @@ public:
 
     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() {}
 
     atomic(const atomic&)                     = 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() {