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

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merged 5 commits into from
Dec 5, 2024
Merged

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

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0750a60
Implement LWG-4169
frederick-vs-ja Nov 24, 2024
e682b08
Workaround for EDG
frederick-vs-ja Nov 24, 2024
b726bc1
Revert "Workaround for EDG"
frederick-vs-ja Nov 25, 2024
ca9beb8
Simplify `_Atomic_storage` converting constructors
CaseyCarter Nov 28, 2024
576324e
Remove paranoia `static_assert`s
CaseyCarter Nov 28, 2024
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13 changes: 11 additions & 2 deletions stl/inc/atomic
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Original file line number Diff line number Diff line change
Expand Up @@ -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;
Expand Down Expand Up @@ -2803,9 +2804,17 @@ _EXPORT_STD struct atomic_flag { // flag with test-and-set semantics
#endif // _HAS_CXX20

#if 1 // TRANSITION, ABI
atomic<long> _Storage;
atomic<long> _Storage
#if !_HAS_CXX20 && defined(__EDG__)
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{0L}
#endif // ^^^ workaround ^^^
;
#else // ^^^ don't break ABI / break ABI vvv
atomic<bool> _Storage;
atomic<bool> _Storage
#if !_HAS_CXX20 && defined(__EDG__)
{false}
#endif // ^^^ workaround ^^^
;
#endif // ^^^ break ABI ^^^
};

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13 changes: 12 additions & 1 deletion tests/std/tests/Dev11_0863628_atomic_compare_exchange/test.cpp
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Expand Up @@ -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__)
Expand Down Expand Up @@ -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() {
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