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[libc++] Make _Atomic(T) directly use an alias template #168679

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[libc++] Make _Atomic(T) directly use an alias template #168679

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7 changes: 6 additions & 1 deletion libcxx/include/stdatomic.h
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Original file line number Diff line number Diff line change
Expand Up @@ -135,7 +135,12 @@ using std::atomic_signal_fence // see below
# undef _Atomic
# endif

# define _Atomic(_Tp) ::std::atomic<_Tp>
_LIBCPP_BEGIN_NAMESPACE_STD
template <class _Tp>
using __libcpp_atomic_alias _LIBCPP_NODEBUG = atomic<_Tp>;
_LIBCPP_END_NAMESPACE_STD

# define _Atomic(_Tp) ::std::__libcpp_atomic_alias<_Tp>

using std::memory_order _LIBCPP_USING_IF_EXISTS;
using std::memory_order_relaxed _LIBCPP_USING_IF_EXISTS;
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25 changes: 25 additions & 0 deletions libcxx/test/std/atomics/stdatomic.h.syn/alias.template.verify.cpp
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@@ -0,0 +1,25 @@
//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

// REQUIRES: std-at-least-c++23
// UNSUPPORTED: no-threads

// <stdatomic.h>

// template<class T>
// using std-atomic = std::atomic<T>; // exposition only
//
// #define _Atomic(T) std-atomic<T>

// Verify that _Atomic(T) directly uses an alias template but not the std::atomic class template.
// See also https://llvm.org/PR168579.

#include <stdatomic.h>

struct _Atomic(int) x;
// expected-error-re@-1{{alias template '{{.*}}' cannot be referenced with the 'struct' specifier}}
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