Undeprecate ATOMIC_FLAG_INIT in C++

[aaron.ballman]

Summary

C++20 deprecated ATOMIC_FLAG_INIT thinking it was deprecated in C when it wasn't. It is expected to be undeprecated in C++23 as part of LWG3659 (https://wg21.link/LWG3659), which is currently Tentatively Ready as of yesterday.

I think similar changes should happen on the libc++ side, basically to undo parts of 4e730aeb731cb9b7116ed51bbef662d363f72b66, so I've included some of the libc++ folks here to see if they agree.

[tambre]

Makes sense to me, thanks!

[aaron.ballman]

In D129362#3638485, @tambre wrote:

Makes sense to me, thanks!

Thanks! Would you mind handling the libc++ side like last time, or do you prefer I handle it as part of the changes in this patch?

[tambre]

In D129362#3638896, @aaron.ballman wrote:

In D129362#3638485, @tambre wrote:

Makes sense to me, thanks!

Thanks! Would you mind handling the libc++ side like last time, or do you prefer I handle it as part of the changes in this patch?

Yeah, sure, see D129380.

[aaron.ballman]

In D129362#3639193, @tambre wrote:

In D129362#3638896, @aaron.ballman wrote:

In D129362#3638485, @tambre wrote:

Makes sense to me, thanks!

Thanks! Would you mind handling the libc++ side like last time, or do you prefer I handle it as part of the changes in this patch?

Yeah, sure, see D129380.

Thank you! :-)

[aaron.ballman]

Committed in 3cfa32a71ecfbc1bf993358e32b916cf3483299f

clang/docs/ReleaseNotes.rst

- As per "Conditionally Trivial Special Member Functions" (P0848), it is
   of the paper about other special member functions is not yet implemented.
 
 C++2b Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 
 - Implemented `P2128R6: Multidimensional subscript operator <https://wg21.link/P2128R6>`_.
 - Implemented `P0849R8: auto(x): decay-copy in the language <https://wg21.link/P0849R8>`_.
 - Implemented `P2242R3: Non-literal variables (and labels and gotos) in constexpr functions <https://wg21.link/P2242R3>`_.
+- Implemented `LWG3659: Consider ATOMIC_FLAG_INIT undeprecation <https://wg21.link/LWG3659>`_.
 
 CUDA/HIP Language Changes in Clang
 ----------------------------------
 
 - Added `__noinline__` as a keyword to avoid diagnostics due to usage of
   `__attribute__((__noinline__))` in CUDA/HIP programs.
 
 Objective-C Language Changes in Clang

clang/lib/Headers/stdatomic.h

 #define atomic_fetch_and(object, operand) __c11_atomic_fetch_and(object, operand, __ATOMIC_SEQ_CST)
 #define atomic_fetch_and_explicit __c11_atomic_fetch_and
 
 /* 7.17.8 Atomic flag type and operations */
 
 typedef struct atomic_flag { atomic_bool _Value; } atomic_flag;
 
 #define ATOMIC_FLAG_INIT { 0 }
-#if __cplusplus >= 202002L && !defined(_CLANG_DISABLE_CRT_DEPRECATION_WARNINGS)
-/* ATOMIC_FLAG_INIT was deprecated in C++20 but is not deprecated in C. */
-#pragma clang deprecated(ATOMIC_FLAG_INIT)
-#endif
 
 /* These should be provided by the libc implementation. */
 #ifdef __cplusplus
 bool atomic_flag_test_and_set(volatile atomic_flag *);
 bool atomic_flag_test_and_set_explicit(volatile atomic_flag *, memory_order);
 #else
 _Bool atomic_flag_test_and_set(volatile atomic_flag *);
 _Bool atomic_flag_test_and_set_explicit(volatile atomic_flag *, memory_order);

clang/test/Headers/stdatomic-deprecations.c

 // RUN: %clang_cc1 -fsyntax-only -std=c11 %s -verify=okay
 // RUN: %clang_cc1 -fsyntax-only -std=c17 %s -verify
 // RUN: %clang_cc1 -fsyntax-only -std=c++11 -x c++ %s -verify=okay
 // RUN: %clang_cc1 -fsyntax-only -std=c++20 -x c++ %s -verify=cxx,expected
 // RUN: %clang_cc1 -fsyntax-only -std=c++20 -x c++ %s -verify=okay -D_CLANG_DISABLE_CRT_DEPRECATION_WARNINGS
 // RUN: %clang_cc1 -fsyntax-only -std=c17 %s -verify=okay -D_CLANG_DISABLE_CRT_DEPRECATION_WARNINGS
 //
 // okay-no-diagnostics
 
 #include <stdatomic.h>
 
 void func(void) {
   _Atomic int i = ATOMIC_VAR_INIT(12); // expected-warning {{macro 'ATOMIC_VAR_INIT' has been marked as deprecated}} \
                                        // expected-note@stdatomic.h:* {{macro marked 'deprecated' here}}
-  #if defined(ATOMIC_FLAG_INIT) // cxx-warning {{macro 'ATOMIC_FLAG_INIT' has been marked as deprecated}} \
-                                // cxx-note@stdatomic.h:* {{macro marked 'deprecated' here}}
+  #if defined(ATOMIC_FLAG_INIT) // Ok, deprecated in C++20, undeprecated in C++23 via LWG4659.
   #endif
 }