Mark ATOMIC_VAR_INIT and ATOMIC_FLAG_INIT as deprecated

C17 deprecated ATOMIC_VAR_INIT with the resolution of DR 485. C++
followed suit when adopting P0883R2 for C++20, but additionally chose
to deprecate ATOMIC_FLAG_INIT at the same time despite the macro still
being required in C. This patch marks both macros as deprecated when
appropriate to do so.

clang/docs/ReleaseNotes.rst

@@ -169,6 +169,11 @@ C Language Changes in Clang
   which adds ``*_WIDTH`` macros to limits.h and stdint.h to report the bit
   width of various integer datatypes.
 
+- The ``ATOMIC_VAR_INIT`` macro from ``<stdatomic.h>`` is now diagnosed as
+  deprecated in C17 and later. The diagnostic can be disabled by defining the
+  ``_CLANG_DISABLE_CRT_DEPRECATION_WARNINGS`` macro prior to including the
+  header.
+
 C++ Language Changes in Clang
 -----------------------------
 
@@ -176,7 +181,15 @@ C++ Language Changes in Clang
 
 C++20 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
-...
+
+- The ``ATOMIC_VAR_INIT`` and ``ATOMIC_FLAG_INIT`` macros from the C standard
+  library ``<stdatomic.h>`` header are now diagnosed as deprecated in C++20 and
+  later. Note, the behavior is specific to the inclusion of ``<stdatomic.h>``
+  in C++ code; the STL ``<atomic>`` header also controls the behavior of these
+  macros and is not affected by these changes. The ``<stdatomic.h>`` diagnostic
+  can be disabled by defining the ``_CLANG_DISABLE_CRT_DEPRECATION_WARNINGS``
+  macro prior to including the header.
+
 
 C++2b Feature Support
 ^^^^^^^^^^^^^^^^^^^^^

clang/docs/UsersManual.rst

@@ -1132,10 +1132,28 @@ A ``#include`` directive which finds a file relative to the current
 directory is treated as including a system header if the including file
 is treated as a system header.
 
+Controlling Deprecation Diagnostics in Clang-Provided C Runtime Headers
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Clang is responsible for providing some of the C runtime headers that cannot be
+provided by a platform CRT, such as implementation limits or when compiling in
+freestanding mode. Define the ``_CLANG_DISABLE_CRT_DEPRECATION_WARNINGS`` macro
+prior to including such a C runtime header to disable the deprecation warnings.
+Note that the C Standard Library headers are allowed to transitively include
+other standard library headers (see 7.1.2p5), and so the most appropriate use
+of this macro is to set it within the build system using ``-D`` or before any
+include directives in the translation unit.
+
+.. code-block:: c
+
+  #define _CLANG_DISABLE_CRT_DEPRECATION_WARNINGS
+  #include <stdint.h>    // Clang CRT deprecation warnings are disabled.
+  #include <stdatomic.h> // Clang CRT deprecation warnings are disabled.
+
 .. _diagnostics_enable_everything:
 
 Enabling All Diagnostics
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+^^^^^^^^^^^^^^^^^^^^^^^^
 
 In addition to the traditional ``-W`` flags, one can enable **all** diagnostics
 by passing :option:`-Weverything`. This works as expected with

clang/lib/Headers/stdatomic.h

@@ -44,6 +44,11 @@ extern "C" {
 /* 7.17.2 Initialization */
 
 #define ATOMIC_VAR_INIT(value) (value)
+#if (__STDC_VERSION__ >= 201710L || __cplusplus >= 202002L) &&                 \
+    !defined(_CLANG_DISABLE_CRT_DEPRECATION_WARNINGS)
+/* ATOMIC_VAR_INIT was deprecated in C17 and C++20. */
+#pragma clang deprecated(ATOMIC_VAR_INIT)
+#endif
 #define atomic_init __c11_atomic_init
 
 /* 7.17.3 Order and consistency */
@@ -153,6 +158,10 @@ typedef _Atomic(uintmax_t)          atomic_uintmax_t;
 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

clang/test/Headers/stdatomic-deprecations.c

@@ -0,0 +1,18 @@
+// 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}}
+  #endif
+}