[clang] [C2y] Add stdcountof.h (PR #140890)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Aaron Ballman (AaronBallman)

<details>
<summary>Changes</summary>

WG14 N3469 changed _Lengthof to _Countof but it also introduced the <stdcountof.h> header to expose a macro with a non-ugly identifier. GCC vends this header as part of the compiler implementation, so Clang should do the same.

---
Full diff: https://github.com/llvm/llvm-project/pull/140890.diff


3 Files Affected:

- (modified) clang/docs/ReleaseNotes.rst (+3-1) 
- (added) clang/lib/Headers/stdcountof.h (+15) 
- (modified) clang/test/C/C2y/n3469.c (+11-2) 


``````````diff
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index b466f3758e0b6..7b2777f711b8d 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -249,7 +249,9 @@ C2y Feature Support
   a conforming extension in earlier C language modes, but not in C++ language
   modes (``std::extent`` and ``std::size`` already provide the same
   functionality but with more granularity). The feature can be tested via
-  ``__has_feature(c_countof)`` or ``__has_extension(c_countof)``.
+  ``__has_feature(c_countof)`` or ``__has_extension(c_countof)``. This also
+  adds the ``<stdcountof.h>`` header file which exposes the ``countof`` macro
+  which expands to ``_Countof``.
 
 C23 Feature Support
 ^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Headers/stdcountof.h b/clang/lib/Headers/stdcountof.h
new file mode 100644
index 0000000000000..5714e6d6ff860
--- /dev/null
+++ b/clang/lib/Headers/stdcountof.h
@@ -0,0 +1,15 @@
+/*===---- stdcountof.h - Standard header for countof -----------------------===
+ *
+ * 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
+ *
+ *===-----------------------------------------------------------------------===
+ */
+
+#ifndef __STDCOUNTOF_H
+#define __STDCOUNTOF_H
+
+#define countof _Countof
+
+#endif /* __STDCOUNTOF_H */
diff --git a/clang/test/C/C2y/n3469.c b/clang/test/C/C2y/n3469.c
index 3d9ac8e6411e9..4660596614075 100644
--- a/clang/test/C/C2y/n3469.c
+++ b/clang/test/C/C2y/n3469.c
@@ -1,9 +1,9 @@
-// RUN: %clang_cc1 -fsyntax-only -std=c2y -verify %s
+// RUN: %clang_cc1 -fsyntax-only -std=c2y -verify -ffreestanding %s
 
 /* WG14 N3469: Clang 21
  * The Big Array Size Survey
  *
- * This renames _Lengthof to _Countof.
+ * This renames _Lengthof to _Countof and introduces the stdcountof.h header.
  */
 
 void test() {
@@ -12,3 +12,12 @@ void test() {
                                expected-error {{expected expression}}
 }
 
+#ifdef countof
+#error "why is countof defined as a macro?"
+#endif
+
+#include <stdcountof.h>
+
+#ifndef countof
+#error "why is countof not defined as a macro?"
+#endif

``````````

</details>


https://github.com/llvm/llvm-project/pull/140890