gh-128902: Fix check for fallthrough attribute support (#128903)

Clang versions prior to 10 (which corresponds to Apple Clang 12) do not
support the GCC extension syntax __attribute__((fallthrough)), but do
evaluate __has_attribute(fallthrough) to 1 because they support the
C++11 style syntax [[fallthrough]]. The only way to tell if the GCC
style syntax is supported is thus to check the clang version.

Ref: llvm/llvm-project@1e0affb

Author: jmroot

Include/pyport.h

@@ -625,8 +625,13 @@ extern "C" {
 //     case 2: code; break;
 //     }
 //
-// __attribute__((fallthrough)) was introduced in GCC 7.
-#if _Py__has_attribute(fallthrough)
+// __attribute__((fallthrough)) was introduced in GCC 7 and Clang 10 /
+// Apple Clang 12.0. Earlier Clang versions support only the C++11
+// style fallthrough attribute, not the GCC extension syntax used here,
+// and __has_attribute(fallthrough) evaluates to 1.
+#if _Py__has_attribute(fallthrough) && (!defined(__clang__) || \
+    (!defined(__apple_build_version__) && __clang_major__ >= 10) || \
+    (defined(__apple_build_version__) && __clang_major__ >= 12))
 #  define _Py_FALLTHROUGH __attribute__((fallthrough))
 #else
 #  define _Py_FALLTHROUGH do { } while (0)

Misc/NEWS.d/next/Build/2025-01-16-03-35-37.gh-issue-128902.Dt7xtV.rst

@@ -0,0 +1 @@
+Fix compile errors with Clang 9 and older due to lack of ``__attribute__((fallthrough))`` support.