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

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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: 1e0affb6e5
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Joshua Root 2025-01-22 20:25:30 +11:00 • committed by GitHub
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commit edf803345a
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2 changed files with 8 additions and 2 deletions
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9
Include/pyport.h
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@ -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)