Re: Apple LLVM 10 and `__fallthrough__`

[Bruno Haible]

Hi Werner,

> In `lib/dfa.c` I see
> 
> ```
> ...
> # elif (__GNUC__ >= 7) || (__clang_major__ >= 10)
> #  define FALLTHROUGH __attribute__ ((__fallthrough__))
> ...
> ```
> 
> I now wonder whether it would be better to have a special case for
> Apple LLVM to avoid this warning.

Indeed, it's not the first time that we see that the Apple
compiler with a certain LLVM version behaves differently (more
like an older compiler) than the original LLVM compiler with that
version. One can use __apple_build_version__ or __APPLE_CC__ to detect
this situation.

> ```
> #include<stdio.h>
> 
> int main(int argc, char* argv[])
> {
>     switch(argc)
>     {
>     case 3:
>         puts(argv[2]);
>         __attribute__((fallthrough));
>     case 2:
>         puts(argv[1]);
>         __attribute__((__fallthrough__));
>     case 1:
>         puts(argv[0]);
>         /* fall through */
>     default:
>         puts("done");
>     }
> }
> ```
> 
> if compiled with
> 
> ```
> $ llvm-gcc --version
> Apple LLVM version 10.0.0 (clang-1000.10.44.4)
> Target: x86_64-apple-darwin17.7.0
> Thread model: posix
> InstalledDir: /Library/Developer/CommandLineTools/usr/bin
> ```
> 
> yields
> 
> ```
> nicola@Quark:freetype $ clang main.c
> main.c:9:9: warning: declaration does not declare anything 
> [-Wmissing-declarations]
>         __attribute__((fallthrough));
>         ^
> main.c:12:9: warning: declaration does not declare anything 
> [-Wmissing-declarations]
>         __attribute__((__fallthrough__));
>         ^
> 2 warnings generated.
> ```

On the other hand, the same file, compiled by

  Apple clang version 14.0.0 (clang-1400.0.29.202)
  Target: arm64-apple-darwin21.6.0
  Thread model: posix
  InstalledDir: /Library/Developer/CommandLineTools/usr/bin

produces no warnings.

I haven't been able to find out precisely which versions produce the warning,
by looking at https://github.com/apple/llvm-project . So, I'm applying the
conservative patch:


2023-02-26  Bruno Haible  <bruno@clisp.org>

        dfa: Avoid warnings with some Apple clang versions.
        Reported by Werner Lemberg <wl@gnu.org> in
        <https://lists.gnu.org/archive/html/bug-gnulib/2023-02/msg00159.html>.
        * lib/dfa.c (FALLTHROUGH): When __apple_build_version__ is defined,
        ignore __clang_major__.

diff --git a/lib/dfa.c b/lib/dfa.c
index 211e1ed18f..994900fea2 100644
--- a/lib/dfa.c
+++ b/lib/dfa.c
@@ -67,7 +67,10 @@ c_isdigit (char c)
 #ifndef FALLTHROUGH
 # if 201710L < __STDC_VERSION__
 #  define FALLTHROUGH [[__fallthrough__]]
-# elif (__GNUC__ >= 7) || (__clang_major__ >= 10)
+# elif ((__GNUC__ >= 7) \
+        || (defined __apple_build_version__ \
+            ? __apple_build_version__ >= 14000000 \
+            : __clang_major__ >= 10))
 #  define FALLTHROUGH __attribute__ ((__fallthrough__))
 # else
 #  define FALLTHROUGH ((void) 0)