Re: [PATCH] Use -Wimplicit-fallthrough only with GCC >= 7

[Bruno Haible]

Hi José,

> The -Wimplicit-fallthrough switch was introduced in GCC 7.
> 
> Older version of GCC (like 3 and 4) do not complain when they get passed
> an invalid -WFOO option

It depends on the option:

$ gcc-version 3.1 -Wimplicit-fallthrough -S /tmp/empty.c   
$ gcc-version 3.2.2 -Wimplicit-fallthrough -S /tmp/empty.c    
$ gcc-version 3.3.6 -Wimplicit-fallthrough -S /tmp/empty.c    
$ gcc-version 3.4.4 -Wimplicit-fallthrough -S /tmp/empty.c    
$ gcc-version 4.0.4 -Wimplicit-fallthrough -S /tmp/empty.c      
cc1: error: unrecognized command line option "-Wimplicit-fallthrough"
$ gcc-version 4.1.2 -Wimplicit-fallthrough -S /tmp/empty.c      
cc1: error: unrecognized command line option "-Wimplicit-fallthrough"

$ gcc-version 3.1 -Wimplicit-fallthrough -S /tmp/empty.c  
$ gcc-version 3.1 -Wimplicit-bullshit -S /tmp/empty.c                       
$ gcc-version 3.1 -Wimplicit- -S /tmp/empty.c    
$ gcc-version 3.1 -Wimplicit -S /tmp/empty.c 
$ gcc-version 3.1 -Wimplici -S /tmp/empty.c 
cc1: unrecognized option `-Wimplici'

> , but GCC 6 bails out like this:
> 
>   gcc: error: unrecognized command line option '-Wimplicit-fallthrough';
>   did you mean '-Wno-fallthrough'?
> 
> diff --git a/m4/gnulib-common.m4 b/m4/gnulib-common.m4
> index 179aac7aa1..62741f830f 100644
> --- a/m4/gnulib-common.m4
> +++ b/m4/gnulib-common.m4
> @@ -896,7 +896,7 @@ AC_DEFUN([gl_CC_GNULIB_WARNINGS],
>      dnl -Wno-conversion                       >= 3            >= 3.9
>      dnl -Wno-float-conversion                 >= 4.9          >= 3.9
>      dnl -Wno-float-equal                      >= 3            >= 3.9
> -    dnl -Wimplicit-fallthrough                >= 3            >= 3.9
> +    dnl -Wimplicit-fallthrough                >= 7            >= 3.9
>      dnl -Wno-pedantic                         >= 4.8          >= 3.9
>      dnl -Wno-sign-compare                     >= 3            >= 3.9
>      dnl -Wno-sign-conversion                  >= 4.3          >= 3.9
> @@ -911,12 +911,14 @@ AC_DEFUN([gl_CC_GNULIB_WARNINGS],
>        -Wno-cast-qual
>        -Wno-conversion
>        -Wno-float-equal
> -      -Wimplicit-fallthrough
>        -Wno-sign-compare
>        -Wno-undef
>        -Wno-unused-function
>        -Wno-unused-parameter
>        #endif
> +      #if __GNUC__ => 7 || (__clang_major__ + (__clang_minor__ >= 9) > 3)
> +      -Wimplicit-fallthrough
> +      #endif
>        #if __GNUC__ + (__GNUC_MINOR__ >= 9) > 4 || (__clang_major__ + 
> (__clang_minor__ >= 9) > 3)
>        -Wno-float-conversion
>        #endif

Thanks. I applied your patch, with the obligatory typo fix.

Bruno


2022-01-05  Bruno Haible  <bruno@clisp.org>

        Fix last commit.
        Based on a patch by José E. Marchesi <jemarch@gnu.org>, reported in
        <https://lists.gnu.org/archive/html/bug-gnulib/2022-01/msg00036.html>.
        * m4/gnulib-common.m4 (gl_CC_GNULIB_WARNINGS): Don't enable
        -Wimplicit-fallthrough for GCC < 7.

diff --git a/m4/gnulib-common.m4 b/m4/gnulib-common.m4
index 179aac7aa1..afe0836083 100644
--- a/m4/gnulib-common.m4
+++ b/m4/gnulib-common.m4
@@ -896,7 +896,7 @@ AC_DEFUN([gl_CC_GNULIB_WARNINGS],
     dnl -Wno-conversion                       >= 3            >= 3.9
     dnl -Wno-float-conversion                 >= 4.9          >= 3.9
     dnl -Wno-float-equal                      >= 3            >= 3.9
-    dnl -Wimplicit-fallthrough                >= 3            >= 3.9
+    dnl -Wimplicit-fallthrough                >= 7            >= 3.9
     dnl -Wno-pedantic                         >= 4.8          >= 3.9
     dnl -Wno-sign-compare                     >= 3            >= 3.9
     dnl -Wno-sign-conversion                  >= 4.3          >= 3.9
@@ -911,7 +911,6 @@ AC_DEFUN([gl_CC_GNULIB_WARNINGS],
       -Wno-cast-qual
       -Wno-conversion
       -Wno-float-equal
-      -Wimplicit-fallthrough
       -Wno-sign-compare
       -Wno-undef
       -Wno-unused-function
@@ -920,6 +919,9 @@ AC_DEFUN([gl_CC_GNULIB_WARNINGS],
       #if __GNUC__ + (__GNUC_MINOR__ >= 9) > 4 || (__clang_major__ + 
(__clang_minor__ >= 9) > 3)
       -Wno-float-conversion
       #endif
+      #if __GNUC__ >= 7 || (__clang_major__ + (__clang_minor__ >= 9) > 3)
+      -Wimplicit-fallthrough
+      #endif
       #if __GNUC__ + (__GNUC_MINOR__ >= 8) > 4 || (__clang_major__ + 
(__clang_minor__ >= 9) > 3)
       -Wno-pedantic
       #endif