https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100605
--- Comment #1 from Tulio Magno Quites Machado Filho <tuliom at ascii dot
art.br> ---
Interestingly, all the 3 warnings are reported when using -save-temps:
$ gcc -c -save-temps -Wimplicit-fallthrough=5 -Werror=implicit-fallthrough t.c
t.c: In function ‘foo’:
t.c:12:9: error: this statement may fall through
[-Werror=implicit-fallthrough=]
12 | k = 3;
| ~~^~~
t.c:14:5: note: here
14 | case 2:
| ^~~~
t.c:15:9: error: this statement may fall through
[-Werror=implicit-fallthrough=]
15 | k = 8;
| ~~^~~
t.c:16:5: note: here
16 | case 1:
| ^~~~
t.c:17:9: error: this statement may fall through
[-Werror=implicit-fallthrough=]
17 | k = 16;
| ~~^~~~
t.c:18:5: note: here
18 | case 0:
| ^~~~
cc1: some warnings being treated as errors
