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From: "tuliom at ascii dot art.br" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/100605] New: -Wimplicit-fallthrough=5 still recognizes comments
Date: Fri, 14 May 2021 15:31:32 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100605

            Bug ID: 100605
           Summary: -Wimplicit-fallthrough=3D5 still recognizes comments
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tuliom at ascii dot art.br
  Target Milestone: ---

According to the GCC manual:

"-Wimplicit-fallthrough=3D5 doesn=E2=80=99t recognize any comments as fallt=
hrough
comments, only attributes disable the warning."

However, comments are still being recognized and are disabling the warning.
In the following example, there are 3 fall-throughs:

$ cat t.c
#include <stddef.h>
#include <inttypes.h>

uint32_t foo(size_t in)
{
  uint32_t out =3D 0;
  uint32_t k =3D 0;

  switch (in & 3)
    {
    case 3:
      k =3D 3;
      // FALLTHROUGH
    case 2:
      k =3D 8;
    case 1:
      k =3D 16;
    case 0:
    default:
      out =3D k;
      break;
    }

  return out;
}

However, GCC 11 reports only 2 when using -Wimplicit-fallthrough=3D5:

$ gcc-11 -c -Wimplicit-fallthrough=3D5 -Werror=3Dimplicit-fallthrough t.c
t.c: In function =E2=80=98foo=E2=80=99:
t.c:15:9: error: this statement may fall through
[-Werror=3Dimplicit-fallthrough=3D]
   15 |       k =3D 8;
      |       ~~^~~
t.c:16:5: note: here
   16 |     case 1:
      |     ^~~~
t.c:17:9: error: this statement may fall through
[-Werror=3Dimplicit-fallthrough=3D]
   17 |       k =3D 16;
      |       ~~^~~~
t.c:18:5: note: here
   18 |     case 0:
      |     ^~~~
cc1: some warnings being treated as errors=