[Bug c/77817] New: -Wimplicit-fallthrough: cpp directive renders FALLTHRU comment ineffective

["jim at meyering dot net" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77817

            Bug ID: 77817
           Summary: -Wimplicit-fallthrough: cpp directive renders FALLTHRU
                    comment ineffective
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jim at meyering dot net
  Target Milestone: ---

Normally, adding a comment like /* FALLTHRU */ works fine to mark a switch case
that is intended to fall through. But if a cpp directive appears in the
vicinity of that comment, the comment becomes ineffective and we get the
warning anyway.

Here is a minimal example (without the "#undef", that comment *does* suppress
the warning):

int
foo (int x)
{
  switch (x)
    {
    case 1:
      x = 3;
#undef X
      /* fallthrough */
    case 2:
      x = 4;
    }
  return x;
}
$ gcc -c -O -Wimplicit-fallthrough /t/ft.c
/t/ft.c: In function ‘foo’:
/t/ft.c:20:9: warning: this statement may fall through [-Wimplicit-fallthrough]
       x = 3;
       ~~^~~
/t/ft.c:23:5: note: here
     case 2:
     ^~~~

The above was compiled with yesterday's GCC 7:
  gcc version 7.0.0 20161001 (experimental) (GCC)

I encountered an actual instance of this in gnulib's vasnprintf.c:
http://git.savannah.gnu.org/cgit/gnulib.git/tree/lib/vasnprintf.c#n4823