[Bug other/81582] New: -Wimplicit-fallthrough= wrong warning with -save-temps

["gjl at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81582

            Bug ID: 81582
           Summary: -Wimplicit-fallthrough= wrong warning with -save-temps
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: other
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gjl at gcc dot gnu.org
  Target Milestone: ---

int f1 (int x)
{
    switch (x)
    {
    case 1: x++; // FALLTHRU
    case 2: x--; /* FALLTHRU */
    case 3: x++;
        // FALLTHRU
    case 4: return x;
    default:
        break;
    }
    return 0;
}

$ gcc-8 wsitch.c -W -S # magic comments work
$ gcc-8 wsitch.c -W -S -save-temps
wsitch.c: In function ‘f1’:
wsitch.c:5:14: warning: this statement may fall through
[-Wimplicit-fallthrough=]
     case 1: x++; // FALLTHRU
             ~^~
wsitch.c:6:5: note: here
     case 2: x--; /* FALLTHRU */
     ^~~~
wsitch.c:6:14: warning: this statement may fall through
[-Wimplicit-fallthrough=]
     case 2: x--; /* FALLTHRU */
             ~^~
wsitch.c:7:5: note: here
     case 3: x++;
     ^~~~
wsitch.c:7:14: warning: this statement may fall through
[-Wimplicit-fallthrough=]
     case 3: x++;
             ~^~
wsitch.c:9:5: note: here
     case 4: return x;
     ^~~~

-save-temps is used during development and also in safety-critical applications
to retain preprosessed files and assembly code for verification or later
inspection.

Dunno how to handle this properly, maybe specs can switch off
-Wimplicit-fallthrough if -save-temps is on.

At least it should be documented that -Wimplict-fallthrough does not work as
expected if -save-temps (and perhaps other options liek -fpreprocessed) is on. 
-save-temps should mention that it may raise unexpected warnings from
-Wimplicit-fallthrough.

$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/srv/local/gnu/install/gcc-8-host/bin/../libexec/gcc/x86_64-pc-linux-gnu/8.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../../gcc.gnu.org/bootstrap-trunk/configure
--prefix=/local/gnu/install/gcc-8-host --enable-languages=c,c++
--enable-checking=release --disable-bootstrap
Thread model: posix
gcc version 8.0.0 20170724 (experimental) (GCC)