__extension__ doesn't affect -Wpointer-arith

[Jim Kingdon <kingdon at redhat dot com>]

The -Wpointer-arith option becomes useless (or at least, painful to
use) with -O and glibc 2.1.2pre3 because bits/string2.h in the latter
uses pointer arithmetic on void * and __extension__ only turns off
-pedantic, not -Wpointer-arith.

For the reaction of Ulrich Drepper (libc maintainer), see:
  http://sourceware.cygnus.com/ml/libc-alpha/1999-08/msg00113.html

Seems to me that having __extension__ affect -Wpointer-arith would
make sense.

Here's an example, for those who don't feel like downloading the whole
libc in question :-):

bash$ gcc -pedantic foo3.c 
bash$ gcc -Wpointer-arith foo3.c 
foo3.c: In function `main':
foo3.c:10: warning: pointer of type `void *' used in arithmetic
bash$ cat foo3.c
#include <stdio.h>
int
main (int argc, char **argv)
{
  union {
    char b;
    int y;
  } *__u = (void *) &argc;

  __u = __extension__ ((void *)__u + 1);
  printf ("%d\n", __u->b);
  return 0;
}
bash$