[Bug c/105319] New: Unexpected warning with zero-length sized array using -std=gnu99 and -O2

[piannetta at kalrayinc dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105319

            Bug ID: 105319
           Summary: Unexpected warning with zero-length sized array using
                    -std=gnu99 and -O2
           Product: gcc
           Version: 10.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: piannetta at kalrayinc dot com
  Target Milestone: ---

// main.c
#define LEN 0

static volatile int tab[LEN];

int main (int argc, char * argv[]) {
        tab[argc] = 0;
}

The code above builds fine with:
  gcc -Wall -std=gnu99
but reports a warning when compiled with -O2:
  gcc -Wall -std=gnu99 -O2
main.c: In function ‘main’:
main.c:4:12: warning: array subscript argc is outside array bounds of ‘volatile
int[0]’ [-Warray-bounds]
    4 |         tab[argc] = 0;
      |         ~~~^~~~~~
main.c:2:21: note: while referencing ‘tab’
    2 | static volatile int tab[LEN];
      |                     ^~~

I've observed this behavior on gcc 10.3.1 and on gcc 12.0.1 (I have not tested
other versions).

I would have expected that the snippet would compile the same way in both cases
and not produce a warning since gnu99 supports zero-length arrays.