[Bug tree-optimization/102151] Spurious warning by -Warray-bounds when allocating with flexible array member

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102151

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |msebor at gcc dot gnu.org
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=101436
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #3 from Martin Sebor <msebor at gcc dot gnu.org> ---
As Andrew explained, the first operand in the -> expression needs to point to
an object of the type whose member is being accessed or at least as big as one,
and the warning is designed to point out when it's not (arguably, it could be
phrased better).  The following is a small test case to illustrate the warning
(see also pr101436 comment 2 for a similar C++ test case).

$ cat pr102151.c && gcc -O2 -S -Wall pr102151.c
struct S { char a, b; };

extern char c;

void f (void)
{
  struct S *p = &c;
  p->a = 0;
}
pr102151.c: In function ‘f’:
pr102151.c:7:17: warning: initialization of ‘struct S *’ from incompatible
pointer type ‘char *’ [-Wincompatible-pointer-types]
    7 |   struct S *p = &c;
      |                 ^
pr102151.c:8:4: warning: array subscript ‘struct S[0]’ is partly outside array
bounds of ‘char[1]’ [-Warray-bounds]
    8 |   p->a = 0;
      |    ^~
pr102151.c:3:13: note: while referencing ‘c’
    3 | extern char c;
      |             ^