struct [[gnu::may_alias]] A
{
  [[gnu::nonstring]] char x[4];
};

struct B
{
  [[gnu::nonstring]] char x[2];
};

void
f (struct B *p)
{
  if (!__builtin_strncmp (p->x, "AA", 2))
    ;
  else if (!__builtin_strncmp (((struct A *)p)->x, "BBCC", 4))
    asm ("");
}

results in a warning which does not make any sense (to me):

t1.c: In function 'f':
t1.c:16:13: warning: '__builtin_strncmp' argument 1 declared attribute 'nonstring' is smaller than the specified bound 4 [-Wstringop-overread]
   16 |   else if (!__builtin_strncmp (((struct A *)p)->x, "BBCC", 4))
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
t1.c:8:27: note: argument 'x' declared here
    8 |   [[gnu::nonstring]] char x[2];
      |                           ^