char *strncpy (char *, const char *, __SIZE_TYPE__);
struct S
{
  char arr[64];
};

int
foo (struct S *p1, const char *a)
{
  if (a)
    goto err;
  strncpy (p1->arr, a, sizeof p1->arr);
  p1->arr[3] = '\0';
err:
  return 0;
}

$ ./cc1 -quiet t.c -Wstringop-truncation -O2 -g
t.c: In function ‘foo’:
t.c:12:3: warning: ‘strncpy’ specified bound 64 equals destination size [-Wstringop-truncation]
   strncpy (p1->arr, a, sizeof p1->arr);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

the warning is gone without -g.