Consider:

void func(void) {
  int [[attr]] i;
  (void)sizeof(int [[attr]]);
}

This generates four diagnostics, two for each unknown attribute. See https://godbolt.org/z/r6eGx1W6E for an example. The same does not happen for declaration attributes or statement attributes, and it does not happen for unknown type attributes spelled with __attribute__.