Regardless of the presence of a guard, if intra-case dominance is violated, the switch needs to be rejected.

public class Test {
    void test(Object obj) {
       switch (obj) {
          case Integer _, CharSequence _, String _ when obj.hashCode() > 0 -> {
          }
          default -> throw new IllegalStateException("Unexpected
value: " + obj);
       }
    }
}

https://mail.openjdk.org/pipermail/amber-spec-experts/2023-August/003910.html