assertThrows(UnsupportedOperationException.class, () -> units.add(SomeEnum.C));
  }

  @J2ktIncompatible
  @GwtIncompatible // SerializableTester
  public void testImmutableEnumSet_serialized() {
    Set<SomeEnum> units = Sets.immutableEnumSet(SomeEnum.D, SomeEnum.B);

    assertThat(units).containsExactly(SomeEnum.B, SomeEnum.D).inOrder();