assertThrows(
        IllegalArgumentException.class,
        () -> Stream.of(mapEntry(AnEnum.A, 1), mapEntry(AnEnum.A, 11)).collect(collector));
  }

  public void testToImmutableMapMerging() {
    Collector<Entry<AnEnum, Integer>, ?, ImmutableMap<AnEnum, Integer>> collector =
        toImmutableEnumMap(Entry::getKey, Entry::getValue, Integer::sum);