AtomicInteger stringsCloseCount = new AtomicInteger();
    Stream<String> strings = source.onClose(stringsCloseCount::incrementAndGet);
    Stream<String> withIndex = Streams.mapWithIndex(strings, (str, i) -> str + ":" + i);

    withIndex.close();

    assertThat(stringsCloseCount.get()).isEqualTo(1);
  }

  public void testMapWithIndex_intStream() {
    SpliteratorTester.of(