public static <InputT> KeyByBuilder<InputT> of(PCollection<InputT> input) {
    return named(null).of(input);
  }
@Test
  @SuppressWarnings("unchecked")
  public void testTypeHints_typePropagation() {
    final PCollection<String> dataset = TestUtils.createMockDataset(TypeDescriptors.strings());
    final TypeDescriptor<String> keyType = TypeDescriptors.strings();
    final TypeDescriptor<Long> valueType = TypeDescriptors.longs();
    final TypeDescriptor<Long> outputType = TypeDescriptors.longs();
    final PCollection<KV<String, Long>> reduced =
        ReduceByKey.of(dataset)
            .keyBy(s -> s, keyType)
            .valueBy(s -> 1L, valueType)
            .combineBy(Sums.ofLongs())
            .output();
    final ReduceByKey reduce = (ReduceByKey) TestUtils.getProducer(reduced);
    TypePropagationAssert.assertOperatorTypeAwareness(reduce, keyType, valueType, outputType);
  }