@Memoized
    Optional<Integer> negate() {
      methodCount++;
      return number().map(n -> -n);
    }
Type that uses both {@code @SerializableAutoValue} and {@code @Memoized}, showing that the two
extensions work correctly together.
value/src/test/java/com/google/auto/value/extension/serializable/processor/SerializableAutoValueExtensionTest.java