checkNotNull(newValue, "Null value for input %s", t));
        },
        EnumMapAccumulator::combine,
        EnumMapAccumulator::toImmutableMap);
  }

  @IgnoreJRERequirement // see enclosing class (whose annotation Animal Sniffer ignores here...)
  private static class EnumMapAccumulator<K extends Enum<K>, V> {
    private final BinaryOperator<V> mergeFunction;