@J2ktIncompatible
  @GwtIncompatible // used only in GwtIncompatible tests
  private interface Merger {

    ListenableFuture<List<String>> merged(ListenableFuture<String> a, ListenableFuture<String> b);

    Merger allMerger =
        new Merger() {
          @Override
          public ListenableFuture<List<String>> merged(
              ListenableFuture<String> a, ListenableFuture<String> b) {

  @J2ktIncompatible
  @GwtIncompatible // used only in GwtIncompatible tests
  private interface Merger {

    ListenableFuture<List<String>> merged(ListenableFuture<String> a, ListenableFuture<String> b);

    Merger allMerger =
        new Merger() {
          @Override
          public ListenableFuture<List<String>> merged(
              ListenableFuture<String> a, ListenableFuture<String> b) {

      throw new UnsupportedOperationException();
    }

    @Override
    @CheckForNull
    @SuppressWarnings("nullness") // TODO(b/262880368): Remove once we see @NonNull in JDK APIs
    public V merge(
        K key,
        @NonNull V value,
        BiFunction<? super @NonNull V, ? super @NonNull V, ? extends @Nullable V> function) {
      throw new UnsupportedOperationException();
    }

   * iterate over keys in their enum definition order, not encounter order.
   *
   * <p>If the mapped keys contain duplicates, the values are merged using the specified merging
   * function.
   *
   * @since 33.2.0 (available since 21.0 in guava-jre)
   */
  @SuppressWarnings({"AndroidJdkLibsChecker", "Java7ApiChecker"})

    checkNotNull(key);
    checkNotNull(function);
    return compute(key, (k, oldValue) -> (oldValue == null) ? null : function.apply(k, oldValue));
  }

  @Override
  @CheckForNull
  public V merge(
      K key, V newValue, BiFunction<? super V, ? super V, ? extends @Nullable V> function) {
    checkNotNull(key);
    checkNotNull(newValue);
    checkNotNull(function);
    return compute(