return future instanceof FluentFuture
        ? (FluentFuture<V>) future
        : new ForwardingFluentFuture<V>(future);
  }

  /**
   * Simply returns its argument.
   *
   * @deprecated no need to use this
   * @since 28.0
   */
  @Deprecated
  public static <V extends @Nullable Object> FluentFuture<V> from(FluentFuture<V> future) {

  private final CloseableList closeables;
  private final FluentFuture<V> future;

  private ClosingFuture(ListenableFuture<V> future) {
    this(future, new CloseableList());
  }

  private ClosingFuture(ListenableFuture<V> future, CloseableList closeables) {
    this.future = FluentFuture.from(future);
    this.closeables = closeables;
  }

  /**

  "nullness", // TODO(b/147136275): Remove once our checker understands & and |.
})
abstract class AbstractCatchingFuture<
        V extends @Nullable Object, X extends Throwable, F, T extends @Nullable Object>
    extends FluentFuture.TrustedFuture<V> implements Runnable {
  static <V extends @Nullable Object, X extends Throwable> ListenableFuture<V> create(
      ListenableFuture<? extends V> input,
      Class<X> exceptionType,

 * interrupted and cancelled if it times out.
 */
@J2ktIncompatible
@GwtIncompatible
@ElementTypesAreNonnullByDefault
final class TimeoutFuture<V extends @Nullable Object> extends FluentFuture.TrustedFuture<V> {
  static <V extends @Nullable Object> ListenableFuture<V> create(
      ListenableFuture<V> delegate,
      long time,
      TimeUnit unit,
      ScheduledExecutorService scheduledExecutor) {

})
abstract class AbstractTransformFuture<
        I extends @Nullable Object, O extends @Nullable Object, F, T extends @Nullable Object>
    extends FluentFuture.TrustedFuture<O> implements Runnable {
  static <I extends @Nullable Object, O extends @Nullable Object> ListenableFuture<O> createAsync(
      ListenableFuture<I> input,
      AsyncFunction<? super I, ? extends O> function,

     * should throw IllegalStateException only when they use static state.
     *
     * Why do we deviate here? The answer: We want for fluentFuture.getDone() to throw the same
     * exception as Futures.getDone(fluentFuture).
     */
    checkState(future.isDone(), "Future was expected to be done: %s", future);
    return getUninterruptibly(future);
  }

  /**

     * should throw IllegalStateException only when they use static state.
     *
     * Why do we deviate here? The answer: We want for fluentFuture.getDone() to throw the same
     * exception as Futures.getDone(fluentFuture).
     */
    checkState(future.isDone(), "Future was expected to be done: %s", future);
    return getUninterruptibly(future);
  }

  /**