* immediateFuture(null)} except that it is restricted to produce futures of type {@code Void}.
   *
   * @since 29.0
   */
  @SuppressWarnings("unchecked")
  public static ListenableFuture<@Nullable Void> immediateVoidFuture() {
    return (ListenableFuture<@Nullable Void>) ImmediateFuture.NULL;
  }

  /**
   * Returns a {@code ListenableFuture} which has an exception set immediately upon construction.
   *