code: @CheckReturnValue
      @SchedulerSupport(SchedulerSupport.NONE)
      public final <U> Single<T> delaySubscription(SingleSource<U> other) {
          ObjectHelper.requireNonNull(other, "other is null");
          return RxJavaPlugins.onAssembly(new SingleDelayWithSingle<T, U>(this, other));
      }
docs: Delays the actual subscription to the current Single until the given other SingleSource
  signals success.
  <p>If the delaying source signals an error, that error is re-emitted and no subscription
  to the current Single happens.
  <dl>
  <dt><b>Scheduler:</b></dt>
  <dd>{@code delaySubscription} does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  @param <U> the element type of the other source
  @param other the SingleSource that has to complete before the subscription to the
  current Single happens
  @return the new Single instance
  @since 2.0