code: @CheckReturnValue
      @SchedulerSupport(SchedulerSupport.NONE)
      public final <T> Single<T> toSingleDefault(final T completionValue) {
          ObjectHelper.requireNonNull(completionValue, "completionValue is null");
          return RxJavaPlugins.onAssembly(new CompletableToSingle<T>(this, null, completionValue));
      }
docs: Converts this Completable into a Single which when this Completable completes normally,
  emits the given value through onSuccess.
  <p>
  <img width="640" height="583" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.toSingleDefault.png" alt="">
  <dl>
  <dt><b>Scheduler:</b></dt>
  <dd>{@code toSingleDefault} does not operate by default on a particular {@link Scheduler}.</dd>
  </dl>
  @param <T> the value type
  @param completionValue the value to emit when this Completable completes normally
  @return the new Single instance
  @throws NullPointerException if completionValue is null