src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Observable<T> mergeWith(@NonNull CompletableSource other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return RxJavaPlugins.onAssembly(new ObservableMergeWithCompletable<T>(this, other));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Observable<T> mergeWith(@NonNull CompletableSource other) {
        ObjectHelper.requireNonNull(other, "other is null");
        return RxJavaPlugins.onAssembly(new ObservableMergeWithCompletable<T>(this, other));
    }
Relays the items of this Observable and completes only when the other CompletableSource completes
as well.
<p>
<img width="640" height="380" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/merge.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code mergeWith} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>
<p>History: 2.1.10 - experimental
@param other the {@code CompletableSource} to await for completion
@return the new Observable instance
@since 2.2