2.x: Change Completable to CompletableSource (#4924)

Author: vanniktech

src/main/java/io/reactivex/Maybe.java

@@ -2842,7 +2842,7 @@ public final <R> Maybe<R> flatMapSingleElement(final Function<? super T, ? exten
      */
     @CheckReturnValue
     @SchedulerSupport(SchedulerSupport.NONE)
-    public final Completable flatMapCompletable(final Function<? super T, ? extends Completable> mapper) {
+    public final Completable flatMapCompletable(final Function<? super T, ? extends CompletableSource> mapper) {
         ObjectHelper.requireNonNull(mapper, "mapper is null");
         return RxJavaPlugins.onAssembly(new MaybeFlatMapCompletable<T>(this, mapper));
     }

src/main/java/io/reactivex/Single.java

@@ -2097,7 +2097,7 @@ public final <R> Observable<R> flatMapObservable(Function<? super T, ? extends O
      */
     @CheckReturnValue
     @SchedulerSupport(SchedulerSupport.NONE)
-    public final Completable flatMapCompletable(final Function<? super T, ? extends Completable> mapper) {
+    public final Completable flatMapCompletable(final Function<? super T, ? extends CompletableSource> mapper) {
         ObjectHelper.requireNonNull(mapper, "mapper is null");
         return RxJavaPlugins.onAssembly(new SingleFlatMapCompletable<T>(this, mapper));
     }