enhance generics on Observable.onErrorResumeNext and onErrorReturn (#4452)

Author: davidmoten

src/main/java/rx/Observable.java

@@ -8035,7 +8035,7 @@ public final Observable<T> onBackpressureLatest() {
      * @return the original Observable, with appropriately modified behavior
      * @see <a href="http://reactivex.io/documentation/operators/catch.html">ReactiveX operators documentation: Catch</a>
      */
-    public final Observable<T> onErrorResumeNext(final Func1<Throwable, ? extends Observable<? extends T>> resumeFunction) {
+    public final Observable<T> onErrorResumeNext(final Func1<? super Throwable, ? extends Observable<? extends T>> resumeFunction) {
         return lift(new OperatorOnErrorResumeNextViaFunction<T>(resumeFunction));
     }
 
@@ -8111,7 +8111,7 @@ public final Observable<T> onErrorResumeNext(final Observable<? extends T> resum
      * @see <a href="http://reactivex.io/documentation/operators/catch.html">ReactiveX operators documentation: Catch</a>
      */
     @SuppressWarnings("cast")
-    public final Observable<T> onErrorReturn(Func1<Throwable, ? extends T> resumeFunction) {
+    public final Observable<T> onErrorReturn(Func1<? super Throwable, ? extends T> resumeFunction) {
         return lift((Operator<T, T>)OperatorOnErrorResumeNextViaFunction.withSingle(resumeFunction));
     }
 

src/main/java/rx/internal/operators/OperatorOnErrorResumeNextViaFunction.java

@@ -44,9 +44,9 @@
  */
 public final class OperatorOnErrorResumeNextViaFunction<T> implements Operator<T, T> {
 
-    final Func1<Throwable, ? extends Observable<? extends T>> resumeFunction;
+    final Func1<? super Throwable, ? extends Observable<? extends T>> resumeFunction;
 
-    public static <T> OperatorOnErrorResumeNextViaFunction<T> withSingle(final Func1<Throwable, ? extends T> resumeFunction) {
+    public static <T> OperatorOnErrorResumeNextViaFunction<T> withSingle(final Func1<? super Throwable, ? extends T> resumeFunction) {
         return new OperatorOnErrorResumeNextViaFunction<T>(new Func1<Throwable, Observable<? extends T>>() {
             @Override
             public Observable<? extends T> call(Throwable t) {
@@ -76,7 +76,7 @@ public Observable<? extends T> call(Throwable t) {
         });
     }
 
-    public OperatorOnErrorResumeNextViaFunction(Func1<Throwable, ? extends Observable<? extends T>> f) {
+    public OperatorOnErrorResumeNextViaFunction(Func1<? super Throwable, ? extends Observable<? extends T>> f) {
         this.resumeFunction = f;
     }