Align hasCustomOnError behavior of CallbackCompletableObserver with L…

src/main/java/io/reactivex/rxjava3/core/Completable.java

@@ -2996,11 +2996,7 @@ public final Disposable subscribe(
     @NonNull
     @SchedulerSupport(SchedulerSupport.NONE)
     public final Disposable subscribe(@NonNull Action onComplete) {
-        Objects.requireNonNull(onComplete, "onComplete is null");
-
-        CallbackCompletableObserver observer = new CallbackCompletableObserver(onComplete);
-        subscribe(observer);
-        return observer;
+        return subscribe(onComplete, Functions.ON_ERROR_MISSING);
     }
 
     /**

src/main/java/io/reactivex/rxjava3/internal/observers/CallbackCompletableObserver.java

@@ -20,33 +20,24 @@
 import io.reactivex.rxjava3.exceptions.*;
 import io.reactivex.rxjava3.functions.*;
 import io.reactivex.rxjava3.internal.disposables.DisposableHelper;
+import io.reactivex.rxjava3.internal.functions.Functions;
 import io.reactivex.rxjava3.observers.LambdaConsumerIntrospection;
 import io.reactivex.rxjava3.plugins.RxJavaPlugins;
 
 public final class CallbackCompletableObserver
 extends AtomicReference<Disposable>
-        implements CompletableObserver, Disposable, Consumer<Throwable>, LambdaConsumerIntrospection {
+        implements CompletableObserver, Disposable, LambdaConsumerIntrospection {
 
     private static final long serialVersionUID = -4361286194466301354L;
 
     final Consumer<? super Throwable> onError;
     final Action onComplete;
 
-    public CallbackCompletableObserver(Action onComplete) {
-        this.onError = this;
-        this.onComplete = onComplete;
-    }
-
     public CallbackCompletableObserver(Consumer<? super Throwable> onError, Action onComplete) {
         this.onError = onError;
         this.onComplete = onComplete;
     }
 
-    @Override
-    public void accept(Throwable e) {
-        RxJavaPlugins.onError(new OnErrorNotImplementedException(e));
-    }
-
     @Override
     public void onComplete() {
         try {
@@ -86,6 +77,6 @@ public boolean isDisposed() {
 
     @Override
     public boolean hasCustomOnError() {
-        return onError != this;
+        return onError != Functions.ON_ERROR_MISSING;
     }
 }

src/test/java/io/reactivex/rxjava3/internal/observers/CallbackCompletableObserverTest.java

@@ -24,7 +24,7 @@ public final class CallbackCompletableObserverTest extends RxJavaTest {
 
     @Test
     public void emptyActionShouldReportNoCustomOnError() {
-        CallbackCompletableObserver o = new CallbackCompletableObserver(Functions.EMPTY_ACTION);
+        CallbackCompletableObserver o = new CallbackCompletableObserver(Functions.ON_ERROR_MISSING, Functions.EMPTY_ACTION);
 
         assertFalse(o.hasCustomOnError());
     }