fix(core): throw error message when @Output not initialized (#19116)

Closes #3664

PR Close #19116

packages/core/src/view/provider.ts

@@ -137,9 +137,15 @@ export function createDirectiveInstance(view: ViewData, def: NodeDef): any {
   if (def.outputs.length) {
     for (let i = 0; i < def.outputs.length; i++) {
       const output = def.outputs[i];
-      const subscription = instance[output.propName !].subscribe(
-          eventHandlerClosure(view, def.parent !.nodeIndex, output.eventName));
-      view.disposables ![def.outputIndex + i] = subscription.unsubscribe.bind(subscription);
+      const outputObservable = instance[output.propName !];
+      if (typeof outputObservable === 'object') {
+        const subscription = outputObservable.subscribe(
+            eventHandlerClosure(view, def.parent !.nodeIndex, output.eventName));
+        view.disposables ![def.outputIndex + i] = subscription.unsubscribe.bind(subscription);
+      } else {
+        throw new Error(
+            `@Output ${output.propName} not initialized in '${instance.constructor.name}'.`);
+      }
     }
   }
   return instance;

packages/core/test/bundling/hello_world_r2/bundle.golden_symbols.json

@@ -3170,6 +3170,9 @@
   {
     "name": "isObservable"
   },
+  {
+    "name": "isObservable$1"
+  },
   {
     "name": "isPrefixEnd"
   },

packages/core/test/linker/integration_spec.ts

@@ -346,6 +346,20 @@ function declareTests({useJit}: {useJit: boolean}) {
         expect(tc.injector.get(EventDir)).not.toBeNull();
       });
 
+      it('should display correct error message for uninitialized @Output', () => {
+        @Directive({selector: '[uninitializedOuput]'})
+        class UninitializedOuput {
+          @Output() customEvent;
+          doSomething() {
+          }
+        }
+
+        TestBed.configureTestingModule({declarations: [MyComp, UninitializedOuput]});
+        const template = '<p (customEvent)="doNothing()"></p>';
+        TestBed.overrideComponent(MyComp, {set: {template}});
+        TestBed.createComponent(MyComp).toThrowError('@Output customEvent not initialized in \'UninitializedOuput\'.');
+      });
+
       it('should read directives metadata from their binding token', () => {
         TestBed.configureTestingModule({declarations: [MyComp, PrivateImpl, NeedsPublicApi]});
         const template = '<div public-api><div needs-public-api></div></div>';