fix(core): imply @optional flag when a default value is provided (ang…

…ular#47242)

Unify default value handling across injector and node injector: as long
as a default value is provided it has the same effect as specifying the
@optional() flag.

Fixes angular#47109

PR Close angular#47242

packages/core/src/render3/di.ts

@@ -334,7 +334,7 @@ export function injectAttributeImpl(tNode: TNode, attrNameToInject: string): str
 
 function notFoundValueOrThrow<T>(
     notFoundValue: T|null, token: ProviderToken<T>, flags: InjectFlags): T|null {
-  if (flags & InjectFlags.Optional) {
+  if ((flags & InjectFlags.Optional) || notFoundValue !== undefined) {
     return notFoundValue;
   } else {
     throwProviderNotFoundError(token, 'NodeInjector');
@@ -352,7 +352,7 @@ function notFoundValueOrThrow<T>(
  */
 function lookupTokenUsingModuleInjector<T>(
     lView: LView, token: ProviderToken<T>, flags: InjectFlags, notFoundValue?: any): T|null {
-  if (flags & InjectFlags.Optional && notFoundValue === undefined) {
+  if ((flags & InjectFlags.Optional) && notFoundValue === undefined) {
     // This must be set or the NullInjector will throw for optional deps
     notFoundValue = null;
   }

packages/core/test/acceptance/di_spec.ts

@@ -1025,6 +1025,24 @@ describe('di', () => {
           const dirC = fixture.componentInstance.dirC;
           expect(dirC.dirB).toBeNull();
         });
+
+        it('should imply @Optional in presence of a default value', () => {
+          const NON_EXISTING_PROVIDER = new InjectionToken<string>('non-existing');
+
+          @Component({template: ''})
+          class MyComp {
+            value: string|undefined;
+            constructor(injector: Injector) {
+              this.value = injector.get(NON_EXISTING_PROVIDER, 'default', InjectFlags.Host);
+            }
+          }
+
+          const injector = Injector.create({providers: []});
+          expect(injector.get(NON_EXISTING_PROVIDER, 'default', InjectFlags.Host)).toBe('default');
+
+          const fixture = TestBed.createComponent(MyComp);
+          expect(fixture.componentInstance.value).toBe('default');
+        });
       });
 
       it('should check only the current node with @Self', () => {