fix(core): enforce return type for service factory

Updates the `factory` signature in `@Service` to enforce the type of the returned value.

goldens/public-api/core/index.api.md

@@ -1802,12 +1802,15 @@ export const Service: ServiceDecorator;
 // @public
 export interface ServiceDecorator {
     (): TypeDecorator;
-    (options?: {
+    (options: {
         autoProvided: false;
     }): TypeDecorator;
+    <T>(options: {
+        autoProvided?: true;
+        factory: () => T;
+    }): <C extends Type<unknown>>(target: C) => Type<T>;
     (options?: {
         autoProvided?: true;
-        factory?: () => unknown;
     }): TypeDecorator;
 }
 

packages/core/src/di/service.ts

@@ -30,13 +30,21 @@ export interface ServiceDecorator {
    * When `autoProvided` is set to `false`, the service won't be exposed to the dependency
    * injection system automatically. It is up to the user to expose it in a providers list.
    */
-  (options?: {autoProvided: false}): TypeDecorator;
+  (options: {autoProvided: false}): TypeDecorator;
 
   /**
-   * Creates a service that is automatically provided. Passing an optional
+   * Creates a service that is automatically provided and uses
-   * `factory` allows for the runtime value to be replaced.
+   * the value returned from the `factory` function.
    */
-  (options?: {autoProvided?: true; factory?: () => unknown}): TypeDecorator;
+  <T>(options: {
+    autoProvided?: true;
+    factory: () => T;
+  }): <C extends Type<unknown>>(target: C) => Type<T>;
+
+  /**
+   * Creates a service that is automatically provided.
+   */
+  (options?: {autoProvided?: true}): TypeDecorator;
 }
 
 /**

packages/core/test/acceptance/service_spec.ts

@@ -28,7 +28,7 @@ describe('@Service decorator', () => {
   it('should be able to provide an alternate implementation using `factory`', () => {
     @Service({factory: () => ({value: 'alternate'})})
     class MyService {
-      readonly value = 'MyService';
+      value = 'MyService';
     }
 
     @Component({template: ''})
@@ -43,7 +43,7 @@ describe('@Service decorator', () => {
   it('should be able to provide an alternate implementation using `factory` when `autoProvided` is set to true', () => {
     @Service({autoProvided: true, factory: () => ({value: 'alternate'})})
     class MyService {
-      readonly value = 'MyService';
+      value = 'MyService';
     }
 
     @Component({template: ''})
@@ -60,7 +60,7 @@ describe('@Service decorator', () => {
 
     @Service({factory: () => ({value: inject(token)})})
     class MyService {
-      readonly value = 'MyService';
+      value = 'MyService';
     }
 
     @Component({template: ''})
@@ -153,7 +153,7 @@ describe('@Service decorator', () => {
   it('should be able to override a service that has a factory', () => {
     @Service({factory: () => ({value: 'factory'})})
     class MyService {
-      readonly value = 'MyService';
+      value = 'MyService';
     }
 
     @Service()