fix(router): should throw when lazy loaded module doesn't define any routes (#15001)

Closes #14596

PR Close #15001

packages/router/src/router_config_loader.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {Compiler, InjectionToken, Injector, NgModuleFactory, NgModuleFactoryLoader, NgModuleRef} from '@angular/core';
+import {Compiler, InjectionToken, Injector, NgModuleFactory, NgModuleFactoryLoader, NgModuleRef, ɵstringify as stringify} from '@angular/core';
 import {Observable} from 'rxjs/Observable';
 import {fromPromise} from 'rxjs/observable/fromPromise';
 import {of } from 'rxjs/observable/of';
@@ -41,7 +41,16 @@ export class RouterConfigLoader {
 
       const module = factory.create(parentInjector);
 
-      return new LoadedRouterConfig(flatten(module.injector.get(ROUTES)), module);
+      const parentRoutes = new Set(flatten(parentInjector.get(ROUTES)));
+      const moduleRoutes =
+          flatten(module.injector.get(ROUTES)).filter(route => !parentRoutes.has(route));
+
+      if (moduleRoutes.length === 0) {
+        throw new Error(
+            `A lazy loaded module must define at least 1 route, but it seems like the '${stringify(factory.moduleType)}' module hasn't defined any. Have you imported RouterModule.forChild(ROUTES) in this module?`);
+      }
+
+      return new LoadedRouterConfig(moduleRoutes, module);
     });
   }
 

packages/router/test/integration.spec.ts

@@ -2697,6 +2697,27 @@ describe('Integration', () => {
              expect(fixture.nativeElement).toHaveText('lazy-loaded-parent [lazy-loaded-child]');
            })));
 
+    it(`should throw an error when lazy loaded module does not provide any routes`,
+       fakeAsync(inject(
+           [Router, Location, NgModuleFactoryLoader],
+           (router: Router, location: Location, loader: SpyNgModuleFactoryLoader) => {
+
+             @NgModule({})
+             class LazyLoadedModule {
+             }
+
+             loader.stubbedModules = {expected: LazyLoadedModule};
+
+             const fixture = createRoot(router, RootCmp);
+
+             router.resetConfig([{path: 'lazy', loadChildren: 'expected'}]);
+
+             router.navigateByUrl('/lazy');
+             expect(() => advance(fixture))
+                 .toThrowError(
+                     /A lazy loaded module must define at least 1 route, but it seems like the 'LazyLoadedModule' module hasn't defined any/);
+           })));
+
     it('should have 2 injector trees: module and element',
        fakeAsync(inject(
            [Router, Location, NgModuleFactoryLoader],