feat(router): throw a helpful error when misusing forRoot() from a lazy module. (#10996)

modules/@angular/router/src/router_module.ts

@@ -7,7 +7,7 @@
  */
 
 import {APP_BASE_HREF, HashLocationStrategy, Location, LocationStrategy, PathLocationStrategy, PlatformLocation} from '@angular/common';
-import {ANALYZE_FOR_ENTRY_COMPONENTS, APP_BOOTSTRAP_LISTENER, ApplicationRef, Compiler, Inject, Injector, ModuleWithProviders, NgModule, NgModuleFactoryLoader, OpaqueToken, Optional, SystemJsNgModuleLoader} from '@angular/core';
+import {ANALYZE_FOR_ENTRY_COMPONENTS, APP_BOOTSTRAP_LISTENER, ApplicationRef, BaseException, Compiler, Inject, Injector, ModuleWithProviders, NgModule, NgModuleFactoryLoader, OpaqueToken, Optional, SkipSelf, SystemJsNgModuleLoader} from '@angular/core';
 
 import {Route, Routes} from './config';
 import {RouterLink, RouterLinkWithHref} from './directives/router_link';
@@ -32,6 +32,8 @@ export const ROUTER_DIRECTIVES = [RouterOutlet, RouterLink, RouterLinkWithHref,
  */
 export const ROUTER_CONFIGURATION = new OpaqueToken('ROUTER_CONFIGURATION');
 
+export const ROUTER_FORROOT_GUARD = new OpaqueToken('ROUTER_FORROOT_GUARD');
+
 const pathLocationStrategy = {
   provide: LocationStrategy,
   useClass: PathLocationStrategy
@@ -81,11 +83,17 @@ export const ROUTER_PROVIDERS: any[] = [
  */
 @NgModule({declarations: ROUTER_DIRECTIVES, exports: ROUTER_DIRECTIVES})
 export class RouterModule {
+  constructor(@Optional() @Inject(ROUTER_FORROOT_GUARD) guard: any) {}
+
   static forRoot(routes: Routes, config?: ExtraOptions): ModuleWithProviders {
     return {
       ngModule: RouterModule,
       providers: [
-        ROUTER_PROVIDERS, provideRoutes(routes),
+        ROUTER_PROVIDERS, provideRoutes(routes), {
+          provide: ROUTER_FORROOT_GUARD,
+          useFactory: provideForRootGuard,
+          deps: [[Router, new Optional(), new SkipSelf()]]
+        },
         {provide: ROUTER_CONFIGURATION, useValue: config ? config : {}}, {
           provide: LocationStrategy,
           useFactory: provideLocationStrategy,
@@ -109,6 +117,14 @@ export function provideLocationStrategy(
                            new PathLocationStrategy(platformLocationStrategy, baseHref);
 }
 
+export function provideForRootGuard(router: Router): any {
+  if (router) {
+    throw new BaseException(
+        `RouterModule.forRoot() called twice. Lazy loaded modules should use RouterModule.forChild() instead.`);
+  }
+  return 'guarded';
+}
+
 /**
  * @stable
  */

modules/@angular/router/test/integration.spec.ts

@@ -1411,7 +1411,34 @@ describe('Integration', () => {
                       expect(fixture.debugElement.nativeElement)
                           .toHaveText('lazy-loaded-parent [lazy-loaded-child]');
                     })));
+    it('throws an error when forRoot() is used in a lazy context',
+       fakeAsync(inject(
+           [Router, Location, NgModuleFactoryLoader],
+           (router: Router, location: Location, loader: SpyNgModuleFactoryLoader) => {
+             @Component({selector: 'lazy', template: 'should not show'})
+             class LazyLoadedComponent {
+             }
 
+             @NgModule({
+               declarations: [LazyLoadedComponent],
+               imports: [RouterModule.forRoot([{path: 'loaded', component: LazyLoadedComponent}])]
+             })
+             class LoadedModule {
+             }
+
+             loader.stubbedModules = {expected: LoadedModule};
+
+             const fixture = createRoot(router, RootCmp);
+
+             router.resetConfig([{path: 'lazy', loadChildren: 'expected'}]);
+
+             let recordedError: any = null;
+             router.navigateByUrl('/lazy/loaded').catch(err => recordedError = err);
+             advance(fixture);
+             expect(recordedError.message)
+                 .toEqual(
+                     `RouterModule.forRoot() called twice. Lazy loaded modules should use RouterModule.forChild() instead.`);
+           })));
     it('should combine routes from multiple modules into a single configuration',
        fakeAsync(inject(
            [Router, Location, NgModuleFactoryLoader],

tools/public_api_guard/router/index.d.ts

@@ -239,6 +239,7 @@ export declare class RouterLinkWithHref implements OnChanges, OnDestroy {
 
 /** @stable */
 export declare class RouterModule {
+    constructor(guard: any);
     static forChild(routes: Routes): ModuleWithProviders;
     static forRoot(routes: Routes, config?: ExtraOptions): ModuleWithProviders;
 }