feat(common): better error message when non-template element used in NgIf (#22274)

closes #16410

PR Close #22274

packages/common/src/directives/ng_if.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {Directive, EmbeddedViewRef, Input, TemplateRef, ViewContainerRef} from '@angular/core';
+import {Directive, EmbeddedViewRef, Input, TemplateRef, ViewContainerRef, ɵstringify as stringify} from '@angular/core';
 
 
 /**
@@ -118,6 +118,7 @@ export class NgIf {
 
   @Input()
   set ngIfThen(templateRef: TemplateRef<NgIfContext>) {
+    assertTemplate('ngIfThen', templateRef);
     this._thenTemplateRef = templateRef;
     this._thenViewRef = null;  // clear previous view if any.
     this._updateView();
@@ -125,6 +126,7 @@ export class NgIf {
 
   @Input()
   set ngIfElse(templateRef: TemplateRef<NgIfContext>) {
+    assertTemplate('ngIfElse', templateRef);
     this._elseTemplateRef = templateRef;
     this._elseViewRef = null;  // clear previous view if any.
     this._updateView();
@@ -163,3 +165,10 @@ export class NgIfContext {
   public $implicit: any = null;
   public ngIf: any = null;
 }
+
+function assertTemplate(property: string, templateRef: TemplateRef<any>): void {
+  const isTemplateRef = templateRef.createEmbeddedView != null;
+  if (!isTemplateRef) {
+    throw new Error(`${property} must be a TemplateRef, but received '${stringify(templateRef)}'.`);
+  }
+}

packages/common/test/directives/ng_if_spec.ts

@@ -217,6 +217,28 @@ import {expect} from '@angular/platform-browser/testing/src/matchers';
            expect(fixture.nativeElement).toHaveText('false');
          }));
     });
+
+    describe('Type guarding', () => {
+      it('should throw when then block is not template', async(() => {
+           const template = '<span *ngIf="booleanCondition; then thenBlock">IGNORE</span>' +
+               '<div #thenBlock>THEN</div>';
+
+           fixture = createTestComponent(template);
+
+           expect(() => fixture.detectChanges())
+               .toThrowError(/ngIfThen must be a TemplateRef, but received/);
+         }));
+
+      it('should throw when else block is not template', async(() => {
+           const template = '<span *ngIf="booleanCondition; else elseBlock">IGNORE</span>' +
+               '<div #elseBlock>ELSE</div>';
+
+           fixture = createTestComponent(template);
+
+           expect(() => fixture.detectChanges())
+               .toThrowError(/ngIfElse must be a TemplateRef, but received/);
+         }));
+    });
   });
 }