fix(ivy): do not throw if host style is on a template node (#30498)

In View Engine, we would simply ignore host style bindings on template nodes. In Ivy,
we are throwing a "Cannot read length of undefined" error instead. For backwards
compatibility, we should also ignore these bindings rather than blowing up.

PR Close #30498

packages/core/src/render3/styling/host_instructions_queue.ts

@@ -35,9 +35,12 @@ export function registerHostDirective(context: StylingContext, directiveIndex: n
  */
 export function enqueueHostInstruction<T extends Function>(
     context: StylingContext, priority: number, instructionFn: T, instructionFnArgs: ParamsOf<T>) {
-  const buffer: HostInstructionsQueue = context[StylingIndex.HostInstructionsQueue] !;
+  const buffer: HostInstructionsQueue|null = context[StylingIndex.HostInstructionsQueue];
+  // Buffer may be null if host element is a template node. In this case, just ignore the style.
+  if (buffer != null) {
     const index = findNextInsertionIndex(buffer, priority);
     buffer.splice(index, 0, priority, instructionFn, instructionFnArgs);
+  }
 }
 
 /**

packages/core/test/acceptance/styling_spec.ts

@@ -160,4 +160,24 @@ describe('styling', () => {
       tNode: 3,
     });
   });
+
+  it('should not throw if host style binding is on a template node', () => {
+    // This ex is a bit contrived. In real apps, you might have a shared class that is extended both
+    // by components with host elements and by directives on template nodes. In that case, the host
+    // styles for the template directives should just be ignored.
+    @Directive({selector: 'ng-template[styleDir]', host: {'[style.display]': 'display'}})
+    class StyleDir {
+      display = 'block';
+    }
+
+    @Component({selector: 'app-comp', template: `<ng-template styleDir></ng-template>`})
+    class MyApp {
+    }
+
+    TestBed.configureTestingModule({declarations: [MyApp, StyleDir]});
+    expect(() => {
+      const fixture = TestBed.createComponent(MyApp);
+      fixture.detectChanges();
+    }).not.toThrow();
+  });
 });