feat(ivy): support template type check narrowing for *ngFor and *ngIf (#26203)

This commit adds an ngTemplateGuard_ngIf static method to the NgIf
directive and an ngTemplateContextGuard static method to NgFor. The
function of these two static methods is to enable type narrowing
within generated type checking code for consumers of the directives.

PR Close #26203

packages/common/src/directives/ng_for_of.ts

@@ -198,6 +198,16 @@ export class NgForOf<T> implements DoCheck {
       view: EmbeddedViewRef<NgForOfContext<T>>, record: IterableChangeRecord<any>) {
     view.context.$implicit = record.item;
   }
+
+  /**
+   * Assert the correct type of the context for the template that `NgForOf` will render.
+   *
+   * The presence of this method is a signal to the Ivy template type check compiler that the
+   * `NgForOf` structural directive renders its template with a specific context type.
+   */
+  static ngTemplateContextGuard<T>(dir: NgForOf<T>, ctx: any): ctx is NgForOfContext<T> {
+    return true;
+  }
 }
 
 class RecordViewTuple<T> {

packages/common/src/directives/ng_if.ts

@@ -159,6 +159,16 @@ export class NgIf {
 
   /** @internal */
   public static ngIfUseIfTypeGuard: void;
+
+  /**
+   * Assert the correct type of the expression bound to the `ngIf` input within the template.
+   *
+   * The presence of this method is a signal to the Ivy template type check compiler that when the
+   * `NgIf` structural directive renders its template, the type of the expression bound to `ngIf`
+   * should be narrowed in some way. For `NgIf`, it is narrowed to be non-null, which allows the
+   * strictNullChecks feature of TypeScript to work with `NgIf`.
+   */
+  static ngTemplateGuard_ngIf<E>(dir: NgIf, expr: E): expr is NonNullable<E> { return true; }
 }
 
 export class NgIfContext {

packages/core/test/render3/compiler_canonical/small_app_spec.ts

@@ -101,7 +101,8 @@ class ToDoAppComponent {
 
 // NON-NORMATIVE
 (ToDoAppComponent.ngComponentDef as r3.ComponentDef<any>).directiveDefs = () =>
-    [ToDoItemComponent.ngComponentDef, (NgForOf as r3.DirectiveType<NgForOf<any>>).ngDirectiveDef];
+    [ToDoItemComponent.ngComponentDef,
+     (NgForOf as unknown as r3.DirectiveType<NgForOf<any>>).ngDirectiveDef];
 // /NON-NORMATIVE
 
 @Component({

tools/public_api_guard/common/common.d.ts

@@ -263,6 +263,7 @@ export declare class NgForOf<T> implements DoCheck {
     ngForTrackBy: TrackByFunction<T>;
     constructor(_viewContainer: ViewContainerRef, _template: TemplateRef<NgForOfContext<T>>, _differs: IterableDiffers);
     ngDoCheck(): void;
+    static ngTemplateContextGuard<T>(dir: NgForOf<T>, ctx: any): ctx is NgForOfContext<T>;
 }
 
 export declare class NgForOfContext<T> {
@@ -282,6 +283,7 @@ export declare class NgIf {
     ngIfElse: TemplateRef<NgIfContext> | null;
     ngIfThen: TemplateRef<NgIfContext> | null;
     constructor(_viewContainer: ViewContainerRef, templateRef: TemplateRef<NgIfContext>);
+    static ngTemplateGuard_ngIf<E>(dir: NgIf, expr: E): expr is NonNullable<E>;
 }
 
 export declare class NgIfContext {