fix(ivy): narrow NgIf context variables in template type checker (#35125)

When the `NgIf` directive is used in a template, its context variables
can be used to capture the bound value. This is typically used together
with a pipe or function call, where the resulting value is captured in a
context variable. There's two syntax forms available:

1. Binding to `NgIfContext.ngIf` using the `as` syntax:
```html
<span *ngIf="(user$ | async) as user">{{user.name}}</span>
```

2. Binding to `NgIfContext.$implicit` using the `let` syntax:
```html
<span *ngIf="user$ | async; let user">{{user.name}}</span>
```

Because of the semantics of `ngIf`, it is known that the captured
context variable is non-nullable, however the template type checker
would not consider them as such and still report errors when
`strictNullTypes` is enabled.

This commit updates `NgIf`'s context guard to make the types of the
context variables non-nullable, avoiding the issue.

Fixes #34572

PR Close #35125

packages/compiler-cli/test/ngtsc/template_typecheck_spec.ts

@@ -70,7 +70,7 @@ export declare class NgIf<T = unknown> {
   ngIfThen: TemplateRef<NgIfContext<T>> | null;
   constructor(_viewContainer: ViewContainerRef, templateRef: TemplateRef<NgIfContext<T>>);
   static ngTemplateGuard_ngIf: 'binding';
-  static ngTemplateContextGuard<T>(dir: NgIf<T>, ctx: any): ctx is NgIfContext<T>;
+  static ngTemplateContextGuard<T>(dir: NgIf<T>, ctx: any): ctx is NgIfContext<NonNullable<T>>;
   static ɵdir: i0.ɵɵDirectiveDefWithMeta<NgIf<any>, '[ngIf]', never, {'ngIf': 'ngIf'}, {}, never>;
 }
 
@@ -805,6 +805,54 @@ export declare class AnimationEvent {
       env.driveMain();
     });
 
+    it('should check usage of NgIf when using "let" to capture $implicit context variable', () => {
+      env.tsconfig({strictTemplates: true});
+      env.write('test.ts', `
+    import {CommonModule} from '@angular/common';
+    import {Component, NgModule} from '@angular/core';
+
+    @Component({
+      selector: 'test',
+      template: '<div *ngIf="user; let u">{{u.name}}</div>',
+    })
+    class TestCmp {
+      user: {name: string}|null;
+    }
+
+    @NgModule({
+      declarations: [TestCmp],
+      imports: [CommonModule],
+    })
+    class Module {}
+    `);
+
+      env.driveMain();
+    });
+
+    it('should check usage of NgIf when using "as" to capture `ngIf` context variable', () => {
+      env.tsconfig({strictTemplates: true});
+      env.write('test.ts', `
+    import {CommonModule} from '@angular/common';
+    import {Component, NgModule} from '@angular/core';
+
+    @Component({
+      selector: 'test',
+      template: '<div *ngIf="user as u">{{u.name}}</div>',
+    })
+    class TestCmp {
+      user: {name: string}|null;
+    }
+
+    @NgModule({
+      declarations: [TestCmp],
+      imports: [CommonModule],
+    })
+    class Module {}
+    `);
+
+      env.driveMain();
+    });
+
     it('should check basic usage of NgFor', () => {
       env.write('test.ts', `
     import {CommonModule} from '@angular/common';