
NgIf
context variables in template type checker (#36627)
When the `NgIf` directive is used in a template, its context variables can be used to capture the bound value. This is sometimes used in complex expressions, 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="enabled && user as u">{{u.name}}</span> ``` 2. Binding to `NgIfContext.$implicit` using the `let` syntax: ```html <span *ngIf="enabled && user; let u">{{u.name}}</span> ``` Because of the semantics of `ngIf`, it is known that the captured context variable is truthy, however the template type checker would not consider them as such and still report errors when `strict` is enabled. This commit updates `NgIf`'s context guard to make the types of the context variables truthy, avoiding the issue. Based on https://github.com/angular/angular/pull/35125 PR Close #36627
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