Is there a way to add a type assertion / annotation to a template input variable?

Let’s see what angular has similar to that and how it work!

<p *ngFor="let number of [{v: 101},{v: 102}, {v: 103}]">{{number.v}}</p>

We can rewrite it without * magic

<ng-template ngFor let-number [ngForOf]="[{v: 101},{v: 102}, {v: 103}]">
  <p>{{number.v}}</p>
</ng-template>

Without watchers (ngDoCheck) it can be the same as (but ngTemplateOutlet have no typecheck):

<ng-template let-number #templateRef>
  <p>{{number.v}}</p>
</ng-template>
<ng-container *ngTemplateOutlet="templateRef; context: {$implicit: {v: 101}}"></ng-container>
<ng-container *ngTemplateOutlet="templateRef; context: {$implicit: {v: 102}}"></ng-container>
<ng-container *ngTemplateOutlet="templateRef; context: {$implicit: {v: 103}}"></ng-container>

Or we can create it by ourselves

// template
<button (click)=create(templateRef)>create</button>

// TS
constructor(private _viewContainerRef: ViewContainerRef) { ... }

create(templateRef: TemplateRef<{$implicit: {v: number;}}>) {
    this._viewContainerRef.createEmbeddedView(templateRef, {$implicit: {v: 101}});
    this._viewContainerRef.createEmbeddedView(templateRef, {$implicit: {v: 102}});
    this._viewContainerRef.createEmbeddedView(templateRef, {$implicit: {v: 103}});
}

TL;DR

Template typecheck magic happens inside viewContainerRef.createEmbeddedView. (for example ngFor);
But it assume what templateRef accepts.

Angular can compile in AOT:

<p *ngFor="let num of [{v:1}, {v:2}]">
  {{num.does.not.exist.completly}}
</p>

So as i understood we should assume what types templates have but do check when template is instantiated (by createEmbeddedView);