The template type checker generates TypeScript expressions for any
expression that occurs in a template, so that TypeScript can check it
and produce errors. Some expressions as they occur in a template may be
translated into TypeScript code multiple times, for instance a binding
to a directive input that has a template guard.
One example would be the `NgIf` directive, which has a template guard to
narrow the type in the template as appropriate. Given the following
template:
```typescript
@Component({
template: '<div *ngIf="person">{{ person.name }}</div>'
})
class AppComponent {
person?: { name: string };
}
```
A type check block (TCB) with roughly the following structure is
created:
```typescript
function tcb(ctx: AppComponent) {
const t1 = NgIf.ngTypeCtor({ ngIf: ctx.person });
if (ctx.person) {
"" + ctx.person.name;
}
}
```
Notice how the `*ngIf="person"` binding is present twice: once in the
type constructor call and once in the `if` guard. As such, TypeScript
will check both instances and would produce duplicate errors, if any
were found.
Another instance is when the safe navigation operator is used, where an
expression such as `person?.name` is emitted into the TCB as
`person != null ? person!.name : undefined`. As can be seen, the
left-hand side expression `person` occurs twice in the TCB.
This commit adds the ability to insert markers into the TCB that
indicate that any errors within the expression should be ignored. This
is similar to `@ts-ignore`, however it can be applied more granularly.
PR Close#34417
3959511b80