It's possible to pass a directive as an input to itself. Consider:
```html
<some-cmp #ref [value]="ref">
```
Since the template type-checker attempts to infer a type for `<some-cmp>`
using the values of its inputs, this creates a circular reference where the
type of the `value` input is used in its own inference:
```typescript
var _t0 = SomeCmp.ngTypeCtor({value: _t0});
```
Obviously, this doesn't work. To resolve this, the template type-checker
used to generate a `null!` expression when a reference would otherwise be
circular:
```typescript
var _t0 = SomeCmp.ngTypeCtor({value: null!});
```
This effectively asks TypeScript to infer a value for this context, and
works well to resolve this simple cycle. However, if the template
instead tries to use the circular value in a larger expression:
```html
<some-cmp #ref [value]="ref.prop">
```
The checker would generate:
```typescript
var _t0 = SomeCmp.ngTypeCtor({value: (null!).prop});
```
In this case, TypeScript can't figure out any way `null!` could have a
`prop` key, and so it infers `never` as the type. `(never).prop` is thus a
type error.
This commit implements a better fallback pattern for circular references to
directive types like this. Instead of generating a `null!` in place for the
reference, a type is inferred by calling the type constructor again with
`null!` as its input. This infers the widest possible type for the directive
which is then used to break the cycle:
```typescript
var _t0 = SomeCmp.ngTypeCtor(null!);
var _t1 = SomeCmp.ngTypeCtor({value: _t0.prop});
```
This has the desired effect of validating that `.prop` is legal for the
directive type (the type of `#ref`) while also avoiding a cycle.
Fixes#35372Fixes#35603Fixes#35522
PR Close#35622
173a1ac8e4