Two-way bindings are meant to represent a property binding to an input and an event binding to an output, e.g. `[(ngModel)]="foo"` represents `[ngModel]="foo" (ngModelChange)="foo = $event"`. Previously due to a quirk in the template parser, we accidentally supported unassignable expressions in two-way bindings.
In #54154 the quirk was fixed, but we kept support or some common expression because of internal usages. Now the internal usages have been cleaned up so the backwards-compatibility code can be deleted.
Externally a migration was added in #54630 that will automatically fix any places that depended on the old behavior.
BREAKING CHANGE:
Angular only supports writable expressions inside of two-way bindings.
PR Close#55342
As a part of #54154, an old parser behavior came up where two-way bindings were parsed by appending `= $event` to the event side. This was problematic, because it allowed some non-writable expressions to be passed into two-way bindings.
These changes introduce a migration that will change the two-way bindings into two separate input/output bindings that represent the old behavior so that in a future version we can throw a parser error for the invalid expressions.
```ts
// Before
@Component({
template: `<input [(ngModel)]="a && b"/>`
})
export class MyComp {}
// After
@Component({
template: `<input [ngModel]="a && b" (ngModelChange)="a && (b = $event)"/>`
})
export class MyComp {}
```
PR Close#54630
Joey Perrott