Often the types of an `@Input`'s field don't fully reflect the types of
assignable values. This can happen when an input has a getter/setter pair
where the getter always returns a narrow type, and the setter coerces a
wider value down to the narrow type.
For example, you could imagine an input of the form:
```typescript
@Input() get value(): string {
return this._value;
}
set value(v: {toString(): string}) {
this._value = v.toString();
}
```
Here, the getter always returns a `string`, but the setter accepts any value
that can be `toString()`'d, and coerces it to a string.
Unfortunately TypeScript does not actually support this syntax, and so
Angular users are forced to type their setters as narrowly as the getters,
even though at runtime the coercion works just fine.
To support these kinds of patterns (e.g. as used by Material), this commit
adds a compiler feature called "input coercion". When a binding is made to
the 'value' input of a directive like MatInput, the compiler will look for a
static field with the name ngAcceptInputType_value. If such a field is found
the type-checking expression for the input will use the static field's type
instead of the type for the @Input field,allowing for the expression of a
type conversion between the binding expression and the value being written
to the input's field.
To solve the case above, for example, MatInput might write:
```typescript
class MatInput {
// rest of the directive...
static ngAcceptInputType_value: {toString(): string};
}
```
FW-1475 #resolve
PR Close#33243
f1269d98dc