Repository: TypeScript
Issue: Request: Allow abstract classes to implement MappedTypes that are instantiated with type parameters.
Labels: Suggestion,In Discussion

Ok, so the title is a mouthful, but here's an example of what we're trying to accomplish:

```ts
interface DeferredProperty<T> {
    get(): T;
}

type Deferred<T> = {
    [P in keyof T]: DeferredProperty<T[P]>
};
```

Here we use mapped types to express the idea that a interface type might be wrapped with a new type that exposes the same properties as it, except as functions instead of data properties.  So, for example:

```ts
interface Person {
    age: number,
    name: string
}
var deferredPerson: Deferred<Person>;
var age = deferredPerson.age.get();


interface Vehicle {
    numWheels: number,
    cost: number
}
var deferredVehicle: Deferred<Vehicle>;
var cost = deferredVehicle.cost.get();
```

We'd like to make a lot of our types deferable in our system, so we create a simple abstract base class that will help us out by doing some of the work for us.  In other words, we'd like to be able to write:

```ts
abstract class BaseDeferred<T> implements Deferred<T> { //<-- note: currently not legal
    // some common stuff, including concreate methods

    // Maybe some abstract methods.
    // protected abstract whatever(): void;

    // etc.
}
```

We could then do the following:

```ts
class DeferredPerson extends BaseDeferred<Person> {
}
class DeferredVehicle extends BaseDeferred<Vehicle> {
}
```

At this point TypeScript would say: "Hey, DeferredPerson doesn't properly implement ```BaseDeferred<Person>```, it is missing ```age: DeferredProperty<number>``` and ```name: DeferredProperty<string>```.   (And likewise for DeferredVehicle)

However, this isn't currently allowed as we cannot say: ```abstract class BaseDeferred<T> implements Deferred<T>```

This is a somewhat understandable restriction.  After all, how can the compiler actually validate that ```BaseDeferred<T>``` is implementing ```Deferred<T>``` when it cannot know (at this point) how the ```Deferred<T>``` lookup type will expand.

While understandable, it would be nice if this restriction could potentially be lifted for abstract types.  Because the type is abstract, we would like it if the check was only actually done at the time the type was concretely derived from.  So, for example, when someone wrote:

```ts
class DeferredPerson extends BaseDeferred<Person> {
	// Now the compiler create the full type signature for BaseDeferred<Person> and then checked DeferredPerson against it.
}
```
--

The workaround today is to do the following:

```ts
abstract class BaseDeferred<T> { // <-- note: no implements
}

class DeferredPerson extends BaseDeferred<Person> implements Deferred<Person> {
}
class DeferredVehicle extends BaseDeferred<Vehicle> implements Deferred<Vehicle> {
}
```

The compiler now appropriately does the right checks.  This is unpleasant though as it's a very simple thing to miss.  Because all subclasses must implement this type, we would very much like to push that requirement up to the base class and have the enforcement applied uniformly across all subtypes.

--

Thanks much, and i hope everyone is doing great!  We're having a blast with TS, especially (ab)using the type system to express some very interesting things.  The more crazy stuff that can be expressed (especially around constraints and variadic types) the happier we are 🙂