Allow to overwrite generic templates

Search Terms

• overwrite generic this type
• overwrite template argument
• class overwrite generic
• overwrite type generic

Suggestion

Allow to overwrite generic type templates.

Use Cases

I'm currently developing a modular ORM and query builder, where especially in the query builder part it would be necessary to overwrite a generic template argument.

Examples

Here's a super simple example of a query builder. The select method should return a new instance of Query but with a different subset of T (since it limits the fields being selected).

interface Entity {}

class Query<T extends Entity> {
    findOne(): T;

    select(names: (keyof T)[]): this<Partial<T>>; //doesn't work
}

interface User {
    username: string;
    group: string;
    created: Date;
}

const item: User = new Query<User>().findOne();
const item: Partial<User> = new Query<User>().select(['username']).findOne();

This is just to demonstrate the use case. You could of course further describe the type of select by using Pick etc, but I wanted to keep it simple. Please note that I could return Query<Partial<T>> instead, but that would break class inheritance.

To my awareness this use case can't be covered with the newest TypeScript version.

Another use case I had:

interface Query<T> {
    findOne(): T;
}

interface AdapterInterface {
    createQuery<T extends Entity>(classType: Constructor<T>): Query<T>;
}

class Database<A extends AdapterInterface> {
    constructor (private adapter: A) {}

    // here is the magic with the return type, which doesn't work, but is necessary since
    // `T` of A['createQuery'] needs to be inferred from query<T>.
    query<T extends Entity>(classType: Constructor<T>): ReturnType<A['createQuery']><T> {
        return this.adapter.createQuery(classType);
    }
}

class MyAdapterQuery<T> extends Query<T> {
     additionalMethod(): this;
}

class MyAdapter {
    createQuery<T extends Entity>(classType: Constructor<T>): MyAdapterQuery<T> {
         return new MyAdapterQuery<T>();
    }
}


const database = new Database(new MyAdapter);
database.query(User).additionalMethod();

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

Start with the generic-template examples in the issue and review how TypeScript currently models generic class and method return types. Determine the intended type-checking semantics and assess the required compiler design before identifying implementation and test locations; done means the proposed cases type-check without changing emitted JavaScript.