Polymorphic `this` and Generics

First, thank you team for polymorphic this. It is really handy!

I think I ran into a case though that I am finding challenging, when I need to return a this, but the generics might have changed. For example:

class A<T> {
    private items: T[] = [];
    map<U>(callback: (item: T, idx: number, a: this) => U): this {
        // boring implementation details
    });
}

Where I want to be able to change the generic type for the class with a function, but I will be contracting to return the “current” class, but I want to guard different generics. The following seems logical to me, but doesn’t appear to be currently supported:

class A<T> {
    private items: T[] = [];
    map<U>(callback: (item: T, idx: number, a: this) => U): this<U> {
        // boring implementation details
    });
}

Where if no generics arguments are supplied, it is inferred to be the current ones, where as if they are supplied they are substituted.

[dead-claudia]

Yeah, they fixed the inconsistency. The workaround is to use type intersection, but IMHO it’s not a great workaround given it prevents me from assuming this extends T (which is actually useful on occasion, believe it or not), and it’s forced me to recast types and parts of my data model at the type level more than once, in ways that were just boilerplatey hacks. I just wish they would allow extends T and extends this in an F-bounded fashion - it’d make certain type-heavy modeling cases so much easier.

[saschanaz]

I agree that this is related with #6220 and I think TS should be able to express ES6 Promise subclassing behavior.