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.

El issue describe una interacción propuesta entre this polimórfico y la sustitución de tipos genéricos, pero no menciona archivos de implementación, tests ni puntos de entrada. Comienza localizando el manejo de this polimórfico y la instanciación genérica del compilador; se considerará completado cuando se admita el comportamiento solicitado de this<U> con las reglas de inferencia y sustitución indicadas, y esté cubierto por tests.