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.

The issue describes a proposed interaction between polymorphic this and generic type substitution, but names no implementation files, tests, or entry points. Begin by locating the compiler's handling of polymorphic this and generic instantiation; done would mean the requested this<U> behavior is supported with the stated inference and substitution rules, covered by tests.