Implement "this" type for intersection and union types

[datokrat]

TypeScript Version: 1.8.0

It seems that polymorphic 'this' types don't apply as expected to intersection and union types.

Motivating Example: Lightweight Mixins

This example was forked from the proposal on polymorphic 'this' types considering "lightweight mixin"s. Unfortunately, the model can only be extended once because 'this' only includes "real" class extensions and no union type extensions as shown in the example. That's why the current implementation doesn't help us with the specified example.

interface Model {
  extend<T>(t: T): this & T;
}

interface NamedModel extends Model {
  name: string;
}

declare function createNamedModel(): NamedModel;

var extendedModel = createNamedModel().extend({
  age: 30
}).extend({
  job: "programmer"
});
extendedModel.name; // valid
extendedModel.job; // valid
extendedModel.age; // error: Property 'age' does not exist because type is NamedModel & { job: string }

Suggestion

I suggest allowing the following:

interface A {
    getThis(): this;
}

let extended: A & { y: string };
extended.getThis().y;

This would require to assign intersection and union types to "this" instead of only superclasses of A. As far as I can see, this should not be too complicated/invasive, is it? I could not find any explicit decision against this feature, so maybe it's kind of a bug?