Polymorphic "this" for static members #5862

When trying to implement a fairly basic, but polymorphic, active record style model system we run into issues with the type system not respecting this when used in conjunction with a constructor or template/generic.

I've posted before about this here, #5493, and #5492 appears to mention this behavior also.

And here is an SO post this that I made:
http://stackoverflow.com/questions/33443793/create-a-generic-factory-in-typescript-unsolved

I have recycled my example from #5493 into this ticket for further discussion. I don't have an immediate proposal for how to correct this but I wanted an open ticket representing the desire for such a thing.

Here is an example that outlines a model Factory which produces models. If you want to customize the BaseModel that comes back from the Factory you should be able to override it. However this fails because this cannot be used in a static member.

// Typically in a library
export interface InstanceConstructor<T extends BaseModel> {
    new(fac: Factory<T>): T;
}

export class Factory<T extends BaseModel> {
    constructor(private cls: InstanceConstructor<T>) {}

    get() {
        return new this.cls(this);
    }
}

export class BaseModel {

    constructor(private fac: Factory<this>) {}

    refresh() {
        // get returns a new instance, but it should be of
        // type Model, not BaseModel.
        return this.fac.get();
    }
}

// Application Code
export class Model extends BaseModel {
    do() {
        return true;
    }
}

// Kinda sucks that Factory cannot infer the "Model" type
let f = new Factory<Model>(Model);
let a = f.get();

let b = a.refresh();