Polymorphic this type not inferred as expected in super calls #7728

class Base {

    public itemFactory: (i: any) => this;

    public save() {
        var xhr: PromiseLike<any> = fetch(....);
        return xhr.then((value) => this.itemFactory(value)); // becomes PromiseLike<this> here
    }
}

class Extend extends Base {
    public inExtended = true;

    public save() {
        // doing some stuff before calling save
        return super.save();
    }
}


var e = new Extend();
e.save().then(value => value.inExtended); // Compile Error. `inExtended` not in Base
e.itemFactory(new Object()).inExtended = true; // No error

// yet, if I don't override `save` the types are inferred as expected
class Extend2 extends Base {
    public inExtended2 = true;
}

var e2 = new Extend2();
e2.save().then(value => value.inExtended2); // No error
e2.itemFactory(new Object()).inExtended2 = true; // No error