generics - TypeScript generics shows in intellisense but does not compile

Question

I'm trying to use generics in backbone typescript definition. Why does the following code not work?! Try it in the playground.

declare module Backbone {
    class Model{}

    interface ViewOptions<TModel extends Model> {
        model?: TModel;
    }

    class View<TModel extends Model> {
        constructor(options?: ViewOptions<TModel>);
        model: TModel;
    }
}

class MyModel extends Backbone.Model {
}

class MyView extends Backbone.View<MyModel> {
}

// Error: Could not select overload for 'new'' expression.
var myView = new MyView({ model: new MyModel(), num: 1 });

// In typescript playground when you hover on 'model' in
// previous line it correctly shows its type as 'MyModel'

var model = myView.model; // expected to be of type MyModel

Update:

I figured the compiler will pass if I explicitly set the constructor for MyView:

class MyView extends Backbone.View<MyModel> {
    constructor(options?: Backbone.ViewOptions<MyModel>) {
        super();
    }
}

But then what is the use of generics?! Is this a defect in the compiler?!

Answer 1

If you do not specify your constructor the compiler expects TModel and not MyModel which is what you are trying to pass in. Seems like a compiler bug Your understanding of generics is correct.

Additionally be sure to pass over the parameter to the super class :

class MyView extends Backbone.View<MyModel> {
     constructor(options?: Backbone.ViewOptions<MyModel>) {
        super(options); // Pass this on 
    }
}