Consider the following case:

class Entry<T> {
    constructor(callback: (v: T) => number) { }
}
interface Widget {
    list: Entry<{ foo: number }>[];
}

var widget: Widget = {
    list: [
        new Entry((v) => v.foo), // Error accessing v.foo
    ]
};

We get a compiler error in the callback accessing v.foo that foo does not exist on type {} since {} is the default type when it is unable to infer the type. However, in this case we know from the Widget definition that T must be typed to {foo:number}. But, it appears TypeScript is not using this as an inference-site to properly determine T.