Decorated classes should implement the decorator return type.

[pietschy]

This is strongly related to #4881 but I'm not sure if it's feature request or if it's really a bug. Given that the Decorators spec states the decorator can return a new constructor function, it sort of seems like it should work.

The following example uses constructor function types and very nearly works.

View this in playground

interface ConstructorFunction<T> {
    new (...args:any[]):T;
    prototype:T
}

interface Disappearing {
    isVisible():boolean;
    vanish():void;
}

function Disappearing<T>(ctor:ConstructorFunction<T>):ConstructorFunction<T & Disappearing> {
    let result = <ConstructorFunction<T & Disappearing>>ctor;
    result.prototype.isVisible = () => {
        return !this['_hasDisappeared'];
    };
    result.prototype.vanish = () => {
        this['_hasDisappeared'] = true;
    };
    return result;
}

// This seems like it should work
// @Disappearing
// class Cat {
//     constructor(private name:string) {}
// }

// but I have to do this
@Disappearing
class Cat implements Disappearing {
    constructor(private name:string) {}
    isVisible() {return false}
    vanish(){}
}

// let ctor = wrappedBlah(Cat);
let buster = new Cat('Buster');
alert(`Is Buster visible? -> ${buster.isVisible()}`);
buster.vanish();
alert(`Is Buster visible? -> ${buster.isVisible()}`);

[mhegazy]

The issue here is that the tyep system is not aware of any type mutations the decoarator does to the original type. #4881 tracks supporting this. This issue is a duplicate of #4881.

for your scenario. you use interface/class merging to get what you want:

@Disappearing
class Cat {
    constructor(private name:string) {}
}

interface Cat extends Disappearing {}

let buster = new Cat('Buster');
buster.vanish();

[pietschy]

Thanks @mhegazy,

I guess I was hoping this scenario was different since the decorator return types were explicitly defined by the function parameters and return types.

[mhegazy]

i agree that this is helpful, but that is #4881.