Improve error message for decorator type mismatches

In TS 1.5 this:

//Relation.ts
export function Relation(cls: any) {
    return function () {
        console.log(cls);
    }
}

//Foo.ts
import {Relation} from "./Relation";

export class Foo {
    @Relation(Foo)
    info: string;
}

Compiles without error. In TS 1.6, it reports the error:

error TS1240: Unable to resolve signature of property decorator when called as an expression.
  Supplied parameters do not match any signature of call target.

On the decorator call in Foo.ts. This is a very cryptic message. What it wants you to do is change Relation.ts to read like this:

export function Relation(cls: any): PropertyDecorator {
    return function () {
        console.log(cls);
    }
}

or this:

export function Relation(cls: any) {
    return function (target: Object, propertyKey: string | symbol) {
        console.log(cls);
    }
}

It would be very useful for the error message to indicate what exactly is wrong and how it could be fixed. Something akin to

error TS1240: Expression type is not assignable to decorator type [[PropertyDecorator]]. Ensure [[@Relation(Foo)]] has a type assignable to [[PropertyDecorator]].

would be more useful in actually understanding what’s wrong and why you need to change your code.

[stewhouston]

This may be tangential, but I had a tough time finding a solution. I was able to create a class factory by casting the return function. Not sure if this is a valid design pattern, but I am using it for dependency injection similar to what is seen in Angular2.

return function(decoratedClass:any) {
    function wrapConstructor(app) {
        return new decoratedClass(app, ...providers);
    }

    wrapConstructor.prototype = decoratedClass.prototype;

    return <typeof decoratedClass> wrapConstructor;
}

[mhegazy]

Looks like a fair proposal. feel free to send a PR if you like.