Improve error message for decorator type mismatches

[weswigham]

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.

[mhegazy]

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

[aciccarello]

Something similar for TS1238: Unable to resolve signature of class decorator when called as an expression. would also be helpful

[stepancar]

Hello! Whats status of this issue?

[mhegazy]

it is marked as "Accepting PRs", so PRs are welcomed :)

[origin1tech]

As of the date of this post this still seems to be an issue.

The following:

const typeMetadataKey = Symbol('type');

function type(name: string): PropertyDescriptor {
 return Reflect.metadata(typeMetadataKey, name);
}

Produces the error: Unable to resolve signature of class decorator when called as an expression..

When used as below:

class Person {
  @type('string')
  firstName: string;
}

The decorator does work as expected and will compile but gives the error above.

If you have thoughts on how this might be resolved happy to dig into it if someone would like to point to the right direction.

[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;
}

[lizzzp1]

Could I take this one?