Support property decorators for shorthand constructor-parameter syntax

[lu4]

TypeScript Version: 2.0.2

Code
I'm taking advantage of shorthand property declarations syntax via constructor.
The code below contains a problem, it won't work as expected for class City, it such case typescript will treat PropertyDecorator as ParameterDecorator, there is no way to supply both PropertyDecorator and ParameterDecorator simultaneously for such shorthand syntax:

function PropertyDecorator(target: Object, propertyKey: string | symbol) { }
function ParameterDecorator(target: Object, propertyKey: string | symbol, index: number) {}

class Country {
    @PropertyDecorator public name: string;

    public constructor(name: string) {
        this.name = name;
    }
}

class City {
    public constructor(
        @PropertyDecorator public name: string,
        @PropertyDecorator public country: Country) {
    }
}

It looks better to use the following syntax that would allow distinguishing different decorator types:

function PropertyDecorator(target: Object, propertyKey: string | symbol) { }
function ParameterDecorator(target: Object, propertyKey: string | symbol, index: number) {}

class Country {
    public name: string;

    public constructor(name: string) {
        this.name = name;
    }
}

class City {
    public constructor(
        @PropertyDecorator public @ParameterDecorator name: string,
        @PropertyDecorator public @ParameterDecorator country: Country) {
    }
}

[mhegazy]

Since the declaration serves as a parameter and property declaration the use of decorators here is ambiguous; frankly if you show one to me and ask me if it is parameter or property decorator I would not be able to answer. This is the rational behind disallowing it. Moreover the solution is not that complicated, define your property separetely and decorat it.

[lu4]

Than if it's ambiguous why does it fallback to ParameterDecorator? It still remains ambiguous in such case. And what I was proposing is the way to resolve that ambiguity...

[mhegazy]

It still remains ambiguous in such case. And what I was proposing is the way to resolve that ambiguity...

I believe the proposed change is subtle. for humans it is still ambiguous. It would be hard to understand for a user new to this feature. moreover, the other option of unrolling the parameter property declaration to a separate property and parameter declarations is not much harder to write.

[lu4]

Yes I agree, it isn't hard to unroll parameter property declaration, this is the thing I've did myself right after I've understood the problem. But before doing that I've invested half of my day debugging Angular 2 core and then then debugging and playing with transpiled JavaScript.

I don't want everyone else to bump into this problem so the argument that "It would be hard to understand for a user new to this feature" is hard to agree from my personal point of view.

On the other hand it is not clear why this is the intended behavior. There are two independent features in typescript: "parameter property declaration" and "parameter decorators". These features make Typescript such a nice and convenient language. Both of them were mend to be used by language users. If they are mutually exclusive, if one cancels another it harms the whole concept why those features were added to TypeScript.

So what I'm trying to say is that while Typescript decorators are an experimental feature it's better to at least consider this problem rather than bounce it off because later it might be even bigger breaking change problem.

[mhegazy]

But before doing that I've invested half of my day debugging Angular 2 core and then then debugging and playing with transpiled JavaScript.

OK, this is a bug. sorry about that. you should get an error "Decorators are not allowed here". but apparently that is not the case. we should fix this and make the error message more useful.