Make property decorator fabrics share generics with corresponding methods

[finom]

🔍 Search Terms

decorator generic
decorator set type

✅ Viability Checklist

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

There are multiple requests to be able to modify class or its methods type

#4881
#49229

But I haven't found a much simpler request: be able to share property decorator fabric generic with its method.

📃 Motivating Example

There is the decorator fabric example, nothing fancy:

function paramDecoratorFabric<T>(arg: T) {
  return function (target: any, propertyKey: string) {
    console.log('Parameter Decorator called on: ', arg);
  }
}

And there is what I would be happy to be able to achieve:

class ParameterDecoratorExample {
  method<T>(@paramDecoratorFabric<T>('Hello') param: T) {
    return param;
  }
}

Unfortunately, I get the following error:

Which says that the T of @paramDecoratorFabric is kind of a different thing that doesn't belong to the "place" where it was used.

💻 Use Cases

I'm actively working on my RPC-ish library and one of the biggest downsides comparing to other RPC libraries is inability to automatically recognise Zod types provided at the controller. This requires to add the type to the controller method manually but also to create extra variables.

export default class UserController {
    @put()
    @vovkZod(UpdateUserModel, UpdateUserQueryModel)
    static updateUser(
        req: VovkRequest<z.infer<typeof UpdateUserModel>, z.infer<typeof UpdateUserQueryModel>>
    ) {
        // ...
    }
}

I understand that adding ability to modify method/class type by a decorator is quite challenging. But I hope that changing the "scope" of generics isn't that big deal. With the feature I'm requesting I would be able to achieve the goal without defining extra variables:

export default class UserController {
  @put()
  static updateUser<B, Q>(
    @vovkZod<B, Q>(z.object({ /*...*/ }), z.object({ /*...*/ })) req: VovkRequest<B, Q>
  ) {
      // ...
  }
}

The method and the parameter already share B and Q, and since @vovkZod is used "inside" of this definition, it makes sense for it to be able to also use B and Q.

[MartinJohns]

Duplicate of #2685. The reasoning still holds, and ECMAScript decorators don't even support parameter decorators.

[finom]

@MartinJohns right, that's a duplicate. Thanks for the link.