Allow decorated accessor properties to have a different type

[NullVoxPopuli]

🔍 Search Terms

I was pretty sure an issue already existed for this, but I couldn't find it. Hopefully opening a new issue (to maybe be closed again) will improve SEO.

Here is what I searched for:

• decorators change type
• decorators accessor type
• decorators change type accessor

I did find this related issue:

• https://github.com/microsoft/TypeScript/issues/52559

✅ Viability Checklist

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

⭐ Suggestion

Allow decorator's return type, ClassAccessorDecoratorResult to specify a different Value-type than what is on the right-hand side of the equals.

Here is how TS 5.6.3 handles this situation today.

📃 Motivating Example

For example, it should be possible to author this with 0 errors:

class Demo {
  @unwrap accessor num = { wrapped: 2 };
  //               ^? => number

  get usage() {
    expectTypeOf(this.num).toEqualTypeOf<number>();
  }
}

💻 Use Cases

1. Reactivity
2. Hiding complex behaviors from the user
3. Unwrapping private objects into public API (the example in the playground link)

[shigma]

A related issue for method decorators: https://github.com/microsoft/TypeScript/issues/49229