Allow decorated accessor properties to have a different type

🔍 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

• 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

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)

Start with the linked TypeScript playground and related issue #52559 to compare current behavior and prior discussion. Use the motivating Demo example as the acceptance case; done means it type-checks with this.num inferred as number while preserving existing runtime behavior.