allow decorator return function with different returnType

[LongTengDao]

🔍 Search Terms

decorator

✅ 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

use decorator like higher-order function

📃 Motivating Example

function co<T> (method :() => Generator<void, T, void>, context :ClassMethodDecoratorContext) {
    return function () :T {
        const generator = method();
        let result;
        while ( !( result = generator.next() ).done ) ) {}
        return result.value;
    };
}

class X {
    @co
    * method () { return 1; }
}

function reiterable<T> (method :() => Generator<T, void, void>, context :ClassMethodDecoratorContext) {
    return function () :Iterable<T> {
        return { [Symbol.iterator]: method };
    };
}

class X {
    @reiterable
    * method () { yield 1; }
}

💻 Use Cases

1. What do you want to use this for?
2. What shortcomings exist with current approaches?
3. What workarounds are you using in the meantime?

[MartinJohns]

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