Support abstract mixin classes.

Search Terms

Related, but not completely the same: https://github.com/microsoft/TypeScript/issues/29653

Suggestion

Allow mixin classes to be abstract.

Use Cases

For example, by using a mixin, it might require the user to implement a method.

Examples

type Constructor<T = any, A extends any[] = any[]> = new (...a: A) => T

function AwesomeMixin<T extends Constructor>(Base: T = Object) {
    return abstract class Awesome extends Base {
        abstract someMethod(): number
    }
}

playground link

gives:

Cannot find name 'abstract'.

and

Unreachable code detected

Checklist

My suggestion meets these guidelines:

• 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, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

Start with the abstract mixin example and its linked playground, then compare the related issue #29653. Confirm how the compiler currently parses and checks abstract in returned mixin classes; done means the example type-checks without the reported errors while emitted JavaScript remains unchanged.