type Constructor<T = {}> = new (...args: any[]) => T;

class Point {
  constructor(
    public readonly x: number,
    public readonly y: number,
  ) {}
}

// Tagged is a mixin with a type parameter.
const Tagged = <T, B extends Constructor = B>(Base: B) =>
  class extends Base {
    getTag(): T {
      // Just imagine I have a function that returns objects,
      // and I want to case them to T.
      return {} as T
    }
  }

class TaggedPoint extends Tagged(Point) {}

class TaggedPoint extends Tagged<number>(Point) {}

class UsesTheTag<T> extends Tagged<T>(Point) {
//                                 ^
// error: Base class expressions cannot reference
// class type parameters.
  useTag() {
    const tag: T = this.getTag()
  }
}