newtype experiment in typescript

newtype.ts

import { do_ } from "@/utility";

type Newtype<R extends object, S extends keyof R, T> = R[S] extends void
  ? R & T
  : never;

const X1: unique symbol = Symbol();
type X1 = Newtype<{ [X1]: void }, typeof X1, number>;

const X2: unique symbol = Symbol();
type X2 = Newtype<{ [X2]: void }, typeof X2, string>;

function square<A>(x: A, y: A) {
  return [x, y];
}

do_(() => {
  const x1: X1 = 100 as X1;
  const x2: X2 = "hello world" as X2;
  square(x1, x1);
  // @ts-expect-error newtype distinguishes
  square(x1, x2);
});