newtype.ts

type NewType<Name extends string, Type> = Type & {
  readonly "": Type;
};

function to<T extends NewType<string, U>, U = T[""]>(x: U): T {
  return x as T;
}

type Size = NewType<"Size", number>;

function logSize(size: Size): void {
  console.log(size);
}


// const s = to<Size>("42"); // Fails
// const s = to<Size>(); // Fails
// const s = to<Size, string>("42"); // Fails


const s = to<Size>(42); // Works !

// logSize(42) // Fails

logSize(s) // Works !