// building on top of
// https://gist.github.com/utatti/c411d0939094ba490ce6dd78c92ffe4c

type Nat = 0 | { succ: Nat };

type Succ<T extends Nat> = { succ: T };

type Add<X extends Nat, Y extends Nat> =
    X extends Succ<infer R> ? { succ: Add<R, Y> } : Y;

type N0  = 0;
type N1  = Succ<N0>;
type N2  = Succ<N1>;
type N3  = Succ<N2>;
type N4  = Succ<N3>;
type N5  = Succ<N4>;
type N6  = Succ<N5>;
type N7  = Succ<N6>;
type N8  = Succ<N7>;
type N9  = Succ<N8>;

type ToType<K> =
    K extends 0 ? N0 :
    K extends 1 ? N1 :
    K extends 2 ? N2 :
    K extends 3 ? N3 :
    K extends 4 ? N4 :
    K extends 5 ? N5 :
    K extends 6 ? N6 :
    K extends 7 ? N7 :
    K extends 8 ? N8 :
    K extends 9 ? N9 : never;

type ToNumber<N> =
    N extends N0 ? 0 :
    N extends N1 ? 1 :
    N extends N2 ? 2 :
    N extends N3 ? 3 :
    N extends N4 ? 4 :
    N extends N5 ? 5 :
    N extends N6 ? 6 :
    N extends N7 ? 7 :
    N extends N8 ? 8 :
    N extends N9 ? 9 : never;

declare function add<
    XK extends number,
    YK extends number,
>(x: XK, y: YK): ToNumber<Add<ToType<XK>, ToType<YK>>>;

// try commenting out this block, next block will show error
{
    const res: 6 = add(2, 4)
}

{
    const res: 8 = add(1, 7)
}