Type alias FlatArray<Arr, Depth>

FlatArray<Arr, Depth>: {
    done: Arr;
    recur: Arr extends ReadonlyArray<infer InnerArr>
        ? FlatArray<InnerArr, [-1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20][Depth]>
        : Arr;
}[Depth extends -1
    ? "done"
    : "recur"]