type FlatArray<Arr, Depth extends number> = {
  "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"];
...
interface Array<T> {
  flatMap<U, This = undefined> (
    callback: (this: This, value: T, index: number, array: T[]) => U | ReadonlyArray<U>,
    thisArg?: This
  ): U[]

  flat<A, D extends number = 1>(
    this: A,
    depth?: D
  ): FlatArray<A, D>[]
}