Mapped types on tuples and arrays not working recursively

[Voronar]

TypeScript Version: 3.2.2 (ts playground)

Search Terms: recursive, mapped types, tuple, array

Code

type DeepMap<T extends unknown[], R> = {
  [K in keyof T]: T[K] extends unknown[] ? DeepMap<T[K], R> : R;
};

type tup = [string, [string, [string]]];
type arr = string[][];

type t1 = DeepMap<tup, number>;
type t2 = DeepMap<arr, number>;

Expected behavior:

type t1 = [number, [number, [number]]];
type t2 = number[][];

Actual behavior:

type t1 = [number, any];
type t2 = any[];

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