type SubsequenceFromHead<T> = T extends [infer Head] ? [] | [Head] : T extends [infer Head, ...infer Tail]
  ? [] | [Head, ...SubsequenceFromHead<Tail>] : never
type Subsequence<T> = T extends [unknown, ...infer Tail]
  ? SubsequenceFromHead<T> | Subsequence<Tail> : never

import type { Equal, Expect } from '@type-challenges/utils'

type cases = [
  Expect<Equal<Subsequence<[1, 2]>, [] | [1] | [2] | [1, 2]>>,
  Expect<Equal<Subsequence<[1, 2, 3]>, [] | [1] | [2] | [1, 2] | [3] | [1, 3] | [2, 3] | [1, 2, 3] >>,
]