type FillImplActive<
  T extends unknown[],
  N,
  End extends number,
  Pos extends readonly unknown[],
  > =
  End extends Pos["length"]
  ? T
  : T extends [infer T1, ... infer T2]
    ? [N, ...FillImplActive<T2,N,End,[...Pos,0]>]
    : []

type FillImplInactive<
  T extends unknown[],
  N,
  Start extends number,
  End extends number,
  Pos extends readonly unknown[],
  > =
  Start extends Pos["length"]
  ? FillImplActive<T,N,End,Pos>
  : T extends [infer T1, ...infer T2]
    ? [T1, ...FillImplInactive<T2,N,Start,End,[...Pos,0]>]
    : []

type Fill<
  T extends unknown[],
  N,
  Start extends number = 0,
  End extends number = T['length'],
  > = FillImplInactive<T,N,Start,End,[]>

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

type cases = [
  Expect<Equal<Fill<[], 0>, []>>,
  Expect<Equal<Fill<[], 0, 0, 3>, []>>,
  Expect<Equal<Fill<[1, 2, 3], 0, 0, 0>, [1, 2, 3]>>,
  Expect<Equal<Fill<[1, 2, 3], 0, 2, 2>, [1, 2, 3]>>,
  Expect<Equal<Fill<[1, 2, 3], 0>, [0, 0, 0]>>,
  Expect<Equal<Fill<[1, 2, 3], true>, [true, true, true]>>,
  Expect<Equal<Fill<[1, 2, 3], true, 0, 1>, [true, 2, 3]>>,
  Expect<Equal<Fill<[1, 2, 3], true, 1, 3>, [1, true, true]>>,
  Expect<Equal<Fill<[1, 2, 3], true, 10, 0>, [1, 2, 3]>>,
  Expect<Equal<Fill<[1, 2, 3], true, 0, 10>, [true, true, true]>>,
]