inc-dec.ts

type CreateTuple<Len extends number, Acc extends unknown[] = []> = 
  Acc['length'] extends Len
  ? Acc
  : CreateTuple<Len, [...Acc, true]>;
;
type Length<Tup extends Array<unknown>> =
  Tup['length']
;

type Unshift<Tuple extends Array<unknown>, Value> =
  [Value, ...Tuple]
;
type _1 = Unshift<['b', 'c'], 'a'>; // ["a", "b", "c"]

type Rest<Tup extends Array<unknown>> =
  Tup extends [unknown, ...infer Rest]
  ? Rest
  : Tup
;
type _2 = Rest<['a', 'b', 'c']>; // ["b", "c"]

type Inc<Num extends number> =
  Length<Unshift<CreateTuple<Num>, true>>
;
type _3 = Inc<5>; // 6

type Dec<Num extends number> =
  Length<Rest<CreateTuple<Num>>>
;
type _4 = Dec<5>; // 4