type CreateArray<N, D extends readonly 0[] = []> = D["length"] extends N ?
  D:
  CreateArray<N, [...D, 0]>;
type Sum<X, Y> = [...CreateArray<X>, ...CreateArray<Y>]["length"];
type LengthOfString<S extends string, N = 0> = S extends `${infer F}${infer Tail}` ?
  LengthOfString<Tail, Sum<N, 1>>
  :N

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

type cases = [
  Expect<Equal<LengthOfString<''>, 0>>,
  Expect<Equal<LengthOfString<'kumiko'>, 6>>,
  Expect<Equal<LengthOfString<'reina'>, 5>>,
  Expect<Equal<LengthOfString<'Sound! Euphonium'>, 16>>,
]