type Impl<B extends string, E extends string[], Delim extends string> =
  E extends [infer E1 extends string, ...infer E2 extends string[]]
  ? `${B}${Delim}${E1}` | Impl<B,E2,Delim>
  : never
type BE<B extends string, E extends string[]> =
  E extends [] ? B : Impl<B,E,"__">
type BM<B extends string, M extends string[]> =
  M extends [] ? B : Impl<B,M,"--">
type BEM<B extends string, E extends string[], M extends string[]> = BM<BE<B,E>,M>

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

type cases = [
  Expect<Equal<BEM<'btn', ['price'], []>, 'btn__price'>>,
  Expect<Equal<BEM<'btn', ['price'], ['warning', 'success']>, 'btn__price--warning' | 'btn__price--success' >>,
  Expect<Equal<BEM<'btn', [], ['small', 'medium', 'large']>, 'btn--small' | 'btn--medium' | 'btn--large' >>,
]