[Typescript] 119. Extreme - Get Readonly Keys

Implement a generic GetReadonlyKeys<T> that returns a union of the readonly keys of an Object.

For example

interface Todo {
  readonly title: string
  readonly description: string
  completed: boolean
}

type Keys = GetReadonlyKeys<Todo> // expected to be "title" | "description"

/* _____________ Your Code Here _____________ */

type GetReadonlyKeys<T> = keyof {
  [Key in keyof T as Equal<Pick<T, Key>, Readonly<Pick<T, Key>>> extends true ? Key: never]: T[Key]
}

/* _____________ Test Cases _____________ */
import type { Equal, Expect } from '@type-challenges/utils'

type cases = [
  Expect<Equal<'title', GetReadonlyKeys<Todo1>>>,
  Expect<Equal<'title' | 'description', GetReadonlyKeys<Todo2>>>,
]

interface Todo1 {
  readonly title: string
  description: string
  completed: boolean
}

interface Todo2 {
  readonly title: string
  readonly description: string
  completed?: boolean
}