00090-hard-optional-keys.ts

// ============= Test Cases =============
import type { Equal, Expect } from './test-utils'

type cases = [
  Expect<Equal<OptionalKeys<{ a: number; b?: string }>, 'b'>>,
  Expect<Equal<OptionalKeys<{ a: undefined; b?: undefined }>, 'b'>>,
  Expect<
    Equal<
      OptionalKeys<{ a: undefined; b?: undefined; c?: string; d?: null }>,
      'b' | 'c' | 'd'
    >
  >,
  Expect<Equal<OptionalKeys<{}>, never>>
]

// ============= Your Code Here =============
type OptionalKeys<T> = keyof {
  [P in keyof T as T[P] extends Required<T>[P] ? never : P]: T[P]
}

type Test = OptionalKeys<{ a: number; b?: string }>