Implement the advanced util type OptionalKeys<T>, which picks all the optional keys into a union.
import type { Equal, Expect } from '@type-challenges/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>>,
]