declare function identity <T extends unknown>(t: T): T
declare function identity (): undefined

declare class Option<A> {
  chain<B>(f: (a: A) => Option<B>): Option<B>;
}

// Actual type: <T>(option: Option<Option<T>>) => Option<{}>
// Expected type: <T>(option: Option<Option<T>>) => Option<T>
// Disabling second `identity` overload fixes it for some reason.
const optionFlatten = <T>(option: Option<Option<T>>) => option.chain(identity)