/**
@hidden
*/
export type _UndefinableKeys<O extends object> = {
    [K in keyof O]-?: [O[K] & (undefined)] extends [never]
                      ? never
                      : K
}[keyof O]

/**
Get the keys of `O` that are `undefined`
(⚠️ needs `--strictNullChecks` enabled)
@param O
@returns [[Key]]
@example
```ts
```
*/
export type UndefinableKeys<O extends object> =
    O extends unknown
    ? _UndefinableKeys<O>
    : never