/**
Extract all optional keys from the given type.

This is useful when you want to create a new type that contains different type values for the optional keys only.

@example
```
import type {OptionalKeysOf, Except} from 'type-fest';

interface User {
name: string;
surname: string;

luckyNumber?: number;
}

const REMOVE_FIELD = Symbol('remove field symbol');
type UpdateOperation<Entity extends object> = Except<Partial<Entity>, OptionalKeysOf<Entity>> & {
[Key in OptionalKeysOf<Entity>]?: Entity[Key] | typeof REMOVE_FIELD;
};

const update1: UpdateOperation<User> = {
name: 'Alice'
};

const update2: UpdateOperation<User> = {
name: 'Bob',
luckyNumber: REMOVE_FIELD
};
```

@group type-fest
*/
export type OptionalKeysOf<BaseType extends object> = BaseType extends unknown ? (keyof { [Key in keyof BaseType as BaseType extends Record<Key, BaseType[Key]> ? never : Key] : never }) & (keyof BaseType) : never;