This is what I'm trying to achieve:

interface A {
  a: string;
  b?: number;
  c?: number;
}

type B = PickOptionalKeys<A>;
// B should be { b?: number | undefined; c?: number | undefined }

Couldn't make it work using the existing types … somewhat surprisingly. Am I missing something here?

Wrote this, which is mostly copied from ConditionalKeys/ConditionalPick to help me for the moment:

type OptionalKeys<Base> = NonNullable<
  {
    [Key in keyof Base]: undefined extends Base[Key] // This line is the only difference compared to `ConditionalKeys`
      ?  Key
      : never;
  }[keyof Base]
>;

type PickOptionalKeys<Base> = Pick<Base, OptionalKeys<Base>>;

Could totally be part of this package, imo. Or should this be tackled in an even broader sense, having the undefined as a second generic parameter? Can't really see a frequent usecase for that one, though.