export function main(a: string[] | undefined) {
    const z = a //
        ? { a }
        : { b: ["there"] };

    z.a //
        ? z.a.toString()
        : z.b.toString(); // [email protected] error on `z.b` here when `exactOptionalPropertyTypes` is enabled

    // work-around #1
    const zWorkAround: { a: string[], b?: undefined } | { b: string[], a?: undefined } = z;
    zWorkAround.a //
        ? zWorkAround.a.toString()
        : zWorkAround.b.toString();

    // work-around #2
    "a" in z // this is **ONLY** a valid workaround when `exactOptionalPropertyTypes` **IS** enabled
        ? z.a.toString()
        : z.b.toString();
}