function extend<T, U>(base: T, ext: U): T & U {
    const retval = <T & U> {};

    for (const x in base) {
        // Error: Type 'T[keyof T]' is not assignable to type '(T & U)[keyof T]'.
        retval[x] = base[x];
    }

    for (const x in ext) {
        // Error: Type 'U[keyof U]' is not assignable to type '(T & U)[keyof U]'.
        retval[x] = ext[x];
    }

    return retval;
}