address& operator=(const address_v4& a) noexcept;
Postconditions: is_v4() == true and is_v6() == false and to_v4() == a.
Returns: *this
address& operator=(const address_v6& a) noexcept;
Postconditions: is_v4() == false and is_v6() == true and to_v6() == a.
Returns: *this