constexpr address() noexcept;
Postconditions: is_v4() == true, is_v6() == false, and is_unspecified() == true.
constexpr address(const address_v4& a) noexcept;
Effects: Initializes v4_ with a.
Postconditions: is_v4() == true and is_v6() == false.
constexpr address(const address_v6& a) noexcept;
Effects: Initializes v6_ with a.
Postconditions: is_v4() == false and is_v6() == true.