namespace detail {

template<class T, class = void>
struct has_close : std::false_type
{
};

template<class T>
struct has_close<T, boost::void_t<
    decltype(std::declval<T&>().close())>>
    : std::true_type
{
};

template<class Stream>
void
close_stream(Stream& stream)
{
    static_assert(has_close<Stream>::value,
        "Requirements not met: no close() and no customization point");
    stream.close();
}

struct close_stream_impl
{
    template<class Stream>
    void
    operator()(Stream& stream) const
    {
        using detail::close_stream;
        close_stream(stream);
    }
};

} // detail

constexpr
detail::close_stream_impl
close_stream{};