Function fallback

narrow.h

std::string wstr_to_utf8(boost::wstring_view);

namespace detail {
    template<class T, class = void>
    struct narrow_impl
    {
        template<class U>
        static U&& invoke(U&& s) noexcept
        {
            static_assert(std::is_same<T, U&&>::value, "error");
            return std::forward<U>(s);
        }
    };

    template<class T>
    struct narrow_impl<T, void_t<decltype(wstr_to_utf8(std::declval<T>()))>>
    {
        template<class U>
        static auto invoke(U&& s) -> decltype(wstr_to_utf8(std::forward<U>(s)))
        {
            static_assert(std::is_same<T, U&&>::value, "error");
            return wstr_to_utf8(std::forward<U>(s));
        }
    };

}

template<class T>
inline auto narrow(T&& s) -> decltype(detail::narrow_impl<T&&>::invoke(std::forward<T>(s)))
{
    return detail::narrow_impl<T&&>::invoke(std::forward<T>(s));
}