generic_tostring_7.cpp

namespace details
{
    template <typename Type>
    struct HasToString {
    private:
        template<typename T>
        static constexpr auto check(T*) -> typename std::is_same<
                                                                 decltype(std::declval<T>().toString()), 
                                                                 std::string
                                                                >::type;

        template<typename>
        static constexpr std::false_type check(...);

        using type = decltype(check<Type>(nullptr));

    public:
        static constexpr bool value = type::value;
    };
}

template <typename T>
constexpr auto HasToString = details::HasToString<T>::value;

template <typename T>
std::enable_if_t<HasToString<T>, std::string>
    toString(const T& value)
{
    return value.toString();
}