constexpr std::string_view str1 = "It is string which was builded ";
constexpr std::string_view str2 = "as concatenation of two strings";

constexpr auto concat_result = 
    cant::too_constexpr(
        []() -> std::string
        {
            return std::string(str1) + std::string(str2);
        }
);

// It does work!
static_assert(concat_result == "It is string which was builded as concatenation of two strings", "error");