C2065 's' 未声明的标识符

我寻求帮助解决错误:(我的代码行如下

// folder name
    std::string folder = "logs";
    CreateDirectoryA(folder.c_str(), nullptr);
    // time format
    auto t = std::time(nullptr);
    std::ostringstream timefmt;
    timefmt << std::put_time(std::localtime(&t), "%Y%m%d_%H%M%S");
    // filename
    std::string filename = folder + "\\"s + "client_"s + std::to_string(CLIENT_GET_VERSION()) + "_"s + timefmt.str() + ".dmp"; /* HERE IS MY PROBLEM */

    auto hFile = CreateFileA(filename.c_str(), GENERIC_WRITE, FILE_SHARE_READ, 0, CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, 0);
    if (hFile == INVALID_HANDLE_VALUE)
        return;