invalid use of incomplete type struct cv::DefaultDeleter<CvHaarClassifierCascade>

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I am trying to build this c++ sample from docs: https://docs.opencv.org/4.1.2/db/d28/... But I am getting this error:

In file included from /home/stiv2/jsoft/research/casclass/main0.cpp:1:0:
/usr/include/opencv2/objdetect.hpp:166:111: error: invalid use of incomplete type ?struct cv::DefaultDeleter<CvHaarClassifierCascade>?
 template<> CV_EXPORTS void DefaultDeleter<CvHaarClassifierCascade>::operator ()(CvHaarClassifierCascade* obj) const;
                                                                                                               ^~~~~
In file included from /usr/local/include/opencv4/opencv2/core/cvstd.hpp:81:0,
                 from /usr/local/include/opencv4/opencv2/core/base.hpp:58,
                 from /usr/local/include/opencv4/opencv2/core.hpp:54,
                 from /usr/include/opencv2/objdetect.hpp:47,
                 from /home/stiv2/jsoft/research/casclass/main0.cpp:1:
/usr/local/include/opencv4/opencv2/core/cvstd_wrapper.hpp:32:29: note: declaration of ?struct cv::DefaultDeleter<CvHaarClassifierCascade>?
 template<typename Y> struct DefaultDeleter;

How can I fix it and what is the reason for it? I've built opencv with objdetect module.