I want to use OpenCV in QT. The vrsions are opencv 4.9.0 and QT 5.15.2(MSV 2019,64bit).

.pro file

INCLUDEPATH += C:/opencv/build/include
LIBS += -LC:/opencv/build/x64/vc16/lib 
        -lopencv_world490

mainwindow.cpp

#include <opencv2/opencv.hpp>
using namespace cv; 

...
cv::Mat mat; 
...

After build, I get the error:
undefined reference to cv::Mat::Mat()