System information (version)

• OpenCV => 4.3.0_2
• Operating System / Platform => macOS 10.15
• Compiler => clang

Detailed description

When #includeing core/eigen.hpp, compilation fails, unless <Eigen/Dense> or something is included before. Otherwise, compilation fails, because

/usr/local/Cellar/opencv/4.3.0_2/include/opencv4/opencv2/core/eigen.hpp:63:22: error: use of undeclared identifier 'Eigen'
void eigen2cv( const Eigen::Matrix<_Tp, _rows, _cols, _options, _maxRows, _maxCols>& src, OutputArray dst )

Am I missing something here? Is this expected? In that case, this should go into the documentation. Otherwise, #include <Eigen/Dense> should be added to https://github.com/opencv/opencv/blob/master/modules/core/include/opencv2/core/eigen.hpp