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Why incomplete type and forward declaration depending on includes?

I have seen that if I do:

#include <opencv2/core.hpp>
#include <opencv2/highgui.hpp>
#include <opencv2/imgproc.hpp>

I am getting some errors:

/usr/local/include/opencv2/core/mat.hpp:58:28: error: invalid use of incomplete type ‘class cv::Mat’
 inline void Mat::initEmpty()
                            ^

/usr/local/include/opencv2/core/base.hpp:526:18: error: forward declaration of ‘class cv::Mat’
 class CV_EXPORTS Mat;
                  ^

....

And if I change the includes to:

#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>

all the errors (much more than the two mentionned) disappear. I am using OpenCV 2.4 on Ubuntu 14.

Could it be because of the linking? I use CMake and it looks like this:

#...
find_package(OpenCV REQUIRED COMPONENTS
    core
    highgui
    imgproc
)
#...
include_directories(
    ${OpenCV2_INCLUDE_DIRS}
    #...
)
#...
target_link_libraries(${EXECUTABLE_NAME}
    ${OpenCV_LIBS}
    #...
)
install(TARGETS ${EXECUTABLE_NAME} RUNTIME DESTINATION bin)