I tried to build OpenCV 4.0 using opencv_contrib and it didn't work.
xfeatures2d.hpp has the following line, and the error is in
cv::FastFeatureDetector::DetectorType
CV_EXPORTS void FASTForPointSet( InputArray image, CV_IN_OUT std::vector<KeyPoint>& keypoints,
int threshold, bool nonmaxSuppression=true, cv::FastFeatureDetector::DetectorType type=FastFeatureDetector::TYPE_9_16);
I checked inside FasFeatureDetector and its parent, and the class really hasn't a definition for DetectorType.
I'm posting here to see if anyone else had this error, or if it's a bug with using OpenCV 4.0-alpha with opencv_contrib. If anyone can help, thank you.