OpenCV 4.0: class “cv::FastFeatureDetector” has no member “DetectorType” [closed]

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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.