When building rtabmap_ros against rtabmap with this PR (on Ubuntu 20.04 ROS Noetic with local version of OpenCV 4.2+Cuda shadowing OpenCV system binaries), we can get this error:

/usr/bin/ld: /usr/local/lib/libopencv_cudafeatures2d.so.4.2.0: undefined reference to `cv::cuda::device::ThrustAllocator::getAllocator()'
/usr/bin/ld: /usr/local/lib/libopencv_cudafeatures2d.so.4.2.0: undefined reference to `cv::cuda::StreamAccessor::getStream(cv::cuda::Stream const&)'

It seems to be because during linking (see with make VERBOSE=1), the binary /usr/lib/x86_64-linux-gnu/libopencv_core.so.4.2.0 is in the list (because of ros dependency cv_bridge) before /usr/local/lib/libopencv_core.so.4.2.0, so cuda core functions are not found. Well, a workaround if we don't want to rebuild cv_bridge is to make a symlink from /usr/lib/x86_64-linux-gnu/libopencv_core.so.4.2.0 to /usr/local/lib/libopencv_core.so.4.2.0 (backup original binary file to revert afterwards) like this:

/usr/lib/x86_64-linux-gnu$ ls -l libopencv_core*
-rw-r--r-- 1 root root 9124066 Feb 17  2020 libopencv_core.a
lrwxrwxrwx 1 root root      21 Feb 17  2020 libopencv_core.so -> libopencv_core.so.4.2
lrwxrwxrwx 1 root root      38 Aug 29 16:50 libopencv_core.so.4.2 -> /usr/local/lib/libopencv_core.so.4.2.0
lrwxrwxrwx 1 root root      38 Aug 29 16:51 libopencv_core.so.4.2.0 -> /usr/local/lib/libopencv_core.so.4.2.0
-rw-r--r-- 1 root root 3840856 Feb 17  2020 libopencv_core.so.4.2.0.back

Note that this would only work if you rebuild exactly the same OpenCV version than the system one to avoid any conflicts.

Loop closure performance comparison with CudaSift (pretty similar to OpenCV's SURF and SIFT):