Project 'class_loader' specifies '/include' as an include dir

[tf-janr]

CMake Error at /home/janr/.cache/yay/ros-melodic-class-loader/src/build/devel/share/class_loader/cmake/class_loaderConfig.cmake:113 (message):
  Project 'class_loader' specifies '/include' as an include dir, which is not
  found.  It does neither exist as an absolute directory nor in
  '/home/janr/.cache/yay/ros-melodic-class-loader/src/class_loader-0.4.1//include'.
  Check the issue tracker 'https://github.com/ros/class_loader/issues' and
  consider creating a ticket if the problem has not been reported yet.
Call Stack (most recent call first):
  /opt/ros/melodic/share/catkin/cmake/catkinConfig.cmake:76 (find_package)
  test/CMakeLists.txt:4 (find_package)

[davidfreina]

Had the same bug and I have a temporary fix for it.
Go to the ~/.cache/yay/ros-melodic-class-loader/src/class_loader-0.4.1/ folder and rename the 'include' folder to '/include'.
This worked for me under ArchLinux Kernel 5.9.8-zen1-1-zen.

[jianghui-lin]

If you look at the error porint in the problematic cmke file(class_loaderConfig.cmake), there is an unexpected path in the _include_dirs variable -- '/include' which cmake complain can't find it under '/' path.
And if check 'which gcc' is '/bin/gcc' rather than '/usr/bin/gcc', that means your bin path is misordered in $PATH env, and you can try to reorder it with e.g 'export PATH="/usr/bin/:/bin/:/sbin:$PATH"', and try build ROS again.