Catkin cannot handle when the CMake language is set to none

[BryceStevenWilley]

Setting the CMake Language to none (which makes sense if this is a catkin package that is solely providing resources, meshes, configurations, etc.) causes CMake to fail.

The smallest CMakeLists.txt that will cause the issue:

cmake_minimum_required(VERSION 3.0.2)
project(hello_test LANGUAGES NONE)

find_package(catkin REQUIRED)
catkin_package()

This will result in two different errors, one regarding FindThreads and GTest:

CMake Error at /usr/share/cmake-3.10/Modules/FindThreads.cmake:54 (message):
  FindThreads only works if either C or CXX language is enabled
Call Stack (most recent call first):
  /usr/share/cmake-3.10/Modules/FindGTest.cmake:204 (find_package)
  /opt/ros/melodic/share/catkin/cmake/test/gtest.cmake:304 (find_package)
  /opt/ros/melodic/share/catkin/cmake/all.cmake:164 (include)
  /opt/ros/melodic/share/catkin/cmake/catkinConfig.cmake:20 (include)
  CMakeLists.txt:6 (find_package)

and another regarding RT (if built with CATKIN_ENABLE_TESTING=False):

CMake Error at /opt/ros/melodic/share/catkin/cmake/assert.cmake:17 (message):


  Assertion failed: check for file existence, but filename
  (RT_LIBRARY-NOTFOUND) unset.  Message: RT Library

Call Stack (most recent call first):
  /opt/ros/melodic/share/catkin/cmake/tools/rt.cmake:42 (assert_file_exists)
  /opt/ros/melodic/share/catkin/cmake/all.cmake:164 (include)
  /opt/ros/melodic/share/catkin/cmake/catkinConfig.cmake:20 (include)
  CMakeLists.txt:4 (find_package)

The main issue is that catkin tries to find a lot of C++ libraries when it doesn't need to (and when CMake can't).

The easy fix is to just make the project's languages CXX, but to "proper" (and probably too much effort) fix is to not include all of the C++ specific libraries that are necessary.

[dirk-thomas]

It should be possible to wrap the specific logic which assumes that either C or C++ is an enabled language in a conditional checking the global CMake variable ENABLED_LANGUAGES. A pull request to support this would be appreciated.