CMake: Misleading error message in C only projects trying to find Boost

[LeSpocky]

I have multiple CMake library projects here, one of them using Boost threads. One library project ("A") exports its own config together with a find_dependency(Boost COMPONENTS thread) call.

Another library project ("B") uses find_package(A) and this way the find_dependency is executed which calls find_package(Boost COMPONENTS thread) internally. That is then used by a third library ("C"), so you a have a chain of dependencies here.

Problem: library C currently has its LANGUAGES set to C in the project() call. Linking libB in libC is possible, but since the switch from CMake FindBoost module to cmake package config coming with Boost I get a build error, which I could track down to Boost.

Minimal example requires a short CMakeLists.txt only:

cmake_minimum_required(VERSION 3.5)
project(bt
    VERSION 0.1
    LANGUAGES C
)

find_package(Boost REQUIRED CONFIG
    COMPONENTS
        thread
)

The error message is like this:

CMake Error at /usr/lib/x86_64-linux-gnu/cmake/BoostDetectToolset-1.74.0.cmake:5 (string):
  string sub-command REGEX, mode MATCHALL needs at least 5 arguments total to
  command.
Call Stack (most recent call first):
  /usr/lib/x86_64-linux-gnu/cmake/boost_thread-1.74.0/boost_thread-config.cmake:27 (include)
  /usr/lib/x86_64-linux-gnu/cmake/Boost-1.74.0/BoostConfig.cmake:141 (find_package)
  /usr/lib/x86_64-linux-gnu/cmake/Boost-1.74.0/BoostConfig.cmake:258 (boost_find_component)
  CMakeLists.txt:7 (find_package)


Boost toolset is unknown (compiler  )
CMake Error at /usr/lib/x86_64-linux-gnu/cmake/BoostDetectToolset-1.74.0.cmake:5 (string):
  string sub-command REGEX, mode MATCHALL needs at least 5 arguments total to
  command.
Call Stack (most recent call first):
  /usr/lib/x86_64-linux-gnu/cmake/boost_atomic-1.74.0/boost_atomic-config.cmake:27 (include)
  /usr/lib/x86_64-linux-gnu/cmake/boost_thread-1.74.0/boost_thread-config.cmake:100 (find_package)
  /usr/lib/x86_64-linux-gnu/cmake/Boost-1.74.0/BoostConfig.cmake:141 (find_package)
  /usr/lib/x86_64-linux-gnu/cmake/Boost-1.74.0/BoostConfig.cmake:258 (boost_find_component)
  CMakeLists.txt:7 (find_package)


Boost toolset is unknown (compiler  )
Performing Test CMAKE_HAVE_LIBC_PTHREAD
Performing Test CMAKE_HAVE_LIBC_PTHREAD - Success
Found Threads: TRUE  
Configuring incomplete, errors occurred!
See also "/home/adahl/build/tmp/bt/CMakeFiles/CMakeOutput.log".

Note: this still happens with most recent Boost, it was just easier to reproduce on my host machine like this.

Note: this only happens if COMPONENTS are involved.

Analysis: the problem is in tools/boost_install/BoostDetectToolset.cmake right in the first line:

string(REGEX MATCHALL "[0-9]+" _BOOST_COMPILER_VERSION ${CMAKE_CXX_COMPILER_VERSION})

In my case ${CMAKE_CXX_COMPILER_VERSION} is empty here (C only project), leading to that error message. I would propose to check for that variable before using it and bailout with a meaningful error message if it is found to be empty.

[pdimov]

Something like this: d85e1b5?

[LeSpocky]

Something like this: d85e1b5?

Wow you are really quick. I could go with that. Opened my own PR meanwhile (#75), lunch break was between issue and pull request. Sorry for the redundancy.

[pdimov]

Please don't move my issues around, @mclow ; it was in the right place.