[CMake] find_package(LLVM CONFIG) may fail when the project does not use C

[lzaoral]

1. Build LLVM 14 rc1 with LLVM_ENABLE_FFI and LLVM_ENABLE_TERMINFO options enabled.
2. Create a C++ only CMake project :

cmake_minimum_required(VERSION 3.16)  # just to silence the warning when this line is missing
project(test LANGUAGES CXX)
find_package(LLVM CONFIG REQUIRED)

3. Configure this project:

$ cmake -Bbuild . -DLLVM_DIR=/root/llvm-project-14.0.0rc1.src/llvm/build/lib/cmake/llvm/
-- The CXX compiler identification is GNU 12.0.1
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
CMake Error at /usr/share/cmake/Modules/Internal/CheckSourceCompiles.cmake:44 (message):
  check_source_compiles: C: needs to be enabled before use.
Call Stack (most recent call first):
  /usr/share/cmake/Modules/CheckCSourceCompiles.cmake:76 (cmake_check_source_compiles)
  /root/llvm-project-14.0.0rc1.src/llvm/build/lib/cmake/llvm/FindFFI.cmake:44 (check_c_source_compiles)
  /root/llvm-project-14.0.0rc1.src/llvm/build/lib/cmake/llvm/LLVMConfig.cmake:145 (find_package)
  CMakeLists.txt:3 (find_package)


-- Could NOT find FFI (missing: HAVE_FFI_CALL)
CMake Error at /usr/share/cmake/Modules/Internal/CheckSourceCompiles.cmake:44 (message):
  check_source_compiles: C: needs to be enabled before use.
Call Stack (most recent call first):
  /usr/share/cmake/Modules/CheckCSourceCompiles.cmake:76 (cmake_check_source_compiles)
  /root/llvm-project-14.0.0rc1.src/llvm/build/lib/cmake/llvm/FindTerminfo.cmake:21 (check_c_source_compiles)
  /root/llvm-project-14.0.0rc1.src/llvm/build/lib/cmake/llvm/LLVMConfig.cmake:152 (find_package)
  CMakeLists.txt:3 (find_package)


-- Could NOT find Terminfo (missing: Terminfo_LINKABLE)
-- Found ZLIB: /usr/lib64/libz.so (found version "1.2.11")
-- Found LibXml2: /usr/lib64/libxml2.so (found version "2.9.12")
-- Configuring incomplete, errors occurred!
See also "/root/test/build/CMakeFiles/CMakeOutput.log".

Changing the project(test LANGUAGES CXX) line to project(test LANGUAGES C CXX) makes the error go away, but that's just a work-around than a solution.

CMake version: 3.22.2
Introduced by: https://reviews.llvm.org/D114327

[lzaoral]

Ping.

[sjoubert]

+1 For now this completely blocks me to update to LLVM 14 as I really don't want to enable the C language in my all C++ project.

I suppose a solution could be for the FindFFI and FindTerminfo modules to fallback to check_cxx_source_compiles if the C language is not enabled or something like that.

[lzaoral]

Ping?

[ekilmer]

I've opened PR #68549, which should fix this issue.

I did some testing with the minimal example in the OP and a few other projects, and it seems okay.

If anyone wants to try modifying their local installation files and test their projects, please report back with any bugs, suggestions, or react with 👍 (if it works) on the PR page.