Build fails with clang-cl due to override of CMAKE_CXX_COMPILER(?)

[emmenlau]

I'm trying to build json 3.9.1 with clang-cl 10.0.0 (from Visual Studio 2019 current latest) on MSVC. The build fails for me already in the cmake configure stage. I could track the problem down to a line in cmake/download_test_data.cmake that overrides the detected compiler:

json/cmake/download_test_data.cmake

Line 51 in 1bcabd9

set(CXX_VERSION_RESULT "${CXX_VERSION_RESULT}; MSVC_VERSION=${MSVC_VERSION}; MSVC_TOOLSET_VERSION=${MSVC_TOOLSET_VERSION}")

I'm under the impression that the semicolon surrounding ... MSVC_VERSION=${MSVC_VERSION}; ... is interpreted by clang-cl as an end-of-line. There are related reports on the internet that would indicate this being the problem.

Here is the output from cmake:

-- The CXX compiler identification is Clang 10.0.0 with MSVC-like command-line
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: C:/Program Files (x86)/Microsoft Visual Studio/2019/Community/VC/Tools/Llvm/x64/bin/clang-cl.exe - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Using the multi-header code from C:/gitlabci/local/builds/XTJyb_7r/0/json/include/
-- Found Git: C:/cygwin64/bin/git.exe (found version "2.21.0") 
-- Operating system: Windows-10.0.19041; CYGWIN_NT-10.0 Aidan02 3.1.4(0.340/5/3) 2020-02-19 08:49 x86_64 Cygwin
-- Compiler: clang-cl: error: no input files
-- Configuring done
-- Generating done

Here are a few examples of the failling build:

-- Build files have been written to: C:/data/stable-tmp-MSVC-Haswell-7-x64-cl19.27.29111_clang10.0.0/Debug/json
[1/99] "clang-cl: error: no input files; MSVC_VERSION=1927; MSVC_TOOLSET_VERSION=142"   -TP  -IC:\gitlabci\local\builds\XTJyb_7r\0\json\test\thirdparty\doctest /MDd /Zi /arch:AVX2 /DDEBUG /DWINVER=_WIN32_WINNT_WIN7 /D_WIN32_WINNT=_WIN32_WINNT_WIN7 /D_ITERATOR_DEBUG_LEVEL=0 /D_CRT_SECURE_NO_DEPRECATE /D_SILENCE_ALL_CXX17_DEPRECATION_WARNINGS    /DWIN32 /D_WINDOWS /W4 /GR /EHsc /wd4566 /wd4996 /MDd /Zi /Ob0 /Od /RTC1 /bigobj /showIncludes /Fotest\CMakeFiles\doctest_main.dir\src\unit.cpp.obj /Fdtest\CMakeFiles\doctest_main.dir\ -c C:\gitlabci\local\builds\XTJyb_7r\0\json\test\src\unit.cpp
FAILED: test/CMakeFiles/doctest_main.dir/src/unit.cpp.obj 
"clang-cl: error: no input files; MSVC_VERSION=1927; MSVC_TOOLSET_VERSION=142"   -TP  -IC:\gitlabci\local\builds\XTJyb_7r\0\json\test\thirdparty\doctest /MDd /Zi /arch:AVX2 /DDEBUG /DWINVER=_WIN32_WINNT_WIN7 /D_WIN32_WINNT=_WIN32_WINNT_WIN7 /D_ITERATOR_DEBUG_LEVEL=0 /D_CRT_SECURE_NO_DEPRECATE /D_SILENCE_ALL_CXX17_DEPRECATION_WARNINGS    /DWIN32 /D_WINDOWS /W4 /GR /EHsc /wd4566 /wd4996 /MDd /Zi /Ob0 /Od /RTC1 /bigobj /showIncludes /Fotest\CMakeFiles\doctest_main.dir\src\unit.cpp.obj /Fdtest\CMakeFiles\doctest_main.dir\ -c C:\gitlabci\local\builds\XTJyb_7r\0\json\test\src\unit.cpp
CreateProcess failed: The system cannot find the file specified.

I'm a bit puzzled what the override of CXX_VERSION_RESULT is used for. When I remove this line everything seems to work fine... is there a good reason to enforce an override?

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[emmenlau]

Help would be greatly appreciated...

[tan-wei]

I've met the same issue here with Clang-11.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[nlohmann]

Maybe related: #2343

[nlohmann]

Thanks for reporting, and sorry for the late response.

As written in #2343 (comment), the goal of CXX_VERSION_RESULT is to have as much detail on the compiler as possible. I played around a bit with the variables provided by CMake, but every now and then missed a detail.

I am confused why setting variable CXX_VERSION_RESULT influences calling clang-cl. Unfortunately, I cannot reproduce the issue in the CI - https://github.com/nlohmann/json/runs/1625144503?check_suite_focus=true#step:3:1 runs without problems:

cmake -S . -B build -G "Visual Studio 16 2019" -A x64 -T ClangCL -DJSON_BuildTests=On
-- The CXX compiler identification is Clang 10.0.0 with MSVC-like command-line
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: C:/Program Files (x86)/Microsoft Visual Studio/2019/Enterprise/VC/Tools/Llvm/x64/bin/clang-cl.exe - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Using the single-header code from D:/a/json/json/single_include/
-- Found Git: C:/Program Files/Git/bin/git.exe (found version "2.29.2.windows.3") 
-- Operating system: Windows-10.0.17763; MSYS_NT-10.0-17763 fv-az68-689 3.1.7-340.x86_64 2020-10-23 13:08 UTC x86_64 Msys
-- Compiler: clang-cl: error: no input files; MSVC_VERSION=1928; MSVC_TOOLSET_VERSION=142
-- Configuring done
-- Generating done
-- Build files have been written to: D:/a/json/json/build

Can you please re-try with the latest develop versions to make sure the issue still persists?

[emmenlau]

Dear @nlohmann thanks for the kind help! Indeed its like you say, the problem comes up only with v3.9.1 (current latest release), but not with the latest develop version! I have no problem using develop, so that resolves the issue for me! Thanks!