Build error caused by overwriting CMAKE_CXX_COMPILER

[ongjunjie]

Building unit tests with a combination of CMake and MSVC fails when selecting generators other than Visual Studio Project (ie. NMake, Ninja). The build failure is due to CMAKE_CXX_COMPILER being overwritten.

json/cmake/download_test_data.cmake

Lines 49 to 54 in db78ac1

	if (MSVC)
	execute_process(COMMAND ${CMAKE_CXX_COMPILER} OUTPUT_VARIABLE CXX_VERSION_RESULT OUTPUT_STRIP_TRAILING_WHITESPACE ERROR_VARIABLE CXX_VERSION_RESULT ERROR_STRIP_TRAILING_WHITESPACE)
	set(CMAKE_CXX_COMPILER "${CXX_VERSION_RESULT}; MSVC_VERSION=${MSVC_VERSION}; MSVC_TOOLSET_VERSION=${MSVC_TOOLSET_VERSION}")
	else()
	execute_process(COMMAND ${CMAKE_CXX_COMPILER} --version OUTPUT_VARIABLE CXX_VERSION_RESULT OUTPUT_STRIP_TRAILING_WHITESPACE)
	endif()

I would be happy to open a PR, but I do not understand the intent of the block of code that causes the problem. My first thought is to remove the entire block of code. If the message printed at the end is important, CMake already provides CMAKE_CXX_COMPILER_ID and CMAKE_CXX_COMPILER_VERSION. There's no need to reinvent the wheel.

What is the issue you have?

Can't build unit test, see above.

Please describe the steps to reproduce the issue.

$ mkdir build
$ cd build
$ cmake -G "NMake Makefiles" .. -DJSON_BuildTests=On
$ cmake --build .

Can you provide a small but working code example?

No.

What is the expected behavior?

Project gets built.

And what is the actual behavior instead?

Build fails.

Which compiler and operating system are you using?

• Compiler: Visual Studio 2015
• Operating system: Windows 7

Which version of the library did you use?

• latest release version 3.9.1
• other release - please state the version: ___
• the develop branch

If you experience a compilation error: can you compile and run the unit tests?

• yes
• no - please copy/paste the error message below

test\CMakeFiles\doctest_main.dir\flags.make(5): fatal error U1034: syntax error : separator missing
Stop.
NMAKE : fatal error U1077: '"C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\BIN\amd64\nmake.exe"' : return code '0x2'
Stop.
NMAKE : fatal error U1077: '"C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\BIN\amd64\nmake.exe"' : return code '0x2'
Stop.

[nlohmann]

The goal of

json/cmake/download_test_data.cmake

Lines 49 to 54 in db78ac1

	if (MSVC)
	execute_process(COMMAND ${CMAKE_CXX_COMPILER} OUTPUT_VARIABLE CXX_VERSION_RESULT OUTPUT_STRIP_TRAILING_WHITESPACE ERROR_VARIABLE CXX_VERSION_RESULT ERROR_STRIP_TRAILING_WHITESPACE)
	set(CMAKE_CXX_COMPILER "${CXX_VERSION_RESULT}; MSVC_VERSION=${MSVC_VERSION}; MSVC_TOOLSET_VERSION=${MSVC_TOOLSET_VERSION}")
	else()
	execute_process(COMMAND ${CMAKE_CXX_COMPILER} --version OUTPUT_VARIABLE CXX_VERSION_RESULT OUTPUT_STRIP_TRAILING_WHITESPACE)
	endif()

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. However, looking at

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

it seems this should rather be

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

Overwriting CXX_VERSION_RESULT really makes no sense here. Would this fix your issue?

[ongjunjie]

Yes. Now that line makes sense.