Fix compilation error with Clang, C++17, libc++, ordered_json and NLOHMANN_JSON_SERIALIZE_ENUM

[alexezeder]

Closes #2491.

Caused by the strange behavior (or maybe even a bug) of Clang described in the linked issue #2491 and also on LLVM bug tracker.

There are 2 commits - the first one is to add testing usage of ordered_json with enum (just like in the issue), the second resolves the problem for Clang with libc++ and C++17+. Since there is no CI workflow for that, I made a temporary workflow with Clang, C++17 and libc++ in my fork just to demonstrate the build results of these commits:

commit	Ubuntu	MacOS
add test for enum and ordered_json	❌ failed	❌ failed
guard instantiation of detail::detector in conversion operator of basic_json	✔️ successful	✔️ successful

P.S. I tried to reuse the existing Clang C++20 workflow for the temporary workflow with libc++, since there is no difference in Clang's behavior between C++17 and C++20 standards in this case, but libc++ of the used LLVM version does not provide <span> header, just for information.

Filled checklist here

Pull request checklist

Read the Contribution Guidelines for detailed information.

• Changes are described in the pull request, or an existing issue is referenced.
• The test suite compiles and runs without error.
• Code coverage is 100%. Test cases can be added by editing the test suite.
• The source code is amalgamated; that is, after making changes to the sources in the include/nlohmann directory, run make amalgamate to create the single-header file single_include/nlohmann/json.hpp. The whole process is described here.

Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
• Specifically, I am aware of compilation problems with Microsoft Visual Studio (there even is an issue label for these kind of bugs). I understand that even in 2016, complete C++11 support isn't there yet. But please also understand that I do not want to drop features or uglify the code just to make Microsoft's sub-standard compiler happy. The past has shown that there are ways to express the functionality such that the code compiles with the most recent MSVC - unfortunately, this is not the main objective of the project.
• Please refrain from proposing changes that would break JSON conformance. If you propose a conformant extension of JSON to be supported by the library, please motivate this extension.
• Please do not open pull requests that address multiple issues.

[coveralls]

Coverage decreased (-99.8%) to 0.214% when pulling ae9cfb1 on alexezeder:issue2491 into 18a5f4c on nlohmann:develop.

[nlohmann]

@ alexezeder Can you please update to the latest develop branch? I overworked the CI and added quite some checks.

[alexezeder]

@nlohmann, finally, CI has done its job

[nlohmann]

One minor change request, then it should be good to merge.

[alexezeder]

I don't know what should I do with an error in Windows/mingw:

C:/ProgramData/chocolatey/lib/mingw/tools/install/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/8.1.0/../../../../x86_64-w64-mingw32/bin/as.exe: CMakeFiles\test-conversions.dir\src\unit-conversions.cpp.obj: too many sections (32979)
C:\Users\RUNNER~1\AppData\Local\Temp\ccbxRRFM.s: Assembler messages:
C:\Users\RUNNER~1\AppData\Local\Temp\ccbxRRFM.s: Fatal error: can't write 300 bytes to section .text of CMakeFiles\test-conversions.dir\src\unit-conversions.cpp.obj: 'File too big'
[ 56%] Building CXX object test/CMakeFiles/test-inspection.dir/src/unit-inspection.cpp.obj
C:/ProgramData/chocolatey/lib/mingw/tools/install/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/8.1.0/../../../../x86_64-w64-mingw32/bin/as.exe: CMakeFiles\test-conversions.dir\src\unit-conversions.cpp.obj: too many sections (32979)
C:\Users\RUNNER~1\AppData\Local\Temp\ccbxRRFM.s: Fatal error: can't close CMakeFiles\test-conversions.dir\src\unit-conversions.cpp.obj: File too big

[nlohmann]

Maybe https://digitalkarabela.com/mingw-w64-how-to-fix-file-too-big-too-many-sections/ can help. I will try to add that flag to the CI.

[nlohmann]

Can you please change line 24 of file .github/workflows/windows.yml

from

        run: cmake -S . -B build -G "MinGW Makefiles" -DCMAKE_BUILD_TYPE=Debug -DJSON_BuildTests=On

to

        run: cmake -S . -B build -G "MinGW Makefiles" -DCMAKE_BUILD_TYPE=Debug -DJSON_BuildTests=On -DCMAKE_CXX_FLAGS=-Wa,-mbig-obj

?