optional: use #if instead of #ifndef, fixes #3859

[theodelrieu]

Hi, it's been a while.

Just connected to Github and saw the optional-related issue, tests pass with -DJSON_ImplicitConversions=OFF.

[github-actions]

🔴 Amalgamation check failed! 🔴

The source code has not been amalgamated. @theodelrieu
Please read and follow the Contribution Guidelines.

[nlohmann]

Welcome back! You need Astyle 3.1 to amalgamate.

[github-actions]

🔴 Amalgamation check failed! 🔴

The source code has not been amalgamated. @theodelrieu
Please read and follow the Contribution Guidelines.

[theodelrieu]

Making 3.1 work on macOS is too much of a pain, I've edited the amalgamated file by hand 🤡

[nlohmann]

I downloaded it and built it myself. It's indeed a pain I hope to fix soon.

[theodelrieu]

I downloaded it and built it myself.

So did I, but running it didn't do anything, which is why I tried to edit by hand, which doesn't seem to work either...

[coveralls]

coverage: 99.65% (+0.001%) from 99.649%
when pulling a73a715 on theodelrieu:develop
into a006a7a on nlohmann:develop.

[nlohmann]

Oh, I see we are not executing the test suite with disabled implicit conversions. I will take care of this.

We actually do check with in job ci_test_noimplicitconversions.

[nlohmann]

Looks good to me.

[nlohmann]

Will check #3859 after merging.

[nlohmann]

@theodelrieu AppVeyor complains: https://ci.appveyor.com/project/nlohmann/json/builds/51095672/job/s37fb0m5ggp1qyfc

[theodelrieu]

Seems that JSON_HAS_CPP_17 is defined, but MSVC doesn't set /std:c++17?
Shouldn't that flag be set when invoking cmake?

I don't have a Windows setup, so it'd be nice if a kind soul could take the burden.

[nlohmann]

In .github/external_ci/appveyor.yml, can you try changing

    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
      configuration: Release
      platform: x86
      CXX_FLAGS: "/W4 /WX"
      CMAKE_OPTIONS: "-DJSON_ImplicitConversions=OFF"
      GENERATOR: Visual Studio 16 2019

to

    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
      configuration: Release
      platform: x86
      CXX_FLAGS: "/permissive- /std:c++latest /utf-8 /W4 /WX"
      CMAKE_OPTIONS: "-DJSON_ImplicitConversions=OFF"
      GENERATOR: Visual Studio 16 2019

The AppVeyor build rules predate the ci.cmake scripts which should take care of setting all the flags properly.

[nlohmann]

Uff, AppVeyor is still failing... I will see if I can update the jobs there to the CI.cmake file. If I can't do this tomorrow, it will take some more time.