Use JSON_HAS_CPP_17 only after it has been defined

[remedi]

Fix for Visual Studio "error C2039: 'optional': is not a member of 'std'" #1749

[remedi]

I seem to have messed up the amalgamation. I will try to figure out how the files are combined so that the JSON_HAS_CPP_17 is used only after language standard detection in single_include json.hpp.

[remedi]

Okay now the amalgamation issue is fixed. I am not sure if this a proper place for including, but I moved the #include to macro_scope.hpp just after the language standard detection.

[nlohmann]

Looks good to me.

Good catch!

[nlohmann]

Let's wait for the CI to become green.

[remedi]

Sorry for re-triggering the CI. Fixed committer email.

[coveralls]

Coverage remained the same at 100.0% when pulling 1b846c9 on remedi:feature/optional into 713038f on nlohmann:feature/optional.

[nlohmann]

There still seems to be an issue in the latest AppVeyor builds.

[remedi]

Yes it is the conversion issue that I mentioned in #1749.

However, now there is another error in unit-conversions.cpp lines 1624 and 1633 where there is
no suitable conversion from nlohmann::json to std::vector containing std::optional (1624) or
std::map containing std::optional (1633). I am not familiar with the code base so I am unsure
where exactly the conversion should happen.

I can try looking into this myself today, but I am not sure if I can find the correct place. What I think is missing is implicit conversion from nlohmann::json to a type that can be passed as an argument to std::vector<std::optional<int>> and std::map<std::string, std::optional<int>> constructors.

The compiler errors are:
C:\projects\json\test\src\unit-conversions.cpp(1624): error C2440: '<function-style-cast>': cannot convert from 'nlohmann::json' to 'std::vector<std::optional<int>,std::allocator<_Ty>>'
and
C:\projects\json\test\src\unit-conversions.cpp(1633): error C2440: '<function-style-cast>': cannot convert from 'nlohmann::json' to 'std::map<std::string,std::optional<int>,std::less<_Kty>,std::allocator<std::pair<const _Kty,_Ty>>>'

I was kind of hoping that you would immediately know what would be required.

[nlohmann]

I see. I did not have a look at the error message, but just hoped that the PR would fix them ;-)

I have no idea what the issue is. I am not expert with MSVC, and since GCC and Clang compile the code, it seems OK. However, I think merging your changes into the feature branch is a good idea anyway since it fixes the issue with the macro definition.

[remedi]

Thanks for merging the PR. If you mean that Travis compiles the code on GCC and Clang you would be mistaken since looking at the build logs only 2 test cases (270 assertions) are being run for test-conversions.exe for all builds regardless of the compiler or options used.

When JSON_HAS_CPP_17 is defined there should be 3 test cases (298 assertions). I think there is something wrong with the language standard detection for GCC and Clang so JSON_HAS_CPP_17 is always undefined therefore the problematic unit test code does not seem to be compiled on GCC or Clang at all.