Fix `to_json` for enums when the enum has an unsigned underlying type.

[TheJCAB]

This fixes issue #4236.

The fix is simple. In the relevant to_json overload, we just need to select the correct json type depending on the signedness of the enum's underlying type.

The unit-udt test is also updated to cover this corner case in a way that fails without the fix.

Note that amalgamating the source was a bit of a story for me:

• I ran make pretty but that modified 20 files, performing a significant amount of indentation changes, none of them related to my change.
• I ran make amalgamate, but that did nothing. Apparently, the make rule won't run if the single_include files have already been updated by make pretty. This makes sense but it can be confusing.
• I forced make amalgamate to do the work by touching the file that had the fix.
• I then decided to keep just the minimal needed change: the addition of the fix to the single_include file.

I just am not conversant enough in Linux to know whether I installed astyle correctly (had to clone the source from a beta branch and build, in order to get support for --squeeze-lines). I decided to just let more accustomed contributors deal with this if needed.

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• [X] Changes are described in the pull request, or an existing issue is referenced.
• [X] The test suite compiles and runs without error.
• [X] Code coverage is 100%. Test cases can be added by editing the test suite.
• [X] 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 files single_include/nlohmann/json.hpp and single_include/nlohmann/json_fwd.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 this kind of bug). 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.

[KenSykes]

include

should add since uint64_t is introduced.

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Refers to: tests/src/unit-udt.cpp:21 in 1402ca4. [](commit_id = 1402ca486594e95ae152617ecfed4770ae90603f, deletion_comment = False)

[gregmarr]

include

should add since uint64_t is introduced.

The <cstdint> header is already being included by json.hpp, so this should probably be std::uint64_t.

[TheJCAB]

should add since uint64_t is introduced.

Thanx! Looking through the codebase I saw this is typically not done. Also, I saw that std::uint64_t and plain uint64_t are both used in the tests indistinctly, even within the same file. So I decided to keep the change simple and minimal.

[gregmarr]

If bare uint64_t is already being used, then you should be fine.

[TheJCAB]

@gregmarr: TBH I'm considering it. It looks like is being included via the Hedley library.

Trivia (which we have had to contend with recently 😊):

• The standard dictates that provides std::blah_t types whereas provides blah_t types.
• It also says that both headers can optionally choose to also provide the other form.
• When building with the Microsoft Visual C++ compiler, both headers are equivalent: they provide both forms.
• When building with Clang (not sure about GCC) it adheres to the minimal specification so the optional forms are not provided by either header.

As for this json library: it includes both (via Hedley) so it provides both. Arguably, std::blah_t is more strictly correct for C++, so I'll make this change if other changes are required, or if the owner asks for it.

Thanx!

[nlohmann]

Yes, please always add the std part.

[coveralls]

coverage: 100.0%. remained the same when pulling 5af4ec4f6bcedad87c4afca42f88e534befcc52c on TheJCAB:issue4236 into 3780b41dd070436f3f55327b0a88f27a52e2dfa8 on nlohmann:develop.

[TheJCAB]

@nlohmann thanx for the review. I hope I got everything proper this time.

[nlohmann]

Thanks a lot!

[TheJCAB]

Thanks a lot!

My pleasure. This library is a neat piece of work, thanks!