nlohmann::to_json method not acting as expected with nlohmann::adl_serializer specialization.

[NovaSmoof]

I could not get the nlohmann::to_json method to work consistently with specializations of the nlohmann::adl_serializer which I assumed would work from the way described in the readme.

What is the issue you have?

I was using the How do I convert third-party types explanation and I specialized the adl_serializer in the nlohmann namespace with my third party types, Eigen's Vectors and Quaternions, I also did for some of my own types because they were move-constructible only. It sort of works. I would attempt to call nlohmann::to_json on my class's to to_json them. Because that appeared to work. (I looked at the how to make your own serializer for a small amount of guidance on implementing the functions) I did the nlohmann::to_json method because it was intuitive to me but it doesn't compile with nlohmann::to_json(json, Quaternion{}), things appear to compile when I do json = Quaternion{} though so I'm going to use that for now.

Please describe the steps to reproduce the issue.

1. Define a specialization of the nlohmann::adl_serializer with some type
2. Call the nlohmann::to_json on a type defined like this.
3. Coin flip if it will compile or not.

Can you provide a small but working code example?

This is a repository I set up very quickly to show off the smallest case I had of this occurring. It should be cross platform in terms of files. It's simply a main file that includes your include directory and eigen's include directory.
https://github.com/NovaSmoof/Eigen-nlohmann-conflict

What is the expected behavior?

that I can call nlohmann::to_json after specializing the adl_serializer.

And what is the actual behavior instead?

That I could not.

Which compiler and operating system are you using?

• Compiler: MSVC C++ from Visual Studio Community 2019 version 16.11.10
• Operating system: Windows 10

Which version of the library did you use?

I test both. Neither worked for MSVC.

• latest release version 3.10.5
• other release - please state the version: ___
• the develop branch

I tested with both and it failed with both.

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

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

I didn't get a compilation error, but my project's settings are really weird and it wouldn't build your unit tests right and your instructions didn't work. I tried it for an hour. And if you would like me to I'd be more than happy to attempt again, but as of right now I can't get many of them to run. I had difficulties downloading the data.

[nlohmann]

You're not supposed to call to_json or from_json directly. That is, instead of calling

nlohmann::adl_serializer<Thing>::to_json(json["Thing"], thing);
nlohmann::to_json(json["Thing"], thing);
json["Thing"] = thing;

call

json["Thing"] = thing;

Making these changes let me compile your example code. As the main is empty, I have no idea what to expect running it though.

[nlohmann]

@NovaSmoof Do you need further assistance?

[falbrechtskirchinger]

Note that Eigen had some issues with versions 3.10.4 and 3.10.5 (#3267). I think it had to do with the string type constraints, i.e., a similar issue as #3602.