C++ 23 Module Support

[ToddThomson]

Thank-you for this C++ json project. It truly is a needed component.

I wanted to share an issue that I had today with a c++23 modules based project. I used:
CPMAddPackage( NAME nlohmann_json VERSION 3.12.0 GITHUB_REPOSITORY nlohmann/json OPTIONS "JSON_BuildTests OFF" "JSON_Install OFF" "JSON_MultipleHeaders OFF" )

to add the package to my project.

I found that using the json package with VS2026 in a module file ( .ixx ) by including the json.hpp I could not get any json API calls to compile.

After reviewing the issues I found that the dev branch had a fix for the issue. Thank-you to all involved!

To resolve the issue ( before the next release comes out ) you can simply add the latest json.hpp as a 3rdparty dependency and then add the json.cppm ( which I renamed to json.ixx ) to your module source files. After that just import the module!
import nlohmann.json;

I hope that helps out anyone with a similar issue.