Cmake target_link_libraries unable to find nlohmann_json since version 3.2.0

[SylvainCorlay]

We updated from 3.1.2 to 3.2.0. The update causes the target_link_libraries cmake command to not properly include nlohmann_json, while find_package(nlohmann_json REQUIRED) did not result in an error.

After some investigation, it seems that the issue was the addition of namespaces to target names which is a backward incompatible change.

[SylvainCorlay]

cc @chuckatkins.

[nlohmann]

Thanks for reporting and sorry for the inconvenience!

The change that introduced the namespace was introduced by @chuckatkins in #1048, and I assumed that the change is fine given that Chuck is with Kitware. Maybe he can comment on the reasons for adding a namespace as I apparently did not understand the repercussions of the change.

[SylvainCorlay]

Yes, this is good cmake practice. Just giving a heads up because it is a breaking change for anyone using cmake to build against nlohmann_json.

[chuckatkins]

Indeed sorry about the breaking change. The namespace was to transition to more modern CMake practices if namespaced target names being external and non-namespaced target names being "local". Ideally, you'll want to transition to using the namespaced target name when consuming it externally through the config file. In the meantime though, I think I have a quick workaround that can provide both.

[chuckatkins]

I'll put a version check in place so it's treated as a backward compatibility thing. That way if someone calls find_package(nlohmann_json) then both targets will get pulled in but if they call find_package(nlohmann_json 3.2.0) then it will only be the namespaced target. @SylvainCorlay would that work for you?

[SylvainCorlay]

On our side we started requiring 3.2.0 and adopting the new syntax, so it is not necessary for us. Maybe a note in the release notes?

I maintain the conda package and we have patched some recipes that depend on it to pin on 3.1.2.