Codegen bug?

@ Hogge - I know this late, but someone might find it useful. I had the same issue (after importing the Terrain Water Erosion addon and all its dependencies). To uncheck 'Auto-referenced' ,as Aviryx suggested, you need to open the Unity.Properties.Codegen file in a code editor and change "autoReferenced": true, to "autoReferenced": false,

Assembly definitions are used to organise scripts in a project into assemblies. When you create one Unity compiles a separate managed assembly from all the scripts in that folder (You can only have 1 assembly file per folder).

Auto Referenced specifies if all the predefined assemblies should reference the project assembly (this matches how precompiled assemblies are referenced by predefinied assemblies).

If you are trying to get rid of it then you need to uninstall the entire package and possibly restart unity. If you are trying to get it to work then you could try to select the CodeGen assembly file and uncheck "auto-referenced" to see if that works (might require another restart of Unity) or delete the codegen assembly file.

or you could try creating a new asmdef and copy/pasting to see if the cause is something to do with Unity.