This issue is happening in 2019.3.0b6, and after I removed the asset store package that was throwing the error I got it again in one of my own scripts.

I was incorrect. unsealing the classes did not workaround the problem. It appears to be intermittent. Still looking for a workaround.

The problem appears to be related to serialized fields in sealed partial classes. One possible workaround, which seems to resolve the build error, is to remove the sealed keyword.

Again, I'm on 2018.4.0f1 and my failing case looks like...

// There are 4 files that make this partial class
// None contain public fields, all contain [SerializeField] private fields
public sealed partial class LoaderService
: ServiceMonoBehaviour<LoaderService>
{
}

// No serialized fields here
public abstract class ServiceMonoBehaviour<TService> :
ServiceMonoBehaviour,
IService<TService>
where TService : ServiceMonoBehaviour<TService>
{
}

// No serialized fields here
public abstract class ServiceMonoBehaviour
: MonoBehaviour,
IService
{
}

Cleaning all intermediates/temps does not resolve the build error, nor does building in VS before the command-line build. Note that ProBuilder NuGet package also has the problem in several classes...

namespace UnityEngine.ProBuilder
{
public sealed partial class ProBuilderMesh

FWIW, I'm seeing this with 2018.4.0f1. Same partial class problem with one of my classes and a nuget package, on clean x64 standalone command-line build. Trying to work around it.

Issue is present in 2018.4.0f1 LTS as well. Just updated from 2018.1.6f1 which did not have the issue.

Restarting Unity fixes it, and allows commandline gradlebuilding just fine. Issue seems to reoccur if scripting define symbols get modified, or recompilation in general.

I can confirm that is still present in 2018.2.12f1.

I was able to reproduce the issue as follows:

- open the project with Unity Ediror
- build
- exit the Editor
- make a modification with another text editor to a C# class that is marked with the [Serializable] attribute (e.g., inverting the order of 2 public properties)
- build the project in batch mode (i.e., C:\Unity\2018.2.12\Editor\Unity.exe -batch -projectPath D:\path\to\project\issue...)
- build fails

Same here. If I build project from editor all works fine, but if I use command line batch build, then build process finished with errors like:
Fields serialized in target platform, class 'UnityEngine.Timeline.TrackAsset'

I can confirm that the issue is still present in Unity 2018.

This issue is happening in 2018.2.14f1 as well, so it's clearly not fixed.