Clean fails?

Xcode 8.1, macOS Sierra 10.12.1

While building a demo project for my plugin, I tried to build UE4 so I could debug through the file browser code. That worked once, but hasn’t worked since, and fails with this message:

ERROR: No modules found to build. All requested binaries were already built.

Xcode keeps the latest build failures around, which is a bit of visual noise that gets on my nerves, so I tried doing a Product->Clean. I got the same error. It seems like that shouldn’t be the case. Is that expected behaviour?