Commit 5c1c0a2b2e40fd78eb218a4180e8add1a4871729 is a relatively simple Kotlin upgrade but it appears to break Eclipse.
The classpath contains items for the build directory:
Changing settings.gradle to downgrade to 1.8.10 fixes the issue.
Comment From: wilkinsona
I suspect this is caused by a combination of Kotlin's Gradle plugin and Buildship. @dsyer reported seeing something similar in a Java project using Boot's AOT plugin. In that case, I suspect a build ship bug as it wasn't deduplicating bin/main and build/classes/main. The former is the Eclipse-specific equivalent of the latter and only the former is needed.