Looks like ConfigAvoidanceTest is failing for the modern implementation. You can put @Category(ExcludeFromPluginGradleModern.class) on it, or you can detect this:

and change the test as necessary. Feel free to modify GradleIntegrationHarness as necessary if it makes testing easier.

I didn't have a chance to investigate: does it makes sense that this test fails?

Passes on JDK 11: https://scans.gradle.com/s/doieryq3v5kp6
Fails on JDK8: https://scans.gradle.com/s/ue2kfzw3ipgam

I would expect it to fail for both.

and InputChanges was introduced in 5.4

Wow, what an elegant solution! This looks ready to merge to me, feel free to push merge whenever you'd like (or ask me if you'd prefer not to accept the committer invite I just sent you).

Thanks @nedtwigg. I've accepted your invite to the Diffplug org, but I want to state up front that I'm very unlikely to have capacity to stay involved in the development of Spotless long-term. My goal is to get the plugin into a good state and work well with the Gradle remote cache, then I'll probably disappear.

But it's certainly nice to be asked :).

Sounds great! Being a committer on the Spotless team comes with no duties, only privileges :) Now you can restart a failed CI job if you think it was flaky, merge-at-will, etc. If you can figure out how to make a project loosely-coupled (as we have for Spotless), then I think the ideas of OPEN open source are useful.