What is causing "exception in phase 'instruction selection' " in Groovy / Grails unit test?

My best approach thus far has been to iteratively comment things out until the code compiles. A grails clean does not fix the issue.

Using this process I've found that the problem is my @TestFor(Class) annotation. I had moved some code from a service to a class in src/groovy. This appears to have caused the annotation to break. I don't really understand why specifically this breaks, but that was sufficient for me to get things working.

So try removing the @TestFor() annotation if your class under test is not a service or controller or similar.

If that doesn't work, comment everything out and add back one piece at a time.