Eclipse 4.11 has a very nasty bug that breaks all reference searches. This way you cannot search for usage of types and methods and you cannot refactor in any way. They have released a patch for this in the experimental Java 12 JDT, but this cannot be installed when Groovy-eclipse is installed. It would be really nice if this patch could be added to the groovy-eclipse patched jdt.

See the bug report for the patch:
https://bugs.eclipse.org/bugs/show_bug.cgi?id=545293