If an annotation that uses @AnnotationCollector is referenced within the workspace (a source reference) an odd error appears in the Groovy editor for the referring class for Groovy 2.5.3+.

In this case, @DomainObject comes from a linked project that has been compiled, so a class file is available for it with complete annotation collection information.

DomainObject.groovy:

package tag

import groovy.transform.*

@CompileStatic
@Immutable(copyWith=true)
@AnnotationCollector
@interface DomainObject {
}