Spock unit test compilation failure with Grails 2.0

This issue is discussed here at Grails mailing list: http://markmail.org/message/4ogcho3v7o25ex4h

The Spock unit test classes are failing to compile with Grails 2.0.0 with the following error:

ClientSpec.groovy: 8: The return type of java.lang.Object mockDomain(java.lang.Class) in foo.Spec is incompatible with void mockDomain(java.lang.Class) in grails.plugin.spock.UnitSpec
. At [-1:-1]
 @ line 8, column 1.
   class ClientSpec extends UnitSpec

Groovy Verifier rejects these unit Spock test classes when it checks co-variant methods, because the following two definitions are defined inconsistently:

1) DomainClassUnitTestMixin:

def mockDomain(Class domainClassToMock, List domains = [])

2) grails.plugin.spock.UnitSpec:

void mockDomain(Class, List)

I am not sure whether to see it is a Grails bug or as a Spock Grails plugin bug, but since even spock-core-0.5-groovy-1.7 defined it with return type "void", I am first trying it wth Grails.