SC: map access different for parameter and local variable of same type

class C implements Map<String,String> {
    @Delegate Map<String,String> impl = [:]
}
def map = new C()
assert map.entry == null
assert map.empty == null
assert map.class == null
assert map.metaClass == null

def test(C map) {
    assert map.entry == null
    assert map.empty == null
    assert map.class == null
    assert map.metaClass == null
}
test(new C())