When you override a method with any number of ref parameters to non-primitive types-things like int and object seem to be okay-you get an error saying it can't figure out which method to override.

class X():
        pass

abstract class A():
        virtual def foo(ref x as X):
                pass

class B(A):
        override def foo(ref x as X):
                pass

x = X()
a as A = B()
a.foo(x)

Here's the error when run under booi with 8.2:

test.boo(9,18): BCE0060: Boo.Lang.Compiler.CompilerError: 'B.foo(X)': no suitable method found to override.