```class A class B open class C open class D val a...

# announcements

[CFrei]

class A
class B
open class C
open class D
val a: A = A()
val c: C = C()

a is B // <-- as expected: error
c is D // <-- not expected: no error

As expected I get an "Incompatible types B and A"-error at compile time for the line a is B. But not for the next line c is D. Is there any chance that c could be of type D or is this a missing feature in the compiler? Does anyone have an example?

[diesieben07]

Someone could later introduce D as a superclass of C.

[diesieben07]

That is not possible in the first case, because B is not open (i.e. final).

[CFrei]

Thanks.... but how could I introduce a superclass later?

[CFrei]

I have the feeling I oversee something, but I can't create a common subclass of C and D and/or hide from the compiler that these classes might already have some common super classes - or can I?

[diesieben07]

class C : D()
Now c is D is true.

[diesieben07]

And yes, the compiler could know that this is not a case and make it an error, however that would be somewhat strange, as normally introducing a superclass is not seen as a backwards-incompatible change, but with this it would be.

[Michael de Kaste]

the is keyword probably only tests on open-ness of a class. Even though we can logically conclude c is D to not be the case in this specific instance. nothing can is a final class, and thus you get an error