A script like
will fail since the local variable c is of a nonserializable type. You need to explicitly say
Since groovy-cps demands that local variables may be serialized, any locally defined classes must also be serializable (unless they are only ever being passed to binary methods: legal but unlikely).
Perhaps the transformer could implicitly add implements Serializable to any class declarations it finds. Generally this is harmless: if the class in fact contains some nonserializable fields, trying to serialize an instance will still fail in a predictable way.