I wanted to put override modifier in there for exactly the same reason you suggest but that's what you try to do that:

```
scala> abstract class A { def foo: Object }
defined class A

scala> type ARefined = A { override def foo: String }
<console>:1: error: illegal start of declaration
       type ARefined = A { override def foo: String }
```

I don't know whether this is by design or not.