class C(x: R) not the same as class C(private val x: R)

``` scala
scala> class C(private val x: Int) { def add(c: C) = x + c.x }
```

```
defined class C
```

``` scala
scala> class C(x: Int) { def add(c: C) = x + c.x }
```

```
 error: type mismatch;
 found   : c.type (with underlying type C)
 required: ?{val x: ?}
```

I agree they are similar in other cases though. :-)
