Omit type parameter in extend clause

As opposed to `scalac`, Dotty lets you write:
```scala
trait Foo[A]
class Bar[A] extends Foo
```

Note that in Dotty and Scala 2, you can write
```scala
class Foo[A]
class Bar[A] extends Foo
```

Here Dotty infers the type parameter toi be `Any` and `scalac` infers `Nothing`.

Does it make sense to infer a type argument when there is no call to the super constructor?