Unsoundness due to missing variance checks for extends clause

This shouldn't compile:
```scala
class Foo[T]
class Bar[-T] extends Foo[T]
```
scalac realises that:
```scala
error: contravariant type T occurs in invariant position in type [-T]Foo[T] {
  def <init>(): Bar[T]
} of class Bar
class Bar[-T] extends Foo[T]
      ^
one error found
````