Unsoundness due to missing variance checks for extends clause

[smarter]

This shouldn't compile:

class Foo[T]
class Bar[-T] extends Foo[T]

scalac realises that:

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

[TomasMikula]

In #3856 I argue that this should compile, and the other way, i.e.

class Foo[-T]
class Bar[T] extends Foo[T]

shouldn't.

[Blaisorblade]

@TomasMikula Added some comments on your idea; TLDR I agree with @smarter, for details see in #3856.

[smarter]

@odersky I see a fix for this issue authored by you in https://github.com/dotty-staging/dotty/commits/fix-%232973 but no corresponding PR, did you forget to open the PR?

[Blaisorblade]

@smarter I already opened #4057 with that fix, but it doesn't work and I couldn't immediately fix it (my comments there explain why): IIRC, if you compute variance by traversing classParents of a ClassInfo, you don't see the annotations because they're just not there.