Add incompat: Inferred return type of an override method

[adpi2]

Fixes #181

class Foo

class RichFoo(foo: Foo) extends Foo {
  def show: String = ""
}

class Parent {
  def foo: Foo = new Foo
}

class Child extends Parent {
  override def foo = new RichFoo(super.foo)
}

object Test {
  (new Child).foo.show // Scala 3 error: value show is not a member of Foo
}

@smarter Is the Scala 3 behavior intentional?

[smarter]

Yes this is intended, the base methods determine the public API, if you want a different API you need to write the type down.