When trying to compile following

class A {
  virtual This a() { return this }
}

class B : A {
  override This a() { return super.a }
}

I get

Cannot return ::A as ::B this

B return type should be substituted instead of This in super.a, shouldn't it?

This returns is one place where auto-casting isn't used by the compiler, rather it really checks that the result type is exactly the type of the containing class. So in that case you need to manually instead a cast to B in the B.a method.