Fix #5001: add test

[liufengyun]

Fix #5001: add test

[allanrenucci]

Can you add the other two tests in #4820? I.e.

trait Foo[A]
class Bar[A] extends Foo

trait SetOps[A, +C <: SetOps[A, C]]  {
  def concat(that: Iterable[A]): C = ???
}

class Set1[A] extends SetOps //should be SetOps[A, Set1[A]]

[allanrenucci]

With scalac:

scala> class Foo extends Any
<console>:11: error: Any does not have a constructor

[liufengyun]

Thanks @allanrenucci , it's addressed in the last commit 9a07924

Addressed in the latest commit.