Add missing test for #4820

Author: liufengyun

tests/neg/i4820b.scala

@@ -0,0 +1,5 @@
+trait SetOps[A, +C <: SetOps[A, C]]  {
+  def concat(that: Iterable[A]): C = ???
+}
+
+class Set1[A] extends SetOps // error: should be SetOps[A, Set1[A]]