Fix #4820: add test

liufengyun · liufengyun · commit 8ae1b1976403 · 2019-03-01T12:28:58.000+01:00
The code no longer type checks after #5027.
diff --git a/tests/neg/i4820.scala b/tests/neg/i4820.scala
@@ -0,0 +1,2 @@
+class Foo[A]
+class Bar[A] extends Foo  // error
