Fix #7920: add test

Author: liufengyun

tests/pos/i7920.scala

@@ -0,0 +1,6 @@
+class Bar {val glass : Int = 0}
+class Foo(val bar : Bar) extends scala.reflect.Selectable
+val f = new Foo(new Bar) {
+  export bar._
+}
+val fGlass = f.glass