Improve test

Author: odersky

tests/pos/i6159.scala

@@ -11,4 +11,10 @@ object O {
   }
   def t(a: T[B]): T[B] = a
   def u(a: T.U[B]): T.U[B] = a
+}
+object O2 {
+  opaque type T[X] = X
+
+  def m(a: T[Int]) = 1
+  def m(a: T[String]) = 2
 }