fix scala#6781: add regression test

Author: bishabosha

tests/pos/i6781.scala

@@ -0,0 +1,12 @@
+enum Nat {
+  case Zero
+  case Succ[N <: Nat](n: N)
+}
+import Nat._
+
+inline def toInt(n: => Nat): Int = inline n match {
+  case Zero => 0
+  case Succ(n1) => toInt(n1) + 1
+}
+
+val natTwo = toInt(Succ(Succ(Zero)))