Add regression test

Closes #6815

Author: nicolasstucki

tests/neg/i6815.scala

@@ -0,0 +1,17 @@
+class Computes[T]
+
+case class Result[T](val computes : Computes[T])
+
+def impl[T](computes : Computes[T]) : Result[T] = {
+  val result =
+    if ??? then {
+      impl(??? : Computes[_])
+    } else {
+      Result(computes)
+    }
+  result match {
+    case Result(r) => {
+      Result(r) // error
+    }
+  }
+}