SI-5440 Test case for exhaustiveness check

Reported against patmatclassic, working in virtpatmat.

Author: retronym

test/files/neg/t5440.check

@@ -0,0 +1,5 @@
+t5440.scala:3: error: match may not be exhaustive.
+It would fail on the following inputs: (List(_), Nil), (Nil, List(_))
+    (list1, list2) match {
+    ^
+one error found

test/files/neg/t5440.flags

@@ -0,0 +1 @@
+-Xfatal-warnings

test/files/neg/t5440.scala

@@ -0,0 +1,7 @@
+object Test {
+  def merge(list1: List[Long], list2: List[Long]): Boolean =
+    (list1, list2) match {
+      case (hd1::_, hd2::_) => true
+      case (Nil, Nil) => true
+    }
+}