Turn 6697 into a neg test

This subtyping check use to work because of the bug in question. Note that
6697 tries to use GADT knowledge that is currently unavailable on expression
patterns (as demonstrated in 6697-2.scala), so I don't think we are in a urge
to support this with match types.

Author: OlivierBlanvillain

tests/pos/6697.scala renamed to tests/neg/6697-1.scala

@@ -3,5 +3,6 @@ object Test {
   case class Of[sup, sub <: sup]() extends Off
   type Sup[O <: Off] = O match { case Of[sup, sub] => sup }
   type Sub[O <: Off] = O match { case Of[sup, sub] => sub }
-  type Copy[O <: Off] = Of[Sup[O], Sub[O]]
+  type Copy[O <: Off] = Of[Sup[O], Sub[O]] // error
+  // Type argument Test.Sub[O] does not conform to upper bound Test.Sup[O]
 }

tests/neg/6697-2.scala

@@ -0,0 +1,18 @@
+object Test {
+  sealed trait Off
+  case class Of[sup, sub <: sup]() extends Off
+
+  def sup[O <: Off](o: O) = o match {
+    case _: Of[sup, sub] =>
+      val a: sub = null.asInstanceOf
+      // Even though we know that sub <: sup from Of's bounds, that knowledge
+      // is lost in the body of pattern matching expressions...
+
+      val b: sup = a // error
+      //  Found:    (a : sub)
+      //  Required: sup
+      //  where:    sub is a type in method sup with bounds <: Test.Of[?, ?]#sup
+
+      ()
+  }
+}