Closes #4603. Review by extempore.

git-svn-id: http://lampsvn.epfl.ch/svn-repos/scala/scala/trunk@25394 5e8d7ff9-d8ef-0310-90f0-a4852d11357a

src/compiler/scala/reflect/internal/Types.scala

@@ -3471,6 +3471,7 @@ A type's typeSymbol should never be inspected directly.
    */
   object rawToExistential extends TypeMap {
     private var expanded = immutable.Set[Symbol]()
+    private var generated = immutable.Set[Type]()
     def apply(tp: Type): Type = tp match {
       case TypeRef(pre, sym, List()) if isRawIfWithoutArgs(sym) =>
         if (expanded contains sym) AnyRefClass.tpe
@@ -3481,8 +3482,10 @@ A type's typeSymbol should never be inspected directly.
         } finally {
           expanded -= sym
         }
-      case ExistentialType(_, _) => // stop to avoid infinite expansions
-        tp
+      case ExistentialType(_, _) if !(generated contains tp) => // to avoid infinite expansions. todo: not sure whether this is needed
+        val result = mapOver(tp)
+        generated += result
+        result
       case _ =>
         mapOver(tp)
     }