diff -r f0899dad4877 -r f13bb379c573 src/Pure/General/cache.scala
--- a/src/Pure/General/cache.scala	Thu May 24 21:13:09 2018 +0200
+++ b/src/Pure/General/cache.scala	Thu May 24 21:21:26 2018 +0200
@@ -18,6 +18,8 @@
 
   def size: Int = table.size
 
+  override def toString: String = "Cache(" + size + ")"
+
   protected def lookup[A](x: A): Option[A] =
   {
     val ref = table.get(x)