diff -r 9efdebe24c65 -r 0ffcad1f6130 src/Pure/System/system_channel.scala
--- a/src/Pure/System/system_channel.scala	Mon Mar 01 20:12:09 2021 +0100
+++ b/src/Pure/System/system_channel.scala	Mon Mar 01 22:22:12 2021 +0100
@@ -25,7 +25,7 @@
 
   override def toString: String = address
 
-  def shutdown() { server.close }
+  def shutdown(): Unit = server.close
 
   def rendezvous(): (OutputStream, InputStream) =
   {