src/Pure/System/command_line.scala

--- a/src/Pure/System/command_line.scala	Mon Mar 01 20:12:09 2021 +0100
+++ b/src/Pure/System/command_line.scala	Mon Mar 01 22:22:12 2021 +0100
@@ -23,7 +23,7 @@
 
   var debug = false
 
-  def tool(body: => Unit)
+  def tool(body: => Unit): Unit =
   {
     val thread =
       Isabelle_Thread.fork(name = "command_line", inherit_locals = true) {