src/Pure/System/options.scala

--- a/src/Pure/System/options.scala	Mon Mar 01 20:12:09 2021 +0100
+++ b/src/Pure/System/options.scala	Mon Mar 01 22:22:12 2021 +0100
@@ -390,7 +390,7 @@
 
   /* save preferences */
 
-  def save_prefs(file: Path = Options.PREFS)
+  def save_prefs(file: Path = Options.PREFS): Unit =
   {
     val defaults: Options = Options.init(prefs = "")
     val changed =