diff -r 9efdebe24c65 -r 0ffcad1f6130 src/Pure/System/posix_interrupt.scala
--- a/src/Pure/System/posix_interrupt.scala	Mon Mar 01 20:12:09 2021 +0100
+++ b/src/Pure/System/posix_interrupt.scala	Mon Mar 01 22:22:12 2021 +0100
@@ -15,7 +15,7 @@
   def handler[A](h: => Unit)(e: => A): A =
   {
     val SIGINT = new Signal("INT")
-    val new_handler = new SignalHandler { def handle(s: Signal) { h } }
+    val new_handler = new SignalHandler { def handle(s: Signal): Unit = h }
     val old_handler = Signal.handle(SIGINT, new_handler)
     try { e } finally { Signal.handle(SIGINT, old_handler) }
   }