diff -r 83a2b6976515 -r c3589f2dff31 src/Pure/Concurrent/future.scala
--- a/src/Pure/Concurrent/future.scala	Sat Jan 09 22:41:08 2021 +0100
+++ b/src/Pure/Concurrent/future.scala	Sun Jan 10 13:04:29 2021 +0100
@@ -34,7 +34,7 @@
 {
   def peek: Option[Exn.Result[A]]
   def is_finished: Boolean = peek.isDefined
-  def get_finished: A = { require(is_finished); Exn.release(peek.get) }
+  def get_finished: A = { require(is_finished, "future not finished"); Exn.release(peek.get) }
   def join_result: Exn.Result[A]
   def join: A = Exn.release(join_result)
   def map[B](f: A => B): Future[B] = Future.fork { f(join) }