diff -r ef8c9b3d5355 -r dde25151c3c1 src/Pure/General/multi_map.scala
--- a/src/Pure/General/multi_map.scala	Thu Mar 04 15:52:08 2021 +0100
+++ b/src/Pure/General/multi_map.scala	Thu Mar 04 15:59:28 2021 +0100
@@ -17,7 +17,7 @@
   def empty[A, B]: Multi_Map[A, B] = empty_val.asInstanceOf[Multi_Map[A, B]]
 
   def from[A, B](entries: IterableOnce[(A, B)]): Multi_Map[A, B] =
-    entries.foldLeft(empty[A, B]) { case (m, (a, b)) => m.insert(a, b) }
+    entries.iterator.foldLeft(empty[A, B]) { case (m, (a, b)) => m.insert(a, b) }
 
   override def newBuilder[A, B]: mutable.Builder[(A, B), Multi_Map[A, B]] = new Builder[A, B]
   private class Builder[A, B] extends mutable.Builder[(A, B), Multi_Map[A, B]]