diff -r 64aad1e46f98 -r 97ccf48c2f0c src/Pure/General/multi_map.scala
--- a/src/Pure/General/multi_map.scala	Fri Mar 27 13:04:15 2020 +0100
+++ b/src/Pure/General/multi_map.scala	Fri Mar 27 22:01:27 2020 +0100
@@ -14,7 +14,7 @@
 object Multi_Map extends ImmutableMapFactory[Multi_Map]
 {
   private val empty_val: Multi_Map[Any, Nothing] = new Multi_Map[Any, Nothing](Map.empty)
-  override def empty[A, B] = empty_val.asInstanceOf[Multi_Map[A, B]]
+  override def empty[A, B]: Multi_Map[A, B] = empty_val.asInstanceOf[Multi_Map[A, B]]
 
   implicit def canBuildFrom[A, B]: CanBuildFrom[Coll, (A, B), Multi_Map[A, B]] =
     new MapCanBuildFrom[A, B]
@@ -63,7 +63,7 @@
 
   override def stringPrefix = "Multi_Map"
 
-  override def empty = Multi_Map.empty
+  override def empty: Multi_Map[A, Nothing] = Multi_Map.empty
   override def isEmpty: Boolean = rep.isEmpty
 
   override def keySet: Set[A] = rep.keySet