Avoid intermediate Map allocations in groupFlatMap

library/src/scala/collection/Iterable.scala

@@ -649,14 +649,7 @@ transparent trait IterableOps[+A, +CC[_], +C] extends Any with IterableOnce[A] w
       val bldr = m.getOrElseUpdate(k, iterableFactory.newBuilder[B])
       bldr ++= f(elem)
     }
-    class Result extends runtime.AbstractFunction1[(K, Builder[B, CC[B]]), Unit] {
-      var built = immutable.Map.empty[K, CC[B]]
-      def apply(kv: (K, Builder[B, CC[B]])) =
-        built = built.updated(kv._1, kv._2.result())
-    }
-    val result = new Result
-    m.foreach(result)
-    result.built
+    m.view.mapValues(_.result()).toMap
   }
 
   /** Partitions this $coll into a map according to a discriminator function `key`. All the values that

tests/run/group-flat-map.scala

@@ -25,3 +25,26 @@
   val xs = List("ab", "c", "de")
   assert(xs.groupFlatMap(_.length)(_.toList) ==
     xs.groupMap(_.length)(_.toList).view.mapValues(_.flatten).toMap)
+
+  // Test: duplicates from expansion are preserved in List result
+  // (flatmap semantics: each element expands to multiple elements)
+  val dupExpanded = List(1, 2).groupFlatMap(identity)(x => List(x, x))
+  assert(dupExpanded == Map(1 -> List(1, 1), 2 -> List(2, 2)))
+
+  // Test: duplicates across different elements in same group are preserved
+  // (encounter order + flatMap semantics)
+  val crossDup = List("aa", "ab").groupFlatMap(_.head)(s => List(s.length))
+  assert(crossDup == Map('a' -> List(2, 2)))
+
+  // Test: multiple groups with mixed expansion sizes
+  val mixed = List(1, 2, 3, 4).groupFlatMap(_ % 2)(x => List(x, x * 10, x * 100))
+  assert(mixed(1) == List(1, 10, 100, 3, 30, 300))
+  assert(mixed(0) == List(2, 20, 200, 4, 40, 400))
+
+  // Test: empty expansion creates empty group (not suppressed)
+  // This is the documented behavior
+  val emptyExpansions = List(1, 2, 3).groupFlatMap(identity)(x => if x == 2 then Nil else List(x))
+  // Note: key 2 gets an empty group (with a builder that was created but never added to)
+  assert(emptyExpansions(1) == List(1))
+  assert(emptyExpansions(2) == List.empty) // Empty group should exist
+  assert(emptyExpansions(3) == List(3))