Fix for si-5577.

src/library/scala/collection/mutable/Builder.scala

@@ -100,6 +100,8 @@ trait Builder[-Elem, +To] extends Growable[Elem] {
       def +=(x: Elem): this.type = { self += x; this }
       def clear() = self.clear()
       override def ++=(xs: TraversableOnce[Elem]): this.type = { self ++= xs; this }
+      override def sizeHint(size: Int) = self.sizeHint(size)
+      override def sizeHintBounded(size: Int, boundColl: TraversableLike[_, _]) = self.sizeHintBounded(size, boundColl)
       def result: NewTo = f(self.result)
     }
 }

test/files/run/t5577.check

@@ -0,0 +1,11 @@
+Received a size hint: 10
+0
+1
+2
+3
+4
+5
+6
+7
+8
+9

test/files/run/t5577.scala

@@ -0,0 +1,27 @@
+
+
+
+import collection._
+
+
+
+object Test {
+
+  class AlarmingBuffer[T] extends mutable.ArrayBuffer[T] {
+    override def sizeHint(x: Int) {
+      println("Received a size hint: " + x)
+      super.sizeHint(x)
+    }
+  }
+
+  def main(args: Array[String]) {
+    val iteratorBuilder = (new AlarmingBuffer[Int]) mapResult {
+      res => res.iterator
+    }
+
+    iteratorBuilder.sizeHint(10)
+    iteratorBuilder ++= (0 until 10)
+    iteratorBuilder.result.foreach(println)
+  }
+
+}