lift mapMaterializedValue to FlowOpsMat

akka-stream/src/main/scala/akka/stream/scaladsl/Flow.scala

@@ -114,7 +114,7 @@ final class Flow[-In, +Out, +Mat](private[stream] override val module: Module)
   /**
    * Transform the materialized value of this Flow, leaving all other properties as they were.
    */
-  def mapMaterializedValue[Mat2](f: Mat ⇒ Mat2): ReprMat[Out, Mat2] =
+  override def mapMaterializedValue[Mat2](f: Mat ⇒ Mat2): ReprMat[Out, Mat2] =
     new Flow(module.transformMaterializedValue(f.asInstanceOf[Any ⇒ Any]))
 
   /**
@@ -1983,6 +1983,11 @@ trait FlowOpsMat[+Out, +Mat] extends FlowOps[Out, Mat] {
   def watchTermination[Mat2]()(matF: (Mat, Future[Done]) ⇒ Mat2): ReprMat[Out, Mat2] =
     viaMat(GraphStages.terminationWatcher)(matF)
 
+  /**
+   * Transform the materialized value of this graph, leaving all other properties as they were.
+   */
+  def mapMaterializedValue[Mat2](f: Mat ⇒ Mat2): ReprMat[Out, Mat2]
+
   /**
    * INTERNAL API.
    */

akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala

@@ -28,7 +28,7 @@ import scala.compat.java8.FutureConverters._
  * a Reactive Streams `Publisher` (at least conceptually).
  */
 final class Source[+Out, +Mat](private[stream] override val module: Module)
-  extends FlowOpsMat[Out, Mat] with Graph[SourceShape[Out], Mat] {
+    extends FlowOpsMat[Out, Mat] with Graph[SourceShape[Out], Mat] {
 
   override type Repr[+O] = Source[O, Mat @uncheckedVariance]
   override type ReprMat[+O, +M] = Source[O, M]
@@ -71,7 +71,7 @@ final class Source[+Out, +Mat](private[stream] override val module: Module)
   /**
    * Transform only the materialized value of this Source, leaving all other properties as they were.
    */
-  def mapMaterializedValue[Mat2](f: Mat ⇒ Mat2): ReprMat[Out, Mat2] =
+  override def mapMaterializedValue[Mat2](f: Mat ⇒ Mat2): ReprMat[Out, Mat2] =
     new Source[Out, Mat2](module.transformMaterializedValue(f.asInstanceOf[Any ⇒ Any]))
 
   /** INTERNAL API */

project/MiMa.scala

@@ -659,6 +659,9 @@ object MiMa extends AutoPlugin {
         ProblemFilters.exclude[DirectMissingMethodProblem]("akka.stream.impl.fusing.Drop.onPush"),
         ProblemFilters.exclude[FinalClassProblem]("akka.stream.stage.GraphStageLogic$Reading"), // this class is private
 
+        // lifting this method to the type where it belongs
+        ProblemFilters.exclude[ReversedMissingMethodProblem]("akka.stream.scaladsl.FlowOpsMat.mapMaterializedValue"),
+
         // #19908 Take is private
         ProblemFilters.exclude[MissingClassProblem]("akka.stream.impl.Stages$Take$"),
         ProblemFilters.exclude[MissingClassProblem]("akka.stream.impl.Stages$Take"),