Fix the case where an identity flow is appended to a Source with materialized value composition (#22442)

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

@@ -45,14 +45,16 @@ final class Source[+Out, +Mat](
   override def via[T, Mat2](flow: Graph[FlowShape[Out, T], Mat2]): Repr[T] = viaMat(flow)(Keep.left)
 
   override def viaMat[T, Mat2, Mat3](flow: Graph[FlowShape[Out, T], Mat2])(combine: (Mat, Mat2) ⇒ Mat3): Source[T, Mat3] = {
-    if (flow.traversalBuilder eq Flow.identityTraversalBuilder) {
-      traversalBuilder.append(LinearTraversalBuilder.empty(), combine).asInstanceOf[Source[T, Mat3]]
-    } else {
-      new Source[T, Mat3](
-        traversalBuilder.append(flow.traversalBuilder, flow.shape, combine),
-        SourceShape(flow.shape.out)
-      )
-    }
+    val toAppend =
+      if (flow.traversalBuilder eq Flow.identityTraversalBuilder)
+        LinearTraversalBuilder.empty()
+      else
+        flow.traversalBuilder
+
+    new Source[T, Mat3](
+      traversalBuilder.append(toAppend, flow.shape, combine),
+      SourceShape(flow.shape.out)
+    )
   }
 
   /**