Scala Akka Streams project fails to compile, "value ~> is not a member of akka.streams.Outlet[In]" - scala

Question

I am using the Akka Streams API in a Scala project, working in Intellij IDEA with the SBT plugin. I have a worker pool Flow as described here: https://doc.akka.io/docs/akka/current/scala/stream/stream-cookbook.html. Here is my code:
package streams
import akka.NotUsed
import akka.stream.scaladsl.{Balance, Flow, GraphDSL, Merge}
import akka.stream.{FlowShape, Graph}
object WorkerPoolFlow {
def apply[In, Out](
worker: Flow[In, Out, Any],
workerCount: Int):
Graph[FlowShape[In, Out], NotUsed] = {
GraphDSL.create() { implicit b =>
val balance = b.add(Balance[In](workerCount, waitForAllDownstreams = true))
val merge = b.add(Merge[Out](workerCount))
for (i <- 0 until workerCount)
balance.out(i) ~> worker.async ~> merge.in(i)
FlowShape(
balance.in,
merge.out)
}
}
}
For some reason the project is now failing to compile, giving this error: value ~> is not a member of akka.stream.Outlet[In].
It compiled fine until today. The only change I am aware of making is installing a Scala linter plugin scalafmt, and importing a few new libraries in build.sbt. Here is my build.sbt:
name := "myProject"
version := "0.1"
scalaVersion := "2.11.11"
unmanagedJars in Compile += file("localDep1.jar")
unmanagedJars in Compile += file("localDep2.jar")
libraryDependencies += "io.spray" %% "spray-json" % "1.3.3"
libraryDependencies += "com.typesafe.akka" %% "akka-actor" % "2.5.6"
libraryDependencies += "com.typesafe.akka" %% "akka-stream" % "2.5.6"
libraryDependencies += "com.typesafe.akka" %% "akka-testkit" % "2.5.6" % Test
libraryDependencies += "com.typesafe.akka" %% "akka-stream-testkit" % "2.5.6" % Test
libraryDependencies += "com.47deg" %% "fetch" % "0.6.0"
libraryDependencies += "ch.qos.logback" % "logback-classic" % "1.2.3"
I have tried reloading SBT, building from SBT outside of IDEA, removing and re-adding dependencies, and cleaning the project, with no luck.

Answer 1

Import GraphDSL.Implicits._:
object WorkerPoolFlow {
def apply[In, Out](
worker: Flow[In, Out, Any],
workerCount: Int):
Graph[FlowShape[In, Out], NotUsed] = {
import GraphDSL.Implicits._
GraphDSL.create() { implicit b =>
...
}
}
}