Why am I getting an overloaded method error, and how can I fix it?

I am building a WebSockets server. The center is a flow that manages the WebSockets messages coming in and out. The Flow needs to take an ActorSource, where that Actor becomes somewhat like the “endpoint” for other actors to send messages into the WebSockets stream.

Right now I have

def webflow(): Flow[Message, Message, _] = Flow.fromGraph(
        GraphDSL.create(ActorSource.actorRef[String](_, _, bufferSize = 5, OverflowStrategy.fail)) { implicit builder => 
            chatSource =>

However, I get
(A) overloaded method error on GraphDSL.create() that I can’t seem to solve
(B) compiler requires that I state the type of chatSource, which I didn’t think I’d need to. Error goes away after I do

I am working with the example from https://doc.akka.io/docs/akka/current/stream/stream-graphs.html#constructing-and-combining-partial-graphs