The if(instanceof) block is not inferring type in a Closure

import groovy.transform.*

@CompileStatic
class Greeting implements Serializable {
    String who
}

import groovy.transform.*

@CompileStatic
class GreetingActor {

  def receive = {
    if(it instanceof Greeting) {
        println "Hello ${it.who}"
    }
  }

}