Files to reproduce https://github.com/pantsbuild/pants/issues/3744

BUILD

MONIX_VER='3.0.0-RC1'
jar_library(name='monix',
            jars=[
              scala_jar(org='io.monix', name='monix', rev=MONIX_VER),
              scala_jar(org='io.monix', name='monix-eval', rev=MONIX_VER),
              scala_jar(org='io.monix', name='monix-reactive', rev=MONIX_VER),
            ])

jar_library(name='scalatest',
            jars=[
              scala_jar(org='org.scalatest', name='scalatest', rev='3.0.4')
            ])

jar_library(name='reactivestreams-test',
            jars=[
              jar(org='org.reactivestreams', name='reactive-streams-tck', rev='1.0.2')
            ])

junit_tests(
  name='consumer_specs',
  sources=['ConsumerReactiveSubscriberSpec.scala'],
  dependencies=[
    ':scalatest',
    ':reactivestreams-test',
    ':monix',
  ]
)

ConsumerReactiveSubscriberSpec.scala

import monix.eval.Callback
import monix.execution.Scheduler
import monix.reactive.Consumer
import org.reactivestreams.Subscriber
import org.reactivestreams.tck.{SubscriberBlackboxVerification, TestEnvironment}
import org.scalatest.testng.TestNGSuiteLike

class ConsumerReactiveSubscriberSpec
  extends SubscriberBlackboxVerification[Int](new TestEnvironment(300))
    with TestNGSuiteLike {

  implicit val scheduler = Scheduler.io()

  override def createSubscriber(): Subscriber[Int] = {
    Consumer.complete[Int].createSubscriber(Callback.empty, scheduler)._1.toReactive
  }

  override def createElement(element: Int): Int = element

}