Why does one work but the other doesn't?

PlayFrameworkIntegrationTestWTF.scala

import org.specs2.mutable._
import org.specs2.runner._
import org.junit.runner._
import play.api.test._
import play.api.test.Helpers._
import org.openqa.selenium.WebDriver

// This compiles
@RunWith(classOf[JUnitRunner])
class IntegrationSpec extends Specification {

  "Application" should {

    "work from within a browser" in new WithBrowser {

      browser.goTo("http://localhost:" + port)

      browser.pageSource must contain("KreateLabs")
    }   
  }

}

// This doesn't -- complains that AppBrowser needs a type variable
@RunWith(classOf[JUnitRunner])
class IntegrationSpec2 extends Specification {
  trait AppBrowser[WEBDRIVER <: WebDriver] extends WithBrowser[WEBDRIVER] {}

  
  "Application" should {

    "work from within a browser" in new AppBrowser {

      browser.goTo("http://localhost:" + port)

      browser.pageSource must contain("KreateLabs")
    }   
  }
}

A number of problems:

• WithBrowser is an abstract class. A trait can't extend an abstract class, it can only extend other traits
• AppBrowser is parameterised, when you use it, you're not declaring that parameter type (the reason WithBrowser works is that the default arguments for the constructor parameters declare what the type of WEBDRIVER is.