title: ScalaTest v3: why require to implement convertToLegacyEqualizer
body: <p>Using <a href="http://www.scalatest.org/install" rel="noreferrer">ScalaTest 3.0.0</a>
  Environment: Scala 2.11.8, sbt 0.13.5, IntelliJ 14.1.4</p>
  
  <p>build.sbt has only</p>
  
  <pre><code>// NOTE: not using org.scalactic
  libraryDependencies += "org.scalatest" %% "scalatest" % "3.0.0" % "test"
  </code></pre>
  
  <p>The test below passed. However, IntelliJ marks a squiggly red line below MyMiniTest with the warning message:</p>
  
  <blockquote>
    <p>Class 'MyMiniTest ' must either be declared abstract or implement
    abstract member 'convertToLegacyEqualizer[T](left: T):
    TripleEqualsSupport.this.LegacyEqualizer[T]' in
    'org.scalactic.TripleEqualsSupport'</p>
  </blockquote>
  
  <pre><code>import org.scalatest.FeatureSpec
  
  class MyMiniTest extends FeatureSpec {
    scenario("A simple test") {
      val a = 12
      assert(a * 3 == 36)
    }
  }
  </code></pre>
  
  <p>What is the reason of this warning and what is the recommended solution to fix it?</p>