title: Failed implicit resolution for Nothing with <:<
body: <p>The following code does not compile on Scala 2.12 / 2.13. Why?</p>
  
  <pre class="lang-scala prettyprint-override"><code>class X[U, T]
  
  object X {
    implicit def genericX[U, T](implicit ev: T &lt;:&lt; U): X[U, T] = new X[U, T]
  }
  
  implicitly[X[AnyRef, String]]  // compiles
  implicitly[X[String, Nothing]] // does not compile
  </code></pre>