title: Error message relating to variance
body: <p>When attempt to type the result of sortBy to RDD : </p>
  
  <pre><code>  val counts: org.apache.spark.rdd.RDD[(String, Int)] = file.flatMap(line =&gt; line.split(" "))
      .map(word =&gt; (word.trim.toLowerCase, 1))
      .reduceByKey(_ + _)   
  val sorted : org.apache.spark.rdd.RDD[Any] = counts.sortBy(-_._2)
  </code></pre>
  
  <p>receive compiler error : </p>
  
  <pre><code>type mismatch;  found   : org.apache.spark.rdd.RDD[(String, Int)]  required: org.apache.spark.rdd.RDD[Any] Note: (String, Int) &lt;: Any, but class RDD is invariant in type T. You may wish to define T as +T instead. (SLS 4.5) Error occurred in an application involving default arguments.
  </code></pre>
  
  <p>What is the meaning of  <code>class RDD is invariant in type T</code> ?</p>
  
  <p>If use instead : </p>
  
  <pre><code>   val sorted : org.apache.spark.rdd.RDD[_] = counts.sortBy(-_._2)
  </code></pre>
  
  <p>Then why is compilation successful ?</p>