src/Pure/Thy/completion.scala

       case _ =>
         Completion.Parse.read(line) match {
           case Some(word) =>
             words_lex.completions(word).map(words_map(_)) match {
               case Nil => None
-              case cs => Some(word, cs.sort(_ < _))
+              case cs => Some(word, cs.sortWith(_ < _))
             }
           case None => None
         }
     }
   }