New option -Ypos-debug, and fixed range position breakage.

(Looks like there is more range position breakage yet, but
this gets the outermost layer.)

Channeling my struggles into a slightly easier future.

  % scalac -Ypos-debug -d /tmp ./src/library/scala/Predef.scala
  ./src/library/scala/Predef.scala:222: warning:  Positioned tree has unpositioned child in phase extmethods
      def x = __resultOfEnsuring
              ^
  parent: #7109  line 222  Select     // (value __resultOfEnsuring in class Ensuring)
   child: #7108            Ident      // (value $this)

  ./src/library/scala/Predef.scala:258: warning:  Positioned tree has unpositioned child in phase extmethods
      def x = __leftOfArrow
              ^
  parent: #7280  line 258  Select     // (value __leftOfArrow in class ArrowAssoc)
   child: #7279            Ident      // (value $this)

  two warnings found

Or try this to really see some output:

  % scalac -Yrangepos -Ypos-debug

1 files changed, 2 insertions, 7 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala b/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala
index 1e17cb2e3f..ec42d251ff 100644
--- a/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala
@@ -1523,16 +1523,11 @@ abstract class RefChecks extends InfoTransform with reflect.internal.transform.R
         case _ => ()
     }
 
-    // verify classes extending AnyVal meet the requirements
-    // (whatever those are to be, but at least: @inline annotation)
+    // Verify classes extending AnyVal meet the requirements
     private def checkAnyValSubclass(clazz: Symbol) = {
-      if ((clazz isSubClass AnyValClass) && (clazz ne AnyValClass) && !isPrimitiveValueClass(clazz)) {
+      if ((clazz isSubClass AnyValClass) && !isPrimitiveValueClass(clazz)) {
         if (clazz.isTrait)
           unit.error(clazz.pos, "Only classes (not traits) are allowed to extend AnyVal")
-      /* [Martin] That one is already taken care of by Typers
-        if (clazz.tpe <:< AnyRefClass.tpe)
-          unit.error(clazz.pos, "Classes which extend AnyVal may not have an ancestor which inherits AnyRef")
-          */
       }
     }