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, 14 insertions, 4 deletions

diff --git a/src/library/scala/reflect/api/Trees.scala b/src/library/scala/reflect/api/Trees.scala
index 181ce85dac..a355207ff0 100644
--- a/src/library/scala/reflect/api/Trees.scala
+++ b/src/library/scala/reflect/api/Trees.scala
@@ -309,7 +309,7 @@ trait Trees { self: Universe =>
    *  quite frequently called modules to reduce ambiguity.
    */
   case class ModuleDef(mods: Modifiers, name: TermName, impl: Template)
-       extends ImplDef
+        extends ImplDef
 
   /** A common base class for ValDefs and DefDefs.
    */
@@ -319,8 +319,13 @@ trait Trees { self: Universe =>
     def rhs: Tree
   }
 
-  /** A value definition (this includes vars as well, which differ from
-   *  vals only in having the MUTABLE flag set in their Modifiers.)
+  /** Broadly speaking, a value definition.  All these are encoded as ValDefs:
+   *
+   *   - immutable values, e.g. "val x"
+   *   - mutable values, e.g. "var x" - the MUTABLE flag set in mods
+   *   - lazy values, e.g. "lazy val x" - the LAZY flag set in mods
+   *   - method parameters, see vparamss in DefDef - the PARAM flag is set in mods
+   *   - explicit self-types, e.g. class A { self: Bar => } - !!! not sure what is set.
    */
   case class ValDef(mods: Modifiers, name: TermName, tpt: Tree, rhs: Tree) extends ValOrDefDef
 
@@ -390,7 +395,6 @@ trait Trees { self: Universe =>
     //   {
     //      def bar  // owner is local dummy
     //   }
-    // System.err.println("TEMPLATE: " + parents)
   }
 
   /** Block of expressions (semicolon separated expressions) */
@@ -741,6 +745,12 @@ trait Trees { self: Universe =>
     case t =>
       sys.error("Not a ClassDef: " + t + "/" + t.getClass)
   }
+  def deriveModuleDef(mdef: Tree)(applyToImpl: Template => Template): ModuleDef = mdef match {
+    case ModuleDef(mods0, name0, impl0) =>
+      treeCopy.ModuleDef(mdef, mods0, name0, applyToImpl(impl0))
+    case t =>
+      sys.error("Not a ModuleDef: " + t + "/" + t.getClass)
+  }
   def deriveCaseDef(cdef: Tree)(applyToBody: Tree => Tree): CaseDef = cdef match {
     case CaseDef(pat0, guard0, body0) =>
       treeCopy.CaseDef(cdef, pat0, guard0, applyToBody(body0))