Initial version of RefChecks

This is still disabled, because the prepare machinery in transform
does not work yet. Concretely, prepare ops need to return a new TreeTransform
but that tree transform has an undefined phaase id.

We need some architectural changes to disentangle transforms from phases.

Author: odersky

src/dotty/tools/dotc/Compiler.scala

@@ -6,7 +6,7 @@ import Contexts._
 import Periods._
 import Symbols._
 import Scopes._
-import typer.{FrontEnd, Typer, Mode, ImportInfo}
+import typer.{FrontEnd, Typer, Mode, ImportInfo, RefChecks}
 import reporting.ConsoleReporter
 import dotty.tools.dotc.core.Phases.Phase
 import dotty.tools.dotc.transform._
@@ -21,8 +21,8 @@ class Compiler {
       List(new FrontEnd),
       List(new FirstTransform),
       List(new SuperAccessors),
-      // pickling and refchecks goes here
-      List(/*new RefChecks,*/ new ElimRepeated, new ElimLocals),
+      // pickling goes here
+      List(/*new RefChecks, */new ElimRepeated, new ElimLocals),
       List(new ExtensionMethods),
       List(new TailRec),
       List(new PatternMatcher,

src/dotty/tools/dotc/core/Phases.scala

@@ -10,7 +10,6 @@ import config.Printers._
 import scala.collection.mutable.{ListBuffer, ArrayBuffer}
 import dotty.tools.dotc.transform.TreeTransforms.{TreeTransformer, TreeTransform}
 import dotty.tools.dotc.transform.TreeTransforms
-import TreeTransforms.Separator
 import Periods._
 
 trait Phases {

src/dotty/tools/dotc/transform/OverridingPairs.scala

@@ -8,15 +8,15 @@ import collection.mutable.HashMap
 import collection.immutable.BitSet
 import scala.annotation.tailrec
 
-/** A class that yields a kind of iterator (`Cursor`),
+/** A module that can produce a kind of iterator (`Cursor`),
  *  which yields all pairs of overriding/overridden symbols
  *  that are visible in some baseclass, unless there's a parent class
  *  that already contains the same pairs.
  *
  *  Adapted from the 2.9 version of OverridingPairs. The 2.10 version is IMO
  *  way too unwieldy to be maintained.
  */
-abstract class OverridingPairs {
+object OverridingPairs {
 
   /** The cursor class
    *  @param base   the base class that contains the overriding pairs
@@ -102,7 +102,7 @@ abstract class OverridingPairs {
     def hasNext: Boolean = curEntry ne null
 
     @tailrec
-    final def next: Unit = {
+    final def next(): Unit = {
       if (curEntry ne null) {
         overriding = curEntry.sym
         if (nextEntry ne null) {