[nomaster] Revert "DummyTree => CannotHaveAttrs"

This reverts commit 838cbe6.

Conflicts:
	src/reflect/scala/reflect/api/Trees.scala

src/compiler/scala/tools/nsc/ast/Positions.scala

@@ -20,7 +20,7 @@ trait Positions extends scala.reflect.internal.Positions {
         // When we prune due to encountering a position, traverse the
         // pruned children so we can warn about those lacking positions.
         t.children foreach { c =>
-          if (!c.canHaveAttrs) ()
+          if (c.isDummy) ()
           else if (c.pos == NoPosition) {
             reporter.warning(t.pos, " Positioned tree has unpositioned child in phase " + globalPhase)
             inform("parent: " + treeSymStatus(t))

src/compiler/scala/tools/nsc/ast/Trees.scala

@@ -337,7 +337,7 @@ trait Trees extends scala.reflect.internal.Trees { self: Global =>
         else
           super.transform {
             tree match {
-              case tree if !tree.canHaveAttrs =>
+              case tree if tree.isDummy =>
                 tree
               case tpt: TypeTree =>
                 if (tpt.original != null)

src/compiler/scala/tools/nsc/ast/parser/Parsers.scala

@@ -2764,7 +2764,7 @@ self =>
       if (in.token == LBRACE) {
         // @S: pre template body cannot stub like post body can!
         val (self, body) = templateBody(isPre = true)
-        if (in.token == WITH && (self eq emptyValDef)) {
+        if (in.token == WITH && self.isEmpty) {
           val earlyDefs: List[Tree] = body flatMap {
             case vdef @ ValDef(mods, _, _, _) if !mods.isDeferred =>
               List(copyValDef(vdef)(mods = mods | Flags.PRESUPER))

src/compiler/scala/tools/nsc/interactive/RangePositions.scala

@@ -144,7 +144,7 @@ self: scala.tools.nsc.Global =>
    */
   private def setChildrenPos(pos: Position, trees: List[Tree]): Unit = try {
     for (tree <- trees) {
-      if (!tree.isEmpty && tree.canHaveAttrs && tree.pos == NoPosition) {
+      if (!tree.isEmpty && tree.pos == NoPosition) {
         val children = tree.children
         if (children.isEmpty) {
           tree setPos pos.focus
@@ -165,7 +165,7 @@ self: scala.tools.nsc.Global =>
    */
   override def atPos[T <: Tree](pos: Position)(tree: T): T = {
     if (pos.isOpaqueRange) {
-      if (!tree.isEmpty && tree.canHaveAttrs && tree.pos == NoPosition) {
+      if (!tree.isEmpty && tree.pos == NoPosition) {
         tree.setPos(pos)
         val children = tree.children
         if (children.nonEmpty) {
@@ -203,7 +203,7 @@ self: scala.tools.nsc.Global =>
 
     def validate(tree: Tree, encltree: Tree): Unit = {
 
-      if (!tree.isEmpty && tree.canHaveAttrs) {
+      if (!tree.isEmpty) {
         if (settings.Yposdebug.value && (settings.verbose.value || settings.Yrangepos.value))
           println("[%10s] %s".format("validate", treeStatus(tree, encltree)))
 

src/compiler/scala/tools/nsc/typechecker/Typers.scala

@@ -53,7 +53,7 @@ trait Typers extends Modes with Adaptations with Tags {
 
   object UnTyper extends Traverser {
     override def traverse(tree: Tree) = {
-      if (tree.canHaveAttrs) {
+      if (!tree.isDummy) {
         tree.tpe = null
         if (tree.hasSymbol) tree.symbol = NoSymbol
       }

src/reflect/scala/reflect/api/Trees.scala

@@ -75,15 +75,8 @@ trait Trees { self: Universe =>
     def isDef: Boolean
 
     /** Is this tree one of the empty trees?
-     *
      *  Empty trees are: the `EmptyTree` null object, `TypeTree` instances that don't carry a type
      *  and the special `emptyValDef` singleton.
-     *
-     *  In the compiler the `isEmpty` check and the derived `orElse` method are mostly used
-     *  as a check for a tree being a null object (`EmptyTree` for term trees and empty TypeTree for type trees).
-     *
-     *  Unfortunately `emptyValDef` is also considered to be `isEmpty`, but this is deemed to be
-     *  a conceptual mistake pending a fix in https://issues.scala-lang.org/browse/SI-6762.
      */
     def isEmpty: Boolean
 
@@ -2923,8 +2916,7 @@ trait Trees { self: Universe =>
       trees mapConserve (tree => transform(tree).asInstanceOf[TypeDef])
     /** Transforms a `ValDef`. */
     def transformValDef(tree: ValDef): ValDef =
-      if (tree eq emptyValDef) tree
-      else transform(tree).asInstanceOf[ValDef]
+      if (tree.isEmpty) tree else transform(tree).asInstanceOf[ValDef]
     /** Transforms a list of `ValDef` nodes. */
     def transformValDefs(trees: List[ValDef]): List[ValDef] =
       trees mapConserve (transformValDef(_))

src/reflect/scala/reflect/internal/Positions.scala

@@ -38,7 +38,7 @@ trait Positions extends api.Positions { self: SymbolTable =>
   protected class DefaultPosAssigner extends PosAssigner {
     var pos: Position = _
     override def traverse(t: Tree) {
-      if (!t.canHaveAttrs) ()
+      if (t.isDummy) ()
       else if (t.pos == NoPosition) {
         t.setPos(pos)
         super.traverse(t)   // TODO: bug? shouldn't the traverse be outside of the if?

src/reflect/scala/reflect/internal/Printers.scala

@@ -435,7 +435,7 @@ trait Printers extends api.Printers { self: SymbolTable =>
         case tree =>
           xprintTree(this, tree)
       }
-      if (printTypes && tree.isTerm && tree.canHaveAttrs) {
+      if (printTypes && tree.isTerm && !tree.isEmpty) {
         print("{", if (tree.tpe eq null) "<null>" else tree.tpe.toString, "}")
       }
     }

src/reflect/scala/reflect/internal/Trees.scala

@@ -36,7 +36,7 @@ trait Trees extends api.Trees { self: SymbolTable =>
     def isDef = false
 
     def isEmpty = false
-    def canHaveAttrs = true
+    def isDummy = false
 
     /** The canonical way to test if a Tree represents a term.
      */
@@ -955,8 +955,9 @@ trait Trees extends api.Trees { self: SymbolTable =>
 
   def ValDef(sym: Symbol): ValDef = ValDef(sym, EmptyTree)
 
-  trait CannotHaveAttrs extends Tree {
-    override def canHaveAttrs = false
+  trait DummyTree extends Tree {
+    override def isEmpty = true
+    override def isDummy = true
 
     private def unsupported(what: String, args: Any*) =
       throw new UnsupportedOperationException(s"$what($args) inapplicable for "+self.toString)
@@ -968,9 +969,9 @@ trait Trees extends api.Trees { self: SymbolTable =>
     override def tpe_=(t: Type) = if (t != NoType) unsupported("tpe_=", t)
   }
 
-  case object EmptyTree extends TermTree with CannotHaveAttrs { override def isEmpty = true; val asList = List(this) }
-  object emptyValDef extends ValDef(Modifiers(PRIVATE), nme.WILDCARD, TypeTree(NoType), EmptyTree) with CannotHaveAttrs
-  object pendingSuperCall extends Apply(Select(Super(This(tpnme.EMPTY), tpnme.EMPTY), nme.CONSTRUCTOR), List()) with CannotHaveAttrs
+  case object EmptyTree extends TermTree with DummyTree { val asList = List(this) }
+  object emptyValDef extends ValDef(Modifiers(PRIVATE), nme.WILDCARD, TypeTree(NoType), EmptyTree) with DummyTree
+  object pendingSuperCall extends Apply(Select(Super(This(tpnme.EMPTY), tpnme.EMPTY), nme.CONSTRUCTOR), List()) with DummyTree
 
   def DefDef(sym: Symbol, mods: Modifiers, vparamss: List[List[ValDef]], rhs: Tree): DefDef =
     atPos(sym.pos) {
@@ -1122,7 +1123,7 @@ trait Trees extends api.Trees { self: SymbolTable =>
         traverse(annot); traverse(arg)
       case Template(parents, self, body) =>
         traverseTrees(parents)
-        if (self ne emptyValDef) traverse(self)
+        if (!self.isEmpty) traverse(self)
         traverseStats(body, tree.symbol)
       case Block(stats, expr) =>
         traverseTrees(stats); traverse(expr)