Make changeOwnerAfter more robust wrt NotDefinedHere errors

It can happen that changeOwner sees symbols that are not defined yet at the phase where
the method is run. Such symbols should be ignored.

Author: odersky

src/dotty/tools/dotc/ast/tpd.scala

@@ -641,7 +641,8 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
         def traverse(tree: Tree)(implicit ctx: Context) = tree match {
           case tree: DefTree =>
             val sym = tree.symbol
-            if (sym.denot(ctx.withPhase(trans)).owner == from) {
+            val prevDenot = sym.denot(ctx.withPhase(trans))
+            if (prevDenot.validFor.containsPhaseId(trans.id) && prevDenot.owner == from) {
               val d = sym.copySymDenotation(owner = to)
               d.installAfter(trans)
               d.transformAfter(trans, d => if (d.owner eq from) d.copySymDenotation(owner = to) else d)
@@ -651,7 +652,7 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
             traverseChildren(tree)
         }
       }
-      traverser.traverse(tree)
+      traverser.traverse(tree)(ctx.withMode(Mode.FutureDefsOK))
       tree
     }