Fix DenotationNotDefinedHere in pattern matcher

This was uncovered by previous commit "Fix isSubType bug ...".
Also removed two redundants ".fresh" calls in "fresh.addMode".

Author: odersky

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

@@ -173,7 +173,7 @@ object Decorators {
       def treatSingleArg(arg: Any) : Any =
         try
           arg match {
-            case arg: Showable => arg.show(ctx.fresh.addMode(Mode.FutureDefsOK))
+            case arg: Showable => arg.show(ctx.addMode(Mode.FutureDefsOK))
             case _ => arg
           }
         catch {

src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -720,7 +720,7 @@ class TreeUnpickler(reader: TastyReader, tastyName: TastyName.Table) {
         case SELECT =>
           def readQual(name: Name) = {
             val localCtx =
-              if (name == nme.CONSTRUCTOR) ctx.fresh.addMode(Mode.InSuperCall) else ctx
+              if (name == nme.CONSTRUCTOR) ctx.addMode(Mode.InSuperCall) else ctx
             readTerm()(localCtx)
           }
           def readRest(name: Name, sig: Signature) = {

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

@@ -21,6 +21,7 @@ import ast.Trees._
 import Applications._
 import TypeApplications._
 import SymUtils._, core.NameOps._
+import typer.Mode
 
 import dotty.tools.dotc.util.Positions.Position
 import dotty.tools.dotc.core.Decorators._
@@ -464,8 +465,9 @@ class PatternMatcher extends MiniPhaseTransform with DenotTransformer {thisTrans
           // all potentially stored subpat binders
           val potentiallyStoredBinders = stored.unzip._1.toSet
           // compute intersection of all symbols in the tree `in` and all potentially stored subpat binders
-          new DeepFolder[Unit]((x: Unit, t:Tree) =>
+          def computeBinders(implicit ctx: Context) = new DeepFolder[Unit]((x: Unit, t:Tree) =>
             if (potentiallyStoredBinders(t.symbol)) usedBinders += t.symbol).apply((), in)
+          computeBinders(ctx.addMode(Mode.FutureDefsOK)) // trigged a NotDefinedHere on $outer when compiler dotc/printing
 
           if (usedBinders.isEmpty) in
           else {