Clean up accesses to Scala-2 inner classes

Since these can shadow each other, we need to refer to them
by a name and a namespace. This is more solid than the system it
replaces, which referred to such symbols symbplically but then fell
back to the (unsafe) simple name on pickling.

Author: odersky

compiler/src/dotty/tools/dotc/core/tasty/TreePickler.scala

@@ -144,10 +144,6 @@ class TreePickler(pickler: TastyPickler) {
       pickleConstant(value)
     case tpe: NamedType =>
       val sym = tpe.symbol
-      def pickleStdRef(name: Name) = {
-        writeByte(if (tpe.isType) TYPEREF else TERMREF)
-        pickleName(name); pickleType(tpe.prefix)
-      }
       def pickleDirectRef() = {
         writeByte(if (tpe.isType) TYPEREFdirect else TERMREFdirect)
         pickleSymRef(sym)
@@ -172,12 +168,9 @@ class TreePickler(pickler: TastyPickler) {
         pickleSymRef(sym); pickleType(tpe.prefix)
       }
       else tpe.designator match {
-        case sym: Symbol =>
-          assert(tpe.symbol.isClass && tpe.symbol.is(Flags.Scala2x), tpe.symbol.showLocated)
-          // Note: vulnerability here, since Scala2x allows several classes with same name and prefix
-          pickleStdRef(sym.name)
         case name: Name =>
-          pickleStdRef(name)
+          writeByte(if (tpe.isType) TYPEREF else TERMREF)
+          pickleName(name); pickleType(tpe.prefix)
         case LocalName(name, space) =>
           writeByte(if (tpe.isType) TYPEREFin else TERMREFin)
           withLength {

compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -733,14 +733,16 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
           case _ =>
         }
         val tycon =
-          if (sym.isClass && sym.is(Scala2x) && !sym.owner.is(Package))
-            // use fixed sym for Scala 2 inner classes, because they might be shadowed
-            TypeRef(pre, sym.asType)
+          if (sym.isClass && sym.is(Scala2x) && sym.owner.isClass && !sym.owner.is(Package))
+            // There can be multiple Scala2 inner classes with the same prefix and name; use a namespace
+            // to pick a particular one.
+            TypeRef(pre, sym.name.asTypeName.withNameSpace(sym.owner.typeRef))
           else if (isLocal(sym) || pre == NoPrefix) {
             val pre1 = if ((pre eq NoPrefix) && (sym is TypeParam)) sym.owner.thisType else pre
             pre1 select sym
           }
-          else TypeRef(pre, sym.name.asTypeName)
+          else
+            TypeRef(pre, sym.name.asTypeName)
         val args = until(end, readTypeRef)
         if (sym == defn.ByNameParamClass2x) ExprType(args.head)
         else if (args.nonEmpty) tycon.safeAppliedTo(EtaExpandIfHK(sym.typeParams, args.map(translateTempPoly)))