Fix #938 - escaping reference.

Need to avoid also symbols in ThisTypes

Author: odersky

src/dotty/tools/dotc/typer/TypeAssigner.scala

@@ -46,10 +46,9 @@ trait TypeAssigner {
         // TODO: measure the cost of using `existsPart`, and if necessary replace it
         // by a `TypeAccumulator` where we have set `stopAtStatic = true`.
         tp existsPart {
-          case tp: NamedType =>
-            forbidden contains tp.symbol
-          case _ =>
-            false
+          case tp: NamedType => forbidden contains tp.symbol
+          case tp: ThisType => forbidden contains tp.cls
+          case _ => false
         }
       def apply(tp: Type): Type = tp match {
         case tp: TermRef if toAvoid(tp) && variance > 0 =>