Fix typo in assertion

Author: odersky

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -125,7 +125,9 @@ object Types {
     def isRef(sym: Symbol)(implicit ctx: Context): Boolean = stripAnnots.stripTypeVar match {
       case this1: TypeRef =>
         this1.info match { // see comment in Namer#typeDefSig
-          case TypeAlias(tp) => assert((tp ne this) && (tp ne this), tp); tp.isRef(sym)
+          case TypeAlias(tp) =>
+            assert((tp ne this) && (tp ne this1), s"$tp / $this")
+            tp.isRef(sym)
           case _ =>  this1.symbol eq sym
         }
       case this1: RefinedOrRecType => this1.parent.isRef(sym)