Fix dealiasing of types in mismatch error messages

Author: ausmarton

compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala

@@ -193,9 +193,9 @@ class PlainPrinter(_ctx: Context) extends Printer {
         "<overloaded " ~ toTextRef(tp) ~ ">"
       case tp: TypeRef =>
         if (printWithoutPrefix.contains(tp.symbol))
-          Str(selectionString(tp))
+          selectionString(tp.dealias.typeSymbol.namedType)
         else
-          toTextPrefixOf(tp) ~ selectionString(tp)
+          toTextPrefixOf(tp) ~ selectionString(tp.dealias.typeSymbol.namedType)
       case tp: TermParamRef =>
         ParamRefNameString(tp) ~ lambdaHash(tp.binder) ~ ".type"
       case tp: TypeParamRef =>