Fix error message

"Expected:" always came out empty: `[]`.

Author: odersky

compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala

@@ -668,8 +668,7 @@ object messages {
     private val msgPrefix = if (actualCount > expectedCount) "Too many" else "Not enough"
 
     //TODO add def simpleParamName to ParamInfo
-    private val expectedArgString = fntpe
-      .widen.typeParams
+    private val expectedArgString = expectedArgs
       .map(_.paramName.unexpandedName.show)
       .mkString("[", ", ", "]")