Add missing case in Formatter

Author: odersky

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

@@ -167,6 +167,8 @@ object Formatting {
     entry match {
       case param: TypeParamRef =>
         s"is a type variable${addendum("constraint", ctx.typeComparer.bounds(param))}"
+      case param: TermParamRef =>
+        s"is a reference to a value parameter"
       case sym: Symbol =>
         val info =
           if (ctx.gadt.bounds.contains(sym))

tests/neg/parser-stability-4.scala

@@ -0,0 +1,2 @@
+class x0{
+      def x0: x0 = (x0 => x0) => x0)  // error // error