Fix for #55

Distringuish between ModifierFlags -- which can appear in trees -- and SourceModifierFlags -- which are
modifiers according to the language spec.

Author: odersky

src/dotty/tools/dotc/core/Flags.scala

@@ -395,10 +395,14 @@ object Flags {
 // --------- Combined Flag Sets and Conjunctions ----------------------
 
   /** Flags representing source modifiers */
-  final val ModifierFlags =
+  final val SourceModifierFlags =
     commonFlags(Private, Protected, Abstract, Final,
      Sealed, Case, Implicit, Override, AbsOverride, Lazy)
 
+  /** Flags representing modifiers that can appear in trees */
+  final val ModifierFlags =
+    SourceModifierFlags | Trait | Module | Param | Synthetic | Package
+
   /** Flags representing access rights */
   final val AccessFlags = Private | Protected | Local
 

src/dotty/tools/dotc/printing/RefinedPrinter.scala

@@ -15,7 +15,7 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
 
   override protected def recursionLimitExceeded() = {}
 
-  protected val PrintableFlags = (ModifierFlags | Label | Module).toCommonFlags
+  protected val PrintableFlags = (SourceModifierFlags | Label | Module).toCommonFlags
 
   /** The closest enclosing DefDef, TypeDef, or ClassDef node */
   private var currentOwner: untpd.Tree = untpd.EmptyTree