Enforce that mods in Modifiers don't add new information

Everything should be reflected in flags and privateWithin already.

Author: odersky

compiler/src/dotty/tools/dotc/ast/untpd.scala

@@ -163,15 +163,26 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
       if (this.flags == flags) this
       else copy(flags = flags)
 
-   def withAddedMod(mod: Mod): Modifiers =
-     if (mods.exists(_ eq mod)) this
-     else withMods(mods :+ mod)
+    def withAddedMod(mod: Mod): Modifiers =
+      if (mods.exists(_ eq mod)) this
+      else withMods(mods :+ mod)
 
-   def withMods(ms: List[Mod]): Modifiers =
-     if (mods eq ms) this
-     else copy(mods = ms)
+    /** Modifiers with given list of Mods. It is checked that
+     *  all modifiers are already accounted for in `flags` and `privateWithin`.
+     */
+    def withMods(ms: List[Mod]): Modifiers = {
+      if (mods eq ms) this
+      else {
+        if (ms.nonEmpty)
+          for (m <- ms)
+            assert(flags.is(m.flags) ||
+                   m.isInstanceOf[Mod.Private] && !privateWithin.isEmpty,
+                   s"unaccounted modifier: $m in $this when adding $ms")
+        copy(mods = ms)
+      }
+    }
 
-   def withAddedAnnotation(annot: Tree): Modifiers =
+    def withAddedAnnotation(annot: Tree): Modifiers =
       if (annotations.exists(_ eq annot)) this
       else withAnnotations(annotations :+ annot)
 

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -2270,7 +2270,7 @@ object Parsers {
     /** EnumCase = `case' (id ClassConstr [`extends' ConstrApps] | ids)
      */
     def enumCase(start: Offset, mods: Modifiers): DefTree = {
-      val mods1 = mods.withAddedMod(atPos(in.offset)(Mod.Enum())) | Case
+      val mods1 = addMod(mods, atPos(in.offset)(Mod.Enum())) | Case
       accept(CASE)
 
       in.adjustSepRegions(ARROW)