Add Mirror.Singleton to enum cases after typer

This makes sure that the mirror type does not leak into the signatures of
these cases.

Author: odersky

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

@@ -117,14 +117,14 @@ object DesugarEnums {
         .withFlags(Override)
     def creator = New(Template(
       constr = emptyConstructor,
-      parents = enumClassRef :: TypeTree(defn.Mirror_SingletonType) :: Nil,
+      parents = enumClassRef :: Nil,
       derived = Nil,
       self = EmptyValDef,
       body = List(enumTagDef, toStringDef) ++ registerCall
     ))
     DefDef(nme.DOLLAR_NEW, Nil,
         List(List(param(nme.tag, defn.IntType), param(nme.name, defn.StringType))),
-        TypeTree(), creator)
+        TypeTree(), creator).withFlags(Private | Synthetic)
   }
 
   /** The return type of an enum case apply method and any widening methods in which
@@ -263,9 +263,7 @@ object DesugarEnums {
         DefDef(nme.toString_, Nil, Nil, TypeTree(defn.StringType), Literal(Constant(name.toString)))
           .withFlags(Override)
       val (tagMeth, scaffolding) = enumTagMeth(CaseKind.Object)
-      val impl1 = cpy.Template(impl)(
-        parents = impl.parents :+ TypeTree(defn.Mirror_SingletonType),
-        body = List(tagMeth, toStringMeth) ++ registerCall)
+      val impl1 = cpy.Template(impl)(body = List(tagMeth, toStringMeth) ++ registerCall)
       val vdef = ValDef(name, TypeTree(), New(impl1)).withMods(mods | Final)
       flatTree(scaffolding ::: vdef :: Nil).withSpan(span)
     }

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

@@ -626,6 +626,7 @@ object Flags {
 
   /** An enum case */
   final val EnumCase: FlagConjunction = allOf(Enum, Case)
+  final val EnumCaseVal: FlagConjunction = allOf(Enum, CaseVal)
 
   /** A term parameter or parameter accessor */
   final val TermParamOrAccessor: FlagSet = Param | ParamAccessor

compiler/src/dotty/tools/dotc/transform/SyntheticMembers.scala

@@ -423,6 +423,10 @@ class SyntheticMembers(thisPhase: DenotTransformer) {
       else if (linked.is(Sealed))
         derive.println(i"$linked is not a sum because ${linked.whyNotGenericSum}")
     }
+    else if (clazz.isAnonymousClass &&
+             (clazz.owner.is(EnumCaseVal) ||
+              clazz.owner.name == nme.DOLLAR_NEW && clazz.owner.is(Synthetic)))
+      addParent(defn.Mirror_SingletonType)
     cpy.Template(impl)(parents = newParents, body = newBody)
   }