Forbid/warn on redundant/illegal object flags

Fix scala#4936.

- Stop using final object in our sources.
- Give error for abstract and sealed objects, warning for final ones (as long as
- scala/bug#11094 is open).

Author: Blaisorblade

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

@@ -10,7 +10,7 @@ import Definitions._
 
 object NameOps {
 
-  final object compactify {
+  object compactify {
     lazy val md5: MessageDigest = MessageDigest.getInstance("MD5")
 
     /** COMPACTIFY

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

@@ -2415,6 +2415,16 @@ object Parsers {
 
     def objectDefRest(start: Offset, mods: Modifiers, name: TermName): ModuleDef = {
       val template = templateOpt(emptyConstructor)
+
+      def flagPos(flag: FlagSet) = mods.mods.find(_.flags == flag).get.pos
+      if (mods is Abstract)
+        syntaxError(hl"""${Abstract} modifier cannot be used for objects""", flagPos(Abstract))
+      if (mods is Sealed)
+        syntaxError(hl"""${Sealed} modifier is redundant for objects""", flagPos(Sealed))
+      // Maybe this should be an error; see https://github.com/scala/bug/issues/11094.
+      if (mods is Final)
+        warning(hl"""${Final} modifier is redundant for objects""", source atPos flagPos(Final))
+
       finalizeDef(ModuleDef(name, template), mods, start)
     }
 

compiler/test/dotty/tools/vulpix/Status.scala

@@ -4,4 +4,4 @@ package vulpix
 sealed trait Status
 final case class Success(output: String) extends Status
 final case class Failure(output: String) extends Status
-final case object Timeout extends Status
+case object Timeout extends Status

doc-tool/src/dotty/tools/dottydoc/model/references.scala

@@ -11,7 +11,7 @@ object references {
   final case class BoundsReference(low: Reference, high: Reference) extends Reference
   final case class NamedReference(title: String, ref: Reference, isByName: Boolean = false, isRepeated: Boolean = false) extends Reference
   final case class ConstantReference(title: String) extends Reference
-  final case object EmptyReference extends Reference
+  case object EmptyReference extends Reference
 
   /** Use MaterializableLink for entities that need be picklable */
   sealed trait MaterializableLink { def title: String }

tests/neg/i3471.scala

@@ -0,0 +1 @@
+sealed object Fun // error

tests/neg/i4936.scala

@@ -0,0 +1,4 @@
+abstract object Foo // error
+sealed final abstract case object Bar // error // error
+
+abstract override object Baz // error