Forbid redundant/illegal object flags

Fix scala#4936.

Author: Blaisorblade

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

@@ -652,6 +652,16 @@ object desugar {
     val impl = mdef.impl
     val mods = mdef.mods
     def isEnumCase = mods.isEnumCase
+
+    def forbidFlag(flag: FlagSet, msg: String): Unit =
+      if (mods is flag)
+        ctx.error(hl"""${flag} modifier $msg for objects""", mods.mods.find(_.flags == flag).get.pos)
+
+    forbidFlag(Abstract, "cannot be used")
+    for (flag <- List(Sealed, Final)) {
+      forbidFlag(flag, "is redundant")
+    }
+
     if (mods is Package)
       PackageDef(Ident(moduleName), cpy.ModuleDef(mdef)(nme.PACKAGE, impl).withMods(mods &~ Package) :: Nil)
     else if (isEnumCase)

tests/neg/i4936.scala

@@ -0,0 +1,2 @@
+abstract object Foo // error
+sealed final abstract case object Bar // error