[JEP-409] Force sealed classes to have a clause

Author: hamzaremmal

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

@@ -810,11 +810,12 @@ object JavaParsers {
         List()
 
 
-    def permittedSubclassesOpt() : List[Tree] =
+    def permittedSubclassesOpt(isSealed: Boolean) : List[Tree] =
       if in.token == PERMITS then
         in.nextToken()
         repsep(() => typ(), COMMA)
       else
+        if isSealed then syntaxError(em"sealed class must have subclasses", false)
         Nil
 
     def classDecl(start: Offset, mods: Modifiers): List[Tree] = {
@@ -830,7 +831,7 @@ object JavaParsers {
         else
           javaLangObject()
       val interfaces = interfacesOpt()
-      val permittedSubclasses = permittedSubclassesOpt()
+      val permittedSubclasses = permittedSubclassesOpt(mods is Flags.Sealed)
       val (statics, body) = typeBody(CLASS, name, tparams)
       val cls = atSpan(start, nameOffset) {
         TypeDef(name, makeTemplate(superclass :: interfaces, body, tparams, true)).withMods(mods)
@@ -895,7 +896,7 @@ object JavaParsers {
         }
         else
           List(javaLangObject())
-      val permittedSubclasses = permittedSubclassesOpt()
+      val permittedSubclasses = permittedSubclassesOpt(mods is Flags.Sealed)
       val (statics, body) = typeBody(INTERFACE, name, tparams)
       val iface = atSpan(start, nameOffset) {
         TypeDef(