chore: explicitely open scala 2 classes when unpickling

Author: hamzaremmal

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

@@ -556,7 +556,7 @@ object Flags {
   val AbstractOverride: FlagSet              = Abstract | Override
   val AbstractSealed: FlagSet                = Abstract | Sealed
   val AbstractOrTrait: FlagSet               = Abstract | Trait
-  val EffectivelyOpenFlags                   = Abstract | JavaDefined | Open | Scala2x | Trait
+  val EffectivelyOpenFlags                   = Abstract | JavaDefined | Open | Trait
   val AccessorOrDeferred: FlagSet            = Accessor | Deferred
   val PrivateAccessor: FlagSet               = Accessor | Private
   val AccessorOrSynthetic: FlagSet           = Accessor | Synthetic

compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -510,8 +510,15 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
     }
 
     def finishSym(sym: Symbol): Symbol = {
-      if (sym.isClass)
+      if sym.isClass then
         sym.setFlag(Scala2x)
+        if !sym.isEffectivelySealed then
+          // We used to assume that `Scala2x` was part of the
+          // Flags.EffectivelyOpenFlags. We dropped this flag from that
+          // FlagSet and now explicitely open all Scala 2 classes which
+          // are not effectively sealed
+          sym.setFlag(Open)
+
       if (!(isRefinementClass(sym) || isUnpickleRoot(sym) || sym.is(Scala2Existential))) {
         val owner = sym.owner
         val canEnter =