diff --git a/compiler/src/dotty/tools/dotc/typer/Namer.scala b/compiler/src/dotty/tools/dotc/typer/Namer.scala
index 07f19b420c1a..661ccc8f628b 100644
--- a/compiler/src/dotty/tools/dotc/typer/Namer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Namer.scala
@@ -249,6 +249,8 @@ class Namer { typer: Typer =>
         var flags = checkFlags(tree.mods.flags)
         if ctx.settings.YcompileScala2Library.value then
           flags |= Scala2x
+          if flags.isOneOf(EffectivelyOpenFlags, butNot = EffectivelyFinalFlags | Sealed) then
+            flags |= Open
         val name = checkNoConflict(tree.name, flags.is(Private), tree.span).asTypeName
         val cls =
           createOrRefine[ClassSymbol](tree, name, flags, ctx.owner,