diff --git a/compiler/src/dotty/tools/dotc/core/SymbolLoaders.scala b/compiler/src/dotty/tools/dotc/core/SymbolLoaders.scala
index 5690720a1b3f..0f01a687f905 100644
--- a/compiler/src/dotty/tools/dotc/core/SymbolLoaders.scala
+++ b/compiler/src/dotty/tools/dotc/core/SymbolLoaders.scala
@@ -51,8 +51,9 @@ object SymbolLoaders {
    */
   def enterClass(
       owner: Symbol, name: PreName, completer: SymbolLoader,
-      flags: FlagSet = EmptyFlags, scope: Scope = EmptyScope)(using Context): Symbol = {
-    val cls = newClassSymbol(owner, name.toTypeName.unmangleClassName.decode, flags, completer, compUnitInfo = completer.compilationUnitInfo)
+      flags: FlagSet = EmptyFlags, scope: Scope = EmptyScope, privateWithin: Symbol = NoSymbol,
+  )(using Context): Symbol = {
+    val cls = newClassSymbol(owner, name.toTypeName.unmangleClassName.decode, flags, completer, privateWithin, compUnitInfo = completer.compilationUnitInfo)
     enterNew(owner, cls, completer, scope)
   }
 
@@ -60,10 +61,13 @@ object SymbolLoaders {
    */
   def enterModule(
       owner: Symbol, name: PreName, completer: SymbolLoader,
-      modFlags: FlagSet = EmptyFlags, clsFlags: FlagSet = EmptyFlags, scope: Scope = EmptyScope)(using Context): Symbol = {
+      modFlags: FlagSet = EmptyFlags, clsFlags: FlagSet = EmptyFlags,
+      scope: Scope = EmptyScope, privateWithin: Symbol = NoSymbol,
+  )(using Context): Symbol = {
     val module = newModuleSymbol(
       owner, name.toTermName.decode, modFlags, clsFlags,
       (module, _) => completer.proxy.withDecls(newScope).withSourceModule(module),
+      privateWithin,
       compUnitInfo = completer.compilationUnitInfo)
     enterNew(owner, module, completer, scope)
     enterNew(owner, module.moduleClass, completer, scope)
@@ -103,13 +107,16 @@ object SymbolLoaders {
    */
   def enterClassAndModule(
       owner: Symbol, name: PreName, completer: SymbolLoader,
-      flags: FlagSet = EmptyFlags, scope: Scope = EmptyScope)(using Context): Unit = {
-    val clazz = enterClass(owner, name, completer, flags, scope)
+      flags: FlagSet = EmptyFlags, scope: Scope = EmptyScope, privateWithin: Symbol = NoSymbol,
+  )(using Context): Unit = {
+    val clazz = enterClass(owner, name, completer, flags, scope, privateWithin)
     val module = enterModule(
       owner, name, completer,
       modFlags = flags.toTermFlags & RetainedModuleValFlags,
       clsFlags = flags.toTypeFlags & RetainedModuleClassFlags,
-      scope = scope)
+      scope = scope,
+      privateWithin = privateWithin,
+    )
   }
 
   /** Enter all toplevel classes and objects in file `src` into package `owner`, provided
diff --git a/compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala b/compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala
index 9e7b59a0cfac..3af0fc6603d5 100644
--- a/compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala
+++ b/compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala
@@ -403,9 +403,10 @@ class ClassfileParser(
 
       val privateWithin = getPrivateWithin(jflags)
 
-      classRoot.setPrivateWithin(privateWithin)
-      moduleRoot.setPrivateWithin(privateWithin)
-      moduleRoot.sourceModule.setPrivateWithin(privateWithin)
+      if privateWithin.exists then
+        classRoot.setPrivateWithin(privateWithin)
+        moduleRoot.setPrivateWithin(privateWithin)
+        moduleRoot.sourceModule.setPrivateWithin(privateWithin)
 
       for (i <- 0 until in.nextChar) parseMember(method = false)
       for (i <- 0 until in.nextChar) parseMember(method = true)
@@ -1059,11 +1060,13 @@ class ClassfileParser(
   private def enterOwnInnerClasses()(using Context, DataReader): Unit = {
     def enterClassAndModule(entry: InnerClassEntry, file: AbstractFile, jflags: Int) =
       SymbolLoaders.enterClassAndModule(
-          getOwner(jflags),
-          entry.originalName,
-          new ClassfileLoader(file),
-          classTranslation.flags(jflags),
-          getScope(jflags))
+        getOwner(jflags),
+        entry.originalName,
+        new ClassfileLoader(file),
+        classTranslation.flags(jflags),
+        getScope(jflags),
+        getPrivateWithin(jflags),
+      )
 
     for entry <- innerClasses.valuesIterator do
       // create a new class member for immediate inner classes
diff --git a/compiler/src/dotty/tools/dotc/printing/Formatting.scala b/compiler/src/dotty/tools/dotc/printing/Formatting.scala
index a36e6f48533a..14b822b11333 100644
--- a/compiler/src/dotty/tools/dotc/printing/Formatting.scala
+++ b/compiler/src/dotty/tools/dotc/printing/Formatting.scala
@@ -109,6 +109,13 @@ object Formatting {
           case Atoms.Range(lo, hi) => CtxShow(s"Range(${toStr(lo.toList)}, ${toStr(hi.toList)})")
       end given
 
+      given Show[ast.untpd.Modifiers] with
+        def show(x: ast.untpd.Modifiers) =
+          CtxShow(s"Modifiers(${toStr(x.flags)}, ${toStr(x.privateWithin)}, ${toStr(x.annotations)}, ${toStr(x.mods)})")
+
+      given Show[ast.untpd.Mod] with
+        def show(x: ast.untpd.Mod) = CtxShow(s"Mod(${toStr(x.flags)})")
+
       given Show[Showable]                            = ShowAny
       given Show[Shown]                               = ShowAny
       given Show[Int]                                 = ShowAny
diff --git a/tests/pos/i21631_joint/AbstractChannel.java b/tests/pos/i21631_joint/AbstractChannel.java
new file mode 100644
index 000000000000..fbcd04549def
--- /dev/null
+++ b/tests/pos/i21631_joint/AbstractChannel.java
@@ -0,0 +1,7 @@
+public abstract class AbstractChannel {
+    protected AbstractChannel() {}
+    protected abstract AbstractUnsafe newUnsafe();
+    protected abstract class AbstractUnsafe {
+        public abstract void connect();
+    }
+}
diff --git a/tests/pos/i21631_joint/i21631.scala b/tests/pos/i21631_joint/i21631.scala
new file mode 100644
index 000000000000..c567d75b1375
--- /dev/null
+++ b/tests/pos/i21631_joint/i21631.scala
@@ -0,0 +1,5 @@
+class Channel extends AbstractChannel() {
+  override def newUnsafe(): AbstractChannel#AbstractUnsafe = new AbstractUnsafe {
+    override def connect(): Unit = ???
+  }
+}
diff --git a/tests/pos/i21631_separ/AbstractChannel_1.java b/tests/pos/i21631_separ/AbstractChannel_1.java
new file mode 100644
index 000000000000..e89767d6d73e
--- /dev/null
+++ b/tests/pos/i21631_separ/AbstractChannel_1.java
@@ -0,0 +1,7 @@
+public abstract class AbstractChannel_1 {
+    protected AbstractChannel_1() {}
+    protected abstract AbstractUnsafe newUnsafe();
+    protected abstract class AbstractUnsafe {
+        public abstract void connect();
+    }
+}
diff --git a/tests/pos/i21631_separ/i21631_2.scala b/tests/pos/i21631_separ/i21631_2.scala
new file mode 100644
index 000000000000..89524e333920
--- /dev/null
+++ b/tests/pos/i21631_separ/i21631_2.scala
@@ -0,0 +1,5 @@
+class Channel extends AbstractChannel_1() {
+  override def newUnsafe(): AbstractChannel_1#AbstractUnsafe = new AbstractUnsafe {
+    override def connect(): Unit = ???
+  }
+}