Fix extending protected nested java classes

compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala

@@ -401,7 +401,8 @@ class ClassfileParser(
       classRoot.setFlag(sflags)
       moduleRoot.setFlag(Flags.JavaDefined | Flags.ModuleClassCreationFlags)
 
-      val privateWithin = getPrivateWithin(jflags)
+      val jflags1 = innerClasses.get(currentClassName.toString).fold(jflags: Int)(_.jflags)
+      val privateWithin = getPrivateWithin(jflags1)
 
       classRoot.setPrivateWithin(privateWithin)
       moduleRoot.setPrivateWithin(privateWithin)

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

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();
+    }
+}

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 = ???
+  }
+}

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();
+    }
+}

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 = ???
+  }
+}