Fix #9780: Tighten condition when to turn superclasses into constructors

compiler/src/dotty/tools/dotc/transform/ExplicitOuter.scala

@@ -101,9 +101,11 @@ class ExplicitOuter extends MiniPhase with InfoTransformer { thisPhase =>
       val parents1 =
         for (parent <- impl.parents) yield
           val parentCls = parent.tpe.classSymbol.asClass
-          parent match // ensure class parent is a constructor
+          parent match
+            // if we are in a regular class and first parent is also a regular class,
+            // make sure we have a contructor
             case parent: TypeTree
-            if !parentCls.is(Trait) && !defn.NotRuntimeClasses.contains(parentCls) =>
+            if !cls.is(Trait) && !parentCls.is(Trait) && !defn.NotRuntimeClasses.contains(parentCls) =>
               New(parent.tpe, Nil).withSpan(impl.span)
             case _ => parent
       cpy.Template(impl)(parents = parents1, body = impl.body ++ newDefs)

tests/pos/i9780.scala

@@ -0,0 +1,18 @@
+import java.awt.event.WindowEvent
+import java.awt.{Window => AWTWindow}
+
+abstract class Window {
+  def peer: AWTWindow with InterfaceMixin
+
+  protected trait InterfaceMixin extends javax.swing.RootPaneContainer
+
+  protected trait SuperMixin extends java.awt.Window {
+    override protected def processWindowEvent(e: WindowEvent): Unit = {
+      super.processWindowEvent(e)
+      if (e.getID == WindowEvent.WINDOW_CLOSING)
+        closeOperation()
+    }
+  }
+
+  def closeOperation(): Unit = ()
+}