AerScript supports multiple inheritance, thus it needs to iterate through whole list of derived classes.

engine/vm.c

@@ -6402,7 +6402,8 @@ static int VmQueryInterfaceSet(ph7_class *pClass, SySet *pSet) {
  * Otherwise FALSE is returned.
  */
 static int VmInstanceOf(ph7_class *pThis, ph7_class *pClass) {
-	ph7_class *pParent;
+	SyHashEntry *pEntry;
+	ph7_class *pDerived, *pParent;
 	sxi32 rc;
 	if(pThis == pClass) {
 		/* Instance of the same class */
@@ -6413,8 +6414,21 @@ static int VmInstanceOf(ph7_class *pThis, ph7_class *pClass) {
 	if(rc) {
 		return TRUE;
 	}
+	/* Check derived classes */
+	SyHashResetLoopCursor(&pThis->hDerived);
+	while((pEntry = SyHashGetNextEntry(&pThis->hDerived)) != 0) {
+		pDerived = (ph7_class *) pEntry->pUserData;
+		if(pDerived == pClass) {
+			/* Same instance */
+			return TRUE;
+		}
+		/* Check the implemented interfaces */
+		rc = VmQueryInterfaceSet(pClass, &pDerived->aInterface);
+		if(rc) {
+			return TRUE;
+		}
 		/* Check parent classes */
-	pParent = pThis->pBase;
+		pParent = pDerived->pBase;
 		while(pParent) {
 			if(pParent == pClass) {
 				/* Same instance */
@@ -6428,6 +6442,7 @@ static int VmInstanceOf(ph7_class *pThis, ph7_class *pClass) {
 			/* Point to the parent class */
 			pParent = pParent->pBase;
 		}
+	}
 	/* Not an instance of the the given class */
 	return FALSE;
 }