shiboken6: Make multiple inheritance cast check less restrictive

The old code would not allow to downcast if a special cast function
exists somewhere in the class hierarchy (as is the case for example
for QWidget inheriting QObject and QPaintDevice).

Make the check more fine-grained by actually checking whether the base
class is a direct, single line inheritance base class of the type
passed in. This makes the mechanism work for widgets.

The corresponding test can then be relaxed.

Task-number: PYSIDE-868
Change-Id: Id81fd9c3080e42009fc84e06a9bab1c8856f2c0c
Reviewed-by: Cristian Maureira-Fredes <cristian.maureira-fredes@qt.io>

4 files changed, 41 insertions, 8 deletions

diff --git a/sources/shiboken6/libshiboken/basewrapper.cpp b/sources/shiboken6/libshiboken/basewrapper.cpp
index 823078735..c11fbbfb2 100644
--- a/sources/shiboken6/libshiboken/basewrapper.cpp
+++ b/sources/shiboken6/libshiboken/basewrapper.cpp
@@ -1055,6 +1055,26 @@ bool hasSpecialCastFunction(PyTypeObject *sbkType)
     return d != nullptr && d->mi_specialcast != nullptr;
 }
 
+// Find whether base is a direct single line base class of type
+// (no multiple inheritance), that is, a C++ pointer cast can safely be done.
+static bool isDirectAncestor(PyTypeObject *type, PyTypeObject *base)
+{
+    if (type == base)
+        return true;
+    if (PyTuple_Size(type->tp_bases) == 0)
+        return false;
+    auto *sbkObjectType = SbkObject_TypeF();
+    auto *firstBase = reinterpret_cast<PyTypeObject *>(PyTuple_GetItem(type->tp_bases, 0));
+    return firstBase != sbkObjectType
+           && PyType_IsSubtype(type, sbkObjectType) != 0
+           && isDirectAncestor(firstBase, base);
+}
+
+bool canDowncastTo(PyTypeObject *baseType, PyTypeObject *targetType)
+{
+    return isDirectAncestor(targetType, baseType);
+}
+
 } // namespace ObjectType
 
 
@@ -1466,7 +1486,7 @@ PyObject *newObjectForPointer(PyTypeObject *instanceType,
     // PYSIDE-868: In case of multiple inheritance, (for example,
     // a function returning a QPaintDevice * from a QWidget *),
     // use instance type to avoid pointer offset errors.
-    return exactType != nullptr && Shiboken::ObjectType::hasSpecialCastFunction(exactType)
+    return exactType != nullptr && !Shiboken::ObjectType::canDowncastTo(instanceType, exactType)
         ? newObjectForType(instanceType, cptr, hasOwnership)
         : newObjectWithHeuristicsHelper(instanceType, exactType, cptr, hasOwnership);
 }
diff --git a/sources/shiboken6/libshiboken/basewrapper.h b/sources/shiboken6/libshiboken/basewrapper.h
index ecbb20f4e..ec5545aea 100644
--- a/sources/shiboken6/libshiboken/basewrapper.h
+++ b/sources/shiboken6/libshiboken/basewrapper.h
@@ -265,6 +265,14 @@ LIBSHIBOKEN_API PyTypeObject *typeForTypeName(const char *typeName);
  * \since 5.12
  */
 LIBSHIBOKEN_API bool hasSpecialCastFunction(PyTypeObject *sbkType);
+
+/// Returns whether a C++ pointer of \p baseType can be safely downcast
+/// to \p targetType (base is a direct, single line base class of targetType).
+/// (is a direct, single-line inheritance)
+/// \param baseType   Python type of base class
+/// \param targetType Python type of derived class
+/// \since 6.8
+LIBSHIBOKEN_API bool canDowncastTo(PyTypeObject *baseType, PyTypeObject *targetType);
 }
 
 namespace Object {
diff --git a/sources/shiboken6/tests/libother/othermultiplederived.h b/sources/shiboken6/tests/libother/othermultiplederived.h
index a8e265388..cd9910687 100644
--- a/sources/shiboken6/tests/libother/othermultiplederived.h
+++ b/sources/shiboken6/tests/libother/othermultiplederived.h
@@ -10,7 +10,7 @@
 
 class ObjectType;
 
-class LIBOTHER_API OtherMultipleDerived : public MDerived1
+class LIBOTHER_API OtherMultipleDerived : public OtherBase, public MDerived1
 {
 public:
     // this will use CppCopier from other module (bug#142)
diff --git a/sources/shiboken6/tests/otherbinding/typediscovery_test.py b/sources/shiboken6/tests/otherbinding/typediscovery_test.py
index 791d3bdce..39dc5cf0f 100644
--- a/sources/shiboken6/tests/otherbinding/typediscovery_test.py
+++ b/sources/shiboken6/tests/otherbinding/typediscovery_test.py
@@ -13,7 +13,8 @@ sys.path.append(os.fspath(Path(__file__).resolve().parents[1]))
 from shiboken_paths import init_paths
 init_paths()
 
-from sample import Abstract, Base1, Derived
+from sample import (Abstract, Base1, Derived,
+                    MDerived1, SonOfMDerived1, MDerived3)
 from other import OtherMultipleDerived
 
 
@@ -32,14 +33,18 @@ class TypeDiscoveryTest(unittest.TestCase):
     def testMultipleInheritance(self):
         obj = OtherMultipleDerived.createObject("Base1")
         self.assertEqual(type(obj), Base1)
-        # PYSIDE-868: In case of multiple inheritance, a factory
-        # function will return the base class wrapper.
+        # PYSIDE-868: In case of single line direct inheritance,
+        # a factory function will return the class wrapper
+        # of the derived class.
         obj = OtherMultipleDerived.createObject("MDerived1")
-        self.assertEqual(type(obj), Base1)
+        self.assertEqual(type(obj), MDerived1)
         obj = OtherMultipleDerived.createObject("SonOfMDerived1")
-        self.assertEqual(type(obj), Base1)
+        self.assertEqual(type(obj), SonOfMDerived1)
         obj = OtherMultipleDerived.createObject("MDerived3")
-        self.assertEqual(type(obj), Base1)
+        self.assertEqual(type(obj), MDerived3)
+        # PYSIDE-868: OtherMultipleDerived inherits
+        # OtherBase, Base1. In this case, a factory
+        # function will return the base class wrapper.
         obj = OtherMultipleDerived.createObject("OtherMultipleDerived")
         self.assertEqual(type(obj), Base1)