Shiboken: Fix subtle bug caused by PyPy support and "bug_825"

When implementing PyPy support, all Shiboken types lost their
hidden extra fields, because they were replaced by shadow dicts.

This creates a problem with multiple inheritance.
In Python < 3.12, the decision which base type will create the
instance is driven simply by the size of the type. In

    class MetaC(MetaA, MetaB):
        pass

when MetaA is derived from type and MetaB is derived from
SbkObjectType, MetaA is not the type that should win.
Fixed by simply adding 1 to the default size of PyType_Type.

Caused by bug_825 corrected version.
The old test is retained as bug_825_old.py .

NOTE: This touches a Python 3.8 bug that was fixed in 3.9.12
and 3.10.4 - unfortunately CI Python is older in all cases.

Task-number: PYSIDE-2230
Change-Id: I6e82cafb83c8351bbbeafcc80e11d5e45568f73d
Pick-to: 6.2 6.5 6.6
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

1 files changed, 10 insertions, 3 deletions

diff --git a/sources/shiboken6/libshiboken/basewrapper.cpp b/sources/shiboken6/libshiboken/basewrapper.cpp
index d33f06431..828c54a89 100644
--- a/sources/shiboken6/libshiboken/basewrapper.cpp
+++ b/sources/shiboken6/libshiboken/basewrapper.cpp
@@ -139,11 +139,18 @@ static PyTypeObject *createObjectTypeType()
     };
 
     // PYSIDE-535: The tp_itemsize field is inherited and does not need to be set.
-    // In PyPy, it _must_ not be set, because it would have the meaning that a
-    // `__len__` field must be defined. Not doing so creates a hard-to-find crash.
+    //             In PyPy, it _must_ not be set, because it would have the meanin
+    //             that a `__len__` field must be defined. Not doing so creates
+    //             a hard-to-find crash.
+    //
+    // PYSIDE-2230: In Python < 3.12, the decision which base class should create
+    //              the instance is arbitrarily drawn by the size of the type.
+    //              Ignoring this creates a bug in the new version of bug_825 that
+    //              selects the wrong metatype.
+    //
     PyType_Spec SbkObjectType_Type_spec = {
         "1:Shiboken.ObjectType",
-        0,
+        static_cast<int>(PyType_Type.tp_basicsize) + 1,           // see above
         0, // sizeof(PyMemberDef), not for PyPy without a __len__ defined
         Py_TPFLAGS_DEFAULT|Py_TPFLAGS_BASETYPE,
         SbkObjectType_Type_slots,