Disallow old-style classes for multiple inheritance

Cython/Utility/ExtensionTypes.c

@@ -23,7 +23,17 @@ static int __Pyx_PyType_Ready(PyTypeObject *t) {
         Py_ssize_t i, n = PyTuple_GET_SIZE(bases);
         for (i = 1; i < n; i++)  /* Skip first base */
         {
-            PyTypeObject *b = (PyTypeObject*)PyTuple_GET_ITEM(bases, i);
+            PyObject *b0 = PyTuple_GET_ITEM(bases, i);
+#if PY_MAJOR_VERSION < 3
+            /* Disallow old-style classes */
+            if (PyClass_Check(b0))
+            {
+                PyErr_Format(PyExc_TypeError, "base class '%.200s' is an old-style class",
+                             PyString_AS_STRING(((PyClassObject*)b0)->cl_name));
+                return -1;
+            }
+#endif
+            PyTypeObject *b = (PyTypeObject*)b0;
             if (!PyType_HasFeature(b, Py_TPFLAGS_HEAPTYPE))
             {
                 PyErr_Format(PyExc_TypeError, "base class '%.200s' is not a heap type",

tests/run/cdef_multiple_inheritance_errors.srctree

@@ -47,8 +47,21 @@ class Py(object):
 cdef class X(Base, Py):
     pass
 
+######## oldstyle.pyx ########
+
+cdef class Base:
+    cdef dict __dict__
+
+class OldStyle:
+    pass
+
+cdef class Foo(Base, OldStyle):
+    pass
+
 ######## runner.py ########
 
+import sys
+
 try:
     import notheaptype
     assert False
@@ -72,3 +85,10 @@ try:
     assert False
 except TypeError as msg:
     assert str(msg) == "extension type 'nodict.X' has no __dict__ slot, but base type 'Py' has: either add 'cdef dict __dict__' to the extension type or add '__slots__ = [...]' to the base type"
+
+try:
+    # This should work on Python 3 but fail on Python 2
+    import oldstyle
+    assert sys.version_info[0] >= 3
+except TypeError as msg:
+    assert str(msg) == "base class 'OldStyle' is an old-style class"