add some safety checks to the shared common types import

Cython/Utility/CommonTypes.c

@@ -14,7 +14,20 @@ static PyTypeObject* __Pyx_FetchCommonType(PyTypeObject* type) {
     Py_INCREF(fake_module);
 
     cached_type = (PyTypeObject*) PyObject_GetAttrString(fake_module, type->tp_name);
-    if (!cached_type) {
+    if (cached_type) {
+        if (!PyType_Check((PyObject*)cached_type)) {
+            PyErr_Format(PyExc_TypeError,
+                "Shared Cython type %.200s is not a type object",
+                type->tp_name);
+            goto bad;
+        }
+        if (cached_type->tp_basicsize != type->tp_basicsize) {
+            PyErr_Format(PyExc_TypeError,
+                "Shared Cython type %.200s has the wrong size, try recompiling",
+                type->tp_name);
+            goto bad;
+        }
+    } else {
         if (!PyErr_ExceptionMatches(PyExc_AttributeError)) goto bad;
         PyErr_Clear();
         if (PyType_Ready(type) < 0) goto bad;
@@ -24,8 +37,13 @@ static PyTypeObject* __Pyx_FetchCommonType(PyTypeObject* type) {
         cached_type = type;
     }
 
-bad:
+done:
     Py_DECREF(fake_module);
     // NOTE: always returns owned reference, or NULL on error
     return cached_type;
+
+bad:
+    Py_XDECREF(cached_type);
+    cached_type = NULL;
+    goto done;
 }