gh-151912: Fix segfault in type() with NULL tp_new metaclasses (#151916)

Author: Santhosh-I

Lib/test/test_descr.py

@@ -815,6 +815,15 @@ class X(C, int()):
             class X(int(), C):
                 pass
 
+    @unittest.skipIf(_testcapi is None, 'need the _testcapi module')
+    def test_type_with_null_new_metaclass(self):
+        metaclass = _testcapi.HeapCTypeMetaclassNullNew
+        base = _testcapi.pytype_fromspec_meta(metaclass)
+
+        # Exercise type_new's metaclass selection path, not a direct call.
+        with self.assertRaisesRegex(TypeError, r"cannot create '.*' instances"):
+            type("Derived", (base,), {})
+
     def test_module_subclasses(self):
         # Testing Python subclass of module...
         log = []

Misc/NEWS.d/next/Core_and_Builtins/2026-06-22-14-48-22.gh-issue-151912.YcxfnU.rst

@@ -0,0 +1 @@
+Fixed a crash in ``type()`` when selecting a metaclass whose ``tp_new`` slot is ``NULL``. Such metaclasses are now rejected with ``TypeError`` instead of causing a NULL pointer dereference.

Objects/typeobject.c

@@ -5031,6 +5031,13 @@ type_new_get_bases(type_new_ctx *ctx, PyObject **type)
 
     if (winner != ctx->metatype) {
         if (winner->tp_new != type_new) {
+            /* Check if tp_new is NULL (cannot instantiate this type) */
+            if (winner->tp_new == NULL) {
+                PyErr_Format(PyExc_TypeError,
+                             "cannot create '%.400s' instances",
+                             winner->tp_name);
+                return -1;
+            }
             /* Pass it to the winner */
             *type = winner->tp_new(winner, ctx->args, ctx->kwds);
             if (*type == NULL) {