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

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) {