gh-159739: Keep generic alias arguments alive while formatting errors (#159981)

When a generic alias contains a list of parameters and an object claiming to be an unpacked type-variable tuple returns a non-tuple from `__typing_subst__`, the error path frees the temporary argument tuple before reading it to format a `TypeError`. That use-after-free can crash the interpreter instead of reporting the invalid substitution.

Author: jelle-openai

Misc/NEWS.d/next/Core_and_Builtins/2026-10-20-00-00-00.gh-issue-159739.a3cc5d.rst

@@ -0,0 +1 @@
+Fix a crash when constructing an error for invalid generic alias arguments.

Objects/genericaliasobject.c

@@ -550,13 +550,13 @@ _Py_subs_parameters(PyObject *self, PyObject *args, PyObject *parameters, PyObje
         }
         if (unpack) {
             if (!PyTuple_Check(arg)) {
-                Py_DECREF(newargs);
-                Py_DECREF(item);
-                Py_XDECREF(tuple_args);
                 PyObject *original = PyTuple_GET_ITEM(args, iarg);
                 PyErr_Format(PyExc_TypeError,
                              "expected __typing_subst__ of %T objects to return a tuple, not %T",
                              original, arg);
+                Py_DECREF(newargs);
+                Py_DECREF(item);
+                Py_XDECREF(tuple_args);
                 Py_DECREF(arg);
                 return NULL;
             }