gh-155053: Fix GenericAlias crash on OOM

Misc/NEWS.d/next/Core_and_Builtins/2026-08-01-19-17-56.gh-issue-155053.sxmlTO.rst

@@ -0,0 +1,2 @@
+Fix a crash in :class:`types.GenericAlias` during substitution of a
+``TypeVarTuple`` under allocation failure.

Objects/genericaliasobject.c

@@ -302,7 +302,8 @@ subs_tvars(PyObject *obj, PyObject *params,
                                     PyTuple_GET_SIZE(arg));
                     if (j < 0) {
                         Py_DECREF(subparams);
-                        Py_DECREF(subargs);
+                        assert(subargs == NULL);
+                        /* tuple_extend() clears subargs on failure. */
                         return NULL;
                     }
                     continue;