[3.15] gh-159739: [typing] Handle generic alias construction errors (GH-159772) (#159909)

(cherry picked from commit 07df106)

Co-authored-by: Jelle Zijlstra <jelle@openai.com>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

Author: 3 people

Misc/NEWS.d/next/Core_and_Builtins/2026-10-19-20-38-17.gh-issue-159739.b62fc9.rst

@@ -0,0 +1,2 @@
+Propagate errors during generic class construction and avoid a crash after
+failed generic alias substitution.

Objects/genericaliasobject.c

@@ -303,7 +303,7 @@ subs_tvars(PyObject *obj, PyObject *params,
                                     PyTuple_GET_SIZE(arg));
                     if (j < 0) {
                         Py_DECREF(subparams);
-                        Py_DECREF(subargs);
+                        assert(subargs == NULL);
                         return NULL;
                     }
                     continue;

Objects/typevarobject.c

@@ -2359,6 +2359,9 @@ _Py_subscript_generic(PyThreadState* unused, PyObject *params)
         return NULL;
     }
     params = unpack_typevartuples(params);
+    if (params == NULL) {
+        return NULL;
+    }
     PyObject *args[2] = {(PyObject *)interp->cached_objects.generic_type, params};
     PyObject *result = call_typing_func_object("_GenericAlias", args, 2);
     Py_DECREF(params);