gh-155053: Fix GenericAlias crash on OOM

Lib/test/test_genericalias.py

@@ -1,6 +1,8 @@
 """Tests for C-implemented GenericAlias."""
 
 import unittest
+from test import support
+from test.support import import_helper
 import pickle
 from array import array
 import copy
@@ -62,10 +64,11 @@
 from string.templatelib import Template, Interpolation
 
 import typing
-from typing import TypeVar, Unpack
+from typing import TypeVar, TypeVarTuple, Unpack
 T = TypeVar('T')
 K = TypeVar('K')
 V = TypeVar('V')
+Ts = TypeVarTuple("Ts")
 
 _UNPACKED_TUPLES = [
     # Unpacked tuple using `*`
@@ -97,6 +100,7 @@
     tuple[*tuple[Unpack[tuple[int, ...]]]],
 ]
 
+_testcapi = import_helper.import_module("_testcapi")
 
 class BaseTest(unittest.TestCase):
     """Test basics."""
@@ -628,6 +632,29 @@ def test_gh150146(self):
                 with self.assertRaises(TypeError):
                     x[*typing.Mapping[..., ...]]
 
+    @support.nomemtest
+    def test_subs_tvars_nomemory(self):
+        alias = dict[str, tuple[*Ts]]
+        key = (int, str)
+
+        # Warm the code path to stabilize the allocation window.
+        # This injection point was determined experimentally for this build.
+        try:
+            dict[str, tuple[int, str]]
+        except Exception:
+            pass
+
+        _testcapi.set_nomemory(24, 25)
+        raised = False
+        try:
+            alias[key]
+        except MemoryError:
+            raised = True
+        finally:
+            _testcapi.remove_mem_hooks()
+
+        self.assertTrue(raised, "MemoryError not raised")
+
 
 class TypeIterationTests(unittest.TestCase):
     _UNITERABLE_TYPES = (list, tuple)

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` when ``tuple_extend()`` fails
+during substitution of a ``TypeVarTuple`` under allocation failure.

Objects/genericaliasobject.c

@@ -302,7 +302,6 @@ subs_tvars(PyObject *obj, PyObject *params,
                                     PyTuple_GET_SIZE(arg));
                     if (j < 0) {
                         Py_DECREF(subparams);
-                        Py_DECREF(subargs);
                         return NULL;
                     }
                     continue;