[Python-checkins] gh-154275: Do not crash on deeply nested `__parameters__` in `GenericAlias` (#154277)

[sobolevn]

https://github.com/python/cpython/commit/1034e07b7d55642c3a46252e6b0a631c2df588d2
commit: 1034e07b7d55642c3a46252e6b0a631c2df588d2
branch: main
author: sobolevn <m...@sobolevn.me>
committer: sobolevn <m...@sobolevn.me>
date: 2026-07-21T01:11:04+03:00
summary:

gh-154275: Do not crash on deeply nested `__parameters__` in `GenericAlias` 
(#154277)

files:
A 
Misc/NEWS.d/next/Core_and_Builtins/2026-07-20-20-16-08.gh-issue-154275.l961cA.rst
M Lib/test/test_typing.py
M Objects/genericaliasobject.c

diff --git a/Lib/test/test_typing.py b/Lib/test/test_typing.py
index c1f340230159db..106ffdede6fd4e 100644
--- a/Lib/test/test_typing.py
+++ b/Lib/test/test_typing.py
@@ -52,6 +52,8 @@
 from test.support import (
     captured_stderr, cpython_only, requires_docstrings, import_helper, 
run_code,
     subTests, EqualToForwardRef,
+    exceeds_recursion_limit, skip_if_huge_c_stack, skip_wasi_stack_overflow,
+    skip_emscripten_stack_overflow,
 )
 from test.typinganndata import (
     ann_module695, mod_generics_cache, _typed_dict_helper,
@@ -5094,6 +5096,17 @@ class MM2(collections.abc.MutableMapping, 
MutableMapping[str, str]):
             pass
         self.assertEqual(MM2.__bases__, (collections.abc.MutableMapping, 
Generic))
 
+    @cpython_only
+    @skip_if_huge_c_stack()
+    @skip_wasi_stack_overflow()
+    @skip_emscripten_stack_overflow()
+    def test_parameters_deep_recursion(self):
+        x = [0]
+        for _ in range(exceeds_recursion_limit()):
+            x = [x]
+        with self.assertRaisesRegex(RecursionError, "in __parameter__ 
calculation"):
+            list[x].__parameters__
+
     def test_orig_bases(self):
         T = TypeVar('T')
         class C(typing.Dict[str, T]): ...
diff --git 
a/Misc/NEWS.d/next/Core_and_Builtins/2026-07-20-20-16-08.gh-issue-154275.l961cA.rst
 
b/Misc/NEWS.d/next/Core_and_Builtins/2026-07-20-20-16-08.gh-issue-154275.l961cA.rst
new file mode 100644
index 00000000000000..998e91c77098e7
--- /dev/null
+++ 
b/Misc/NEWS.d/next/Core_and_Builtins/2026-07-20-20-16-08.gh-issue-154275.l961cA.rst
@@ -0,0 +1,2 @@
+Fix a crash when getting deeply nested ``__parameters__`` from a
+:class:`types.GenericAlias` objects.
diff --git a/Objects/genericaliasobject.c b/Objects/genericaliasobject.c
index 4e85927def7ea7..5caf5c7086c87a 100644
--- a/Objects/genericaliasobject.c
+++ b/Objects/genericaliasobject.c
@@ -186,6 +186,10 @@ PyObject *
 _Py_make_parameters(PyObject *args)
 {
     assert(PyTuple_Check(args) || PyList_Check(args));
+    if (Py_EnterRecursiveCall(" in __parameter__ calculation")) {
+        return NULL;
+    }
+
     const bool is_args_list = PyList_Check(args);
     PyObject *tuple_args = NULL;
     if (is_args_list) {
@@ -210,9 +214,7 @@ _Py_make_parameters(PyObject *args)
         }
         int rc = PyObject_HasAttrWithError(t, &_Py_ID(__typing_subst__));
         if (rc < 0) {
-            Py_DECREF(parameters);
-            Py_XDECREF(tuple_args);
-            return NULL;
+            goto error;
         }
         if (rc) {
             iparam += tuple_add(parameters, iparam, t);
@@ -221,18 +223,14 @@ _Py_make_parameters(PyObject *args)
             PyObject *subparams;
             if (PyObject_GetOptionalAttr(t, &_Py_ID(__parameters__),
                                      &subparams) < 0) {
-                Py_DECREF(parameters);
-                Py_XDECREF(tuple_args);
-                return NULL;
+                goto error;
             }
             if (!subparams && (PyTuple_Check(t) || PyList_Check(t))) {
                 // Recursively call _Py_make_parameters for lists/tuples and
                 // add the results to the current parameters.
                 subparams = _Py_make_parameters(t);
                 if (subparams == NULL) {
-                    Py_DECREF(parameters);
-                    Py_XDECREF(tuple_args);
-                    return NULL;
+                    goto error;
                 }
             }
             if (subparams && PyTuple_Check(subparams)) {
@@ -243,7 +241,7 @@ _Py_make_parameters(PyObject *args)
                     if (_PyTuple_Resize(&parameters, len) < 0) {
                         Py_DECREF(subparams);
                         Py_XDECREF(tuple_args);
-                        return NULL;
+                        goto cleanup;
                     }
                 }
                 for (Py_ssize_t j = 0; j < len2; j++) {
@@ -256,13 +254,19 @@ _Py_make_parameters(PyObject *args)
     }
     if (iparam < len) {
         if (_PyTuple_Resize(&parameters, iparam) < 0) {
-            Py_XDECREF(parameters);
-            Py_XDECREF(tuple_args);
-            return NULL;
+            goto error;
         }
     }
     Py_XDECREF(tuple_args);
+    Py_LeaveRecursiveCall();
     return parameters;
+
+error:
+    Py_XDECREF(parameters);
+    Py_XDECREF(tuple_args);
+cleanup:
+    Py_LeaveRecursiveCall();
+    return NULL;
 }
 
 /* If obj is a generic alias, substitute type variables params

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