gh-159739: Release replaced type alias values

Lib/test/test_type_aliases.py

@@ -1,7 +1,8 @@
 import pickle
 import types
 import unittest
-from test.support import check_syntax_error, run_code
+import weakref
+from test.support import check_syntax_error, gc_collect, run_code
 from test.typinganndata import mod_generics_cache
 
 from typing import (
@@ -131,6 +132,24 @@ def more_generic[T, *Ts, **P]():
         T, Ts, P = more_generic.__type_params__
         self.assertEqual(alias.__value__, tuple[Callable[P, tuple[T, *Ts]], Callable[P2, tuple[T2, *Ts2]]])
 
+    def test_reentrant_value_does_not_leak(self):
+        class Value:
+            pass
+
+        refs = []
+
+        def compute():
+            value = Value()
+            refs.append(weakref.ref(value))
+            if len(refs) == 1:
+                Alias.__value__
+            return value
+
+        type Alias = compute()
+        self.assertIs(Alias.__value__, refs[0]())
+        gc_collect()
+        self.assertIsNone(refs[1]())
+
     def test_subscripting(self):
         type NonGeneric = int
         type Generic[A] = dict[A, A]

Misc/NEWS.d/next/Core_and_Builtins/2026-10-21-19-55-00.gh-issue-159739.tali01.rst

@@ -0,0 +1,2 @@
+Fix a reference leak when the value of a type alias is evaluated
+recursively or concurrently.

Objects/typevarobject.c

@@ -1899,14 +1899,23 @@ typealias_dealloc(PyObject *self)
 static PyObject *
 typealias_get_value(typealiasobject *ta)
 {
-    if (ta->value != NULL) {
-        return Py_NewRef(ta->value);
+    PyObject *result;
+    Py_BEGIN_CRITICAL_SECTION(ta);
+    result = Py_XNewRef(ta->value);
+    Py_END_CRITICAL_SECTION();
+    if (result != NULL) {
+        return result;
     }
-    PyObject *result = PyObject_CallNoArgs(ta->compute_value);
+    result = PyObject_CallNoArgs(ta->compute_value);
     if (result == NULL) {
         return NULL;
     }
+    PyObject *old_value;
+    Py_BEGIN_CRITICAL_SECTION(ta);
+    old_value = ta->value;
     ta->value = Py_NewRef(result);
+    Py_END_CRITICAL_SECTION();
+    Py_XDECREF(old_value);
     return result;
 }