Do not shadow user arguments in generated __new__ by @deprecated (#581)

Viicos · web-flow · commit 88a0c200ceb0 · 2025-04-10T06:51:18.000-07:00
Backport of: python/cpython#132160
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -3,6 +3,10 @@
 - Fix `TypeError` when taking the union of `typing_extensions.TypeAliasType` and a
   `typing.TypeAliasType` on Python 3.12 and 3.13.
   Patch by [Joren Hammudoglu](https://github.com/jorenham).
+- Backport from CPython PR [#132160](https://github.com/python/cpython/pull/132160)
+  to avoid having user arguments shadowed in generated `__new__` by
+  `@typing_extensions.deprecated`.
+  Patch by [Victorien Plot](https://github.com/Viicos).
 
 # Release 4.13.1 (April 3, 2025)
 
diff --git a/src/test_typing_extensions.py b/src/test_typing_extensions.py
@@ -707,6 +707,25 @@ class Child(Base, Mixin):
         instance = Child(42)
         self.assertEqual(instance.a, 42)
 
+    def test_do_not_shadow_user_arguments(self):
+        new_called = False
+        new_called_cls = None
+
+        @deprecated("MyMeta will go away soon")
+        class MyMeta(type):
+            def __new__(mcs, name, bases, attrs, cls=None):
+                nonlocal new_called, new_called_cls
+                new_called = True
+                new_called_cls = cls
+                return super().__new__(mcs, name, bases, attrs)
+
+        with self.assertWarnsRegex(DeprecationWarning, "MyMeta will go away soon"):
+            class Foo(metaclass=MyMeta, cls='haha'):
+                pass
+
+        self.assertTrue(new_called)
+        self.assertEqual(new_called_cls, 'haha')
+
     def test_existing_init_subclass(self):
         @deprecated("C will go away soon")
         class C:
diff --git a/src/typing_extensions.py b/src/typing_extensions.py
@@ -3123,7 +3123,8 @@ def method(self) -> None:
         return arg
 
 
-if hasattr(warnings, "deprecated"):
+# Python 3.13.3+ contains a fix for the wrapped __new__
+if sys.version_info >= (3, 13, 3):
     deprecated = warnings.deprecated
 else:
     _T = typing.TypeVar("_T")
@@ -3203,7 +3204,7 @@ def __call__(self, arg: _T, /) -> _T:
                 original_new = arg.__new__
 
                 @functools.wraps(original_new)
-                def __new__(cls, *args, **kwargs):
+                def __new__(cls, /, *args, **kwargs):
                     if cls is arg:
                         warnings.warn(msg, category=category, stacklevel=stacklevel + 1)
                     if original_new is not object.__new__:
