Do not shadow user arguments in generated __new__ by @deprecated

Backport of: python/cpython#132160

Author: Viicos

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:

src/typing_extensions.py

@@ -3203,7 +3203,7 @@ 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__: