python · JelleZijlstra · Nov 4, 2023 · Oct 28, 2023 · Oct 28, 2023 · Oct 28, 2023
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -3,7 +3,9 @@
 - All parameters on `NewType.__call__` are now positional-only. This means that
   the signature of `typing_extensions.NewType.__call__` now exactly matches the
   signature of `typing.NewType.__call__`. Patch by Alex Waygood.
-- `typing.deprecated` now gives a better error message if you pass a non-`str`
+- Fix bug with using `@deprecated` on a mixin class. Inheriting from a
+  deprecated class now raises a `DeprecationWarning`. Patch by Jelle Zijlstra.
+- `@deprecated` now gives a better error message if you pass a non-`str`
   argument to the `msg` parameter. Patch by Alex Waygood.
 - Exclude `__match_args__` from `Protocol` members,
   this is a backport of https://github.com/python/cpython/pull/110683

diff --git a/doc/index.rst b/doc/index.rst
@@ -553,6 +553,11 @@ Decorators
 
    .. versionadded:: 4.5.0
 
+   .. versionchanged:: 4.9.0
+
+      Inheriting from a deprecated class now also raises a runtime
+      :py:exc:`DeprecationWarning`.
+
 .. decorator:: final
 
    See :py:func:`typing.final` and :pep:`591`. In ``typing`` since 3.8.

diff --git a/src/test_typing_extensions.py b/src/test_typing_extensions.py
@@ -418,6 +418,93 @@ def __new__(cls, x):
         self.assertEqual(instance.x, 42)
         self.assertTrue(new_called)
 
+    def test_mixin_class(self):
+        @deprecated("Mixin will go away soon")
+        class Mixin:
+            pass
+
+        class Base:
+            def __init__(self, a) -> None:
+                self.a = a
+
+        with self.assertWarnsRegex(DeprecationWarning, "Mixin will go away soon"):
+            class Child(Base, Mixin):
+                pass
+
+        instance = Child(42)
+        self.assertEqual(instance.a, 42)
+
+    def test_existing_init_subclass(self):
+        @deprecated("C will go away soon")
+        class C:
+            def __init_subclass__(cls) -> None:
+                cls.inited = True
+
+        with self.assertWarnsRegex(DeprecationWarning, "C will go away soon"):
+            C()
+
+        with self.assertWarnsRegex(DeprecationWarning, "C will go away soon"):
+            class D(C):
+                pass
+
+        self.assertTrue(D.inited)
+        self.assertIsInstance(D(), D)  # no deprecation
+
+    def test_existing_init_subclass_in_base(self):
+        class Base:
+            def __init_subclass__(cls, x) -> None:
+                cls.inited = x
+
+        @deprecated("C will go away soon")
+        class C(Base, x=42):
+            pass
+
+        self.assertEqual(C.inited, 42)
+
+        with self.assertWarnsRegex(DeprecationWarning, "C will go away soon"):
+            C()
+
+        with self.assertWarnsRegex(DeprecationWarning, "C will go away soon"):
+            class D(C, x=3):
+                pass
+
+        self.assertEqual(D.inited, 3)
+
+    def test_init_subclass_has_correct_cls(self):
+        init_subclass_saw = None
+
+        @deprecated("Base will go away soon")
+        class Base:
+            def __init_subclass__(cls) -> None:
+                nonlocal init_subclass_saw
+                init_subclass_saw = cls
+
+        self.assertIsNone(init_subclass_saw)
+
+        with self.assertWarnsRegex(DeprecationWarning, "Base will go away soon"):
+            class C(Base):
+                pass
+
+        self.assertIs(init_subclass_saw, C)
+
+    def test_init_subclass_with_explicit_classmethod(self):
+        init_subclass_saw = None
+
+        @deprecated("Base will go away soon")
+        class Base:
+            @classmethod
+            def __init_subclass__(cls) -> None:
+                nonlocal init_subclass_saw
+                init_subclass_saw = cls
+
+        self.assertIsNone(init_subclass_saw)
+
+        with self.assertWarnsRegex(DeprecationWarning, "Base will go away soon"):
+            class C(Base):
+                pass
+
+        self.assertIs(init_subclass_saw, C)
+
     def test_function(self):
         @deprecated("b will go away soon")
         def b():

diff --git a/src/typing_extensions.py b/src/typing_extensions.py
@@ -2343,21 +2343,45 @@ def decorator(arg: _T, /) -> _T:
                 return arg
             elif isinstance(arg, type):
                 original_new = arg.__new__
-                has_init = arg.__init__ is not object.__init__
 
                 @functools.wraps(original_new)
                 def __new__(cls, *args, **kwargs):
-                    warnings.warn(msg, category=category, stacklevel=stacklevel + 1)
+                    if cls is arg:
+                        warnings.warn(msg, category=category, stacklevel=stacklevel + 1)
                     if original_new is not object.__new__:
                         return original_new(cls, *args, **kwargs)
                     # Mirrors a similar check in object.__new__.
-                    elif not has_init and (args or kwargs):
+                    elif cls.__init__ is object.__init__ and (args or kwargs):
                         raise TypeError(f"{cls.__name__}() takes no arguments")
                     else:
                         return original_new(cls)
 
                 arg.__new__ = staticmethod(__new__)
+
+                original_init_subclass = arg.__init_subclass__
+                # We need slightly different behavior if __init_subclass__
+                # is a bound method (likely if it was implemented in Python)
+                if isinstance(original_init_subclass, _types.MethodType):
+                    original_init_subclass = original_init_subclass.__func__
+
+                    @functools.wraps(original_init_subclass)
+                    def __init_subclass__(*args, **kwargs):
+                        warnings.warn(msg, category=category, stacklevel=stacklevel + 1)
+                        return original_init_subclass(*args, **kwargs)
+
+                    arg.__init_subclass__ = classmethod(__init_subclass__)
+                # Or otherwise, which likely means it's a builtin such as
+                # object's implementation of __init_subclass__.
+                else:
+                    @functools.wraps(original_init_subclass)
+                    def __init_subclass__(*args, **kwargs):
+                        warnings.warn(msg, category=category, stacklevel=stacklevel + 1)
+                        return original_init_subclass(*args, **kwargs)
+
+                    arg.__init_subclass__ = __init_subclass__
+
                 arg.__deprecated__ = __new__.__deprecated__ = msg
+                __init_subclass__.__deprecated__ = msg
                 return arg
             elif callable(arg):
                 @functools.wraps(arg)