contextlib: Remove explicit base class from ExitStack (#7963)

Fixes #7961

Author: JelleZijlstra

stdlib/contextlib.pyi

@@ -163,7 +163,7 @@ class _RedirectStream(AbstractContextManager[_T_io]):
 class redirect_stdout(_RedirectStream[_T_io]): ...
 class redirect_stderr(_RedirectStream[_T_io]): ...
 
-class ExitStack(AbstractContextManager[ExitStack]):
+class ExitStack:
     def __init__(self) -> None: ...
     def enter_context(self, cm: AbstractContextManager[_T]) -> _T: ...
     def push(self, exit: _CM_EF) -> _CM_EF: ...
@@ -179,7 +179,7 @@ if sys.version_info >= (3, 7):
     _ExitCoroFunc: TypeAlias = Callable[[type[BaseException] | None, BaseException | None, TracebackType | None], Awaitable[bool]]
     _ACM_EF = TypeVar("_ACM_EF", bound=AbstractAsyncContextManager[Any] | _ExitCoroFunc)
 
-    class AsyncExitStack(AbstractAsyncContextManager[AsyncExitStack]):
+    class AsyncExitStack:
         def __init__(self) -> None: ...
         def enter_context(self, cm: AbstractContextManager[_T]) -> _T: ...
         async def enter_async_context(self, cm: AbstractAsyncContextManager[_T]) -> _T: ...

test_cases/stdlib/test_contextlib.py

@@ -0,0 +1,18 @@
+from contextlib import ExitStack
+from typing_extensions import assert_type
+
+
+# See issue #7961
+class Thing(ExitStack):
+    pass
+
+
+stack = ExitStack()
+thing = Thing()
+assert_type(stack.enter_context(Thing()), Thing)
+assert_type(thing.enter_context(ExitStack()), ExitStack)
+
+with stack as cm:
+    assert_type(cm, ExitStack)
+with thing as cm2:
+    assert_type(cm2, Thing)