add _GeneratorContextManagerBase.__init__ (#13246)

I think that #6676 showed that Paramspec didn't work, but that
wasn't actually the fault of _GeneratorContextManagerBase.

stdlib/@tests/stubtest_allowlists/common.txt

@@ -11,7 +11,6 @@ _collections_abc.AsyncGenerator.ag_running
 asyncio.__all__
 builtins.dict.get
 collections\.ChainMap\.fromkeys  # https://github.com/python/mypy/issues/17023
-contextlib._GeneratorContextManagerBase.__init__  # skipped in the stubs in favor of its child classes
 http.client.HTTPConnection.response_class  # the actual type at runtime is abc.ABCMeta
 importlib.abc.Loader.exec_module  # See Lib/importlib/_abc.py. Might be defined for backwards compatibility
 importlib.abc.MetaPathFinder.find_spec  # Not defined on the actual class, but expected to exist.

stdlib/contextlib.pyi

@@ -33,8 +33,12 @@ _T_co = TypeVar("_T_co", covariant=True)
 _T_io = TypeVar("_T_io", bound=IO[str] | None)
 _ExitT_co = TypeVar("_ExitT_co", covariant=True, bound=bool | None, default=bool | None)
 _F = TypeVar("_F", bound=Callable[..., Any])
+_G = TypeVar("_G", bound=Generator[Any, Any, Any] | AsyncGenerator[Any, Any], covariant=True)
 _P = ParamSpec("_P")
 
+_SendT_contra = TypeVar("_SendT_contra", contravariant=True, default=None)
+_ReturnT_co = TypeVar("_ReturnT_co", covariant=True, default=None)
+
 _ExitFunc: TypeAlias = Callable[[type[BaseException] | None, BaseException | None, TracebackType | None], bool | None]
 _CM_EF = TypeVar("_CM_EF", bound=AbstractContextManager[Any, Any] | _ExitFunc)
 
@@ -64,16 +68,19 @@ class ContextDecorator:
     def _recreate_cm(self) -> Self: ...
     def __call__(self, func: _F) -> _F: ...
 
-class _GeneratorContextManagerBase: ...
-
-class _GeneratorContextManager(_GeneratorContextManagerBase, AbstractContextManager[_T_co, bool | None], ContextDecorator):
-    # __init__ and all instance attributes are actually inherited from _GeneratorContextManagerBase
-    # adding them there is more trouble than it's worth to include in the stub; see #6676
-    def __init__(self, func: Callable[..., Iterator[_T_co]], args: tuple[Any, ...], kwds: dict[str, Any]) -> None: ...
-    gen: Generator[_T_co, Any, Any]
-    func: Callable[..., Generator[_T_co, Any, Any]]
+class _GeneratorContextManagerBase(Generic[_G]):
+    # Ideally this would use ParamSpec, but that requires (*args, **kwargs), which this isn't. see #6676
+    def __init__(self, func: Callable[..., _G], args: tuple[Any, ...], kwds: dict[str, Any]) -> None: ...
+    gen: _G
+    func: Callable[..., _G]
     args: tuple[Any, ...]
     kwds: dict[str, Any]
+
+class _GeneratorContextManager(
+    _GeneratorContextManagerBase[Generator[_T_co, _SendT_contra, _ReturnT_co]],
+    AbstractContextManager[_T_co, bool | None],
+    ContextDecorator,
+):
     if sys.version_info >= (3, 9):
         def __exit__(
             self, typ: type[BaseException] | None, value: BaseException | None, traceback: TracebackType | None
@@ -93,26 +100,18 @@ if sys.version_info >= (3, 10):
         def __call__(self, func: _AF) -> _AF: ...
 
     class _AsyncGeneratorContextManager(
-        _GeneratorContextManagerBase, AbstractAsyncContextManager[_T_co, bool | None], AsyncContextDecorator
+        _GeneratorContextManagerBase[AsyncGenerator[_T_co, _SendT_contra]],
+        AbstractAsyncContextManager[_T_co, bool | None],
+        AsyncContextDecorator,
     ):
-        # __init__ and these attributes are actually defined in the base class _GeneratorContextManagerBase,
-        # adding them there is more trouble than it's worth to include in the stub (see #6676)
-        def __init__(self, func: Callable[..., AsyncIterator[_T_co]], args: tuple[Any, ...], kwds: dict[str, Any]) -> None: ...
-        gen: AsyncGenerator[_T_co, Any]
-        func: Callable[..., AsyncGenerator[_T_co, Any]]
-        args: tuple[Any, ...]
-        kwds: dict[str, Any]
         async def __aexit__(
             self, typ: type[BaseException] | None, value: BaseException | None, traceback: TracebackType | None
         ) -> bool | None: ...
 
 else:
-    class _AsyncGeneratorContextManager(_GeneratorContextManagerBase, AbstractAsyncContextManager[_T_co, bool | None]):
-        def __init__(self, func: Callable[..., AsyncIterator[_T_co]], args: tuple[Any, ...], kwds: dict[str, Any]) -> None: ...
-        gen: AsyncGenerator[_T_co, Any]
-        func: Callable[..., AsyncGenerator[_T_co, Any]]
-        args: tuple[Any, ...]
-        kwds: dict[str, Any]
+    class _AsyncGeneratorContextManager(
+        _GeneratorContextManagerBase[AsyncGenerator[_T_co, _SendT_contra]], AbstractAsyncContextManager[_T_co, bool | None]
+    ):
         async def __aexit__(
             self, typ: type[BaseException] | None, value: BaseException | None, traceback: TracebackType | None
         ) -> bool | None: ...