Made Listener covariant in the stream type variable (agronholm#464)

For example, when a function expectes Listener[ByteStream], it will accept Listener[SocketStream] or Listener[TLSStream] too.

docs/versionhistory.rst

@@ -25,6 +25,7 @@ This library adheres to `Semantic Versioning 2.0 <http:https://semver.org/>`_.
 
  - The ``TaskStatus`` class is now generic, and should be parametrized to indicate the
  type of the value passed to ``task_status.started()``
+ - The ``Listener`` class is now covariant in its stream type
 - Fixed ``CapacityLimiter`` on the asyncio backend to order waiting tasks in the FIFO
  order (instead of LIFO) (PR by Conor Stevenson)
 - Fixed ``CancelScope.cancel()`` not working on asyncio if called before entering the

src/anyio/abc/_sockets.py

@@ -14,7 +14,7 @@
  TypedAttributeSet,
  typed_attribute,
 )
-from ._streams import ByteStream, Listener, T_Stream, UnreliableObjectStream
+from ._streams import ByteStream, Listener, UnreliableObjectStream
 from ._tasks import TaskGroup
 
 IPAddressType = Union[str, IPv4Address, IPv6Address]
@@ -133,7 +133,9 @@ async def accept(self) -> SocketStream:
  """Accept an incoming connection."""
 
  async def serve(
- self, handler: Callable[[T_Stream], Any], task_group: TaskGroup | None = None
+ self,
+ handler: Callable[[SocketStream], Any],
+ task_group: TaskGroup | None = None,
  ) -> None:
  from .. import create_task_group
 

src/anyio/abc/_streams.py

@@ -10,7 +10,7 @@
 from ._tasks import TaskGroup
 
 T_Item = TypeVar("T_Item")
-T_Stream = TypeVar("T_Stream")
+T_Stream_co = TypeVar("T_Stream_co", covariant=True)
 
 
 class UnreliableObjectReceiveStream(
@@ -186,12 +186,12 @@ async def send_eof(self) -> None:
 AnyByteStream = Union[ObjectStream[bytes], ByteStream]
 
 
-class Listener(Generic[T_Stream], AsyncResource, TypedAttributeProvider):
+class Listener(Generic[T_Stream_co], AsyncResource, TypedAttributeProvider):
  """An interface for objects that let you accept incoming connections."""
 
  @abstractmethod
  async def serve(
- self, handler: Callable[[T_Stream], Any], task_group: TaskGroup | None = None
+ self, handler: Callable[[T_Stream_co], Any], task_group: TaskGroup | None = None
  ) -> None:
  """
  Accept incoming connections as they come in and start tasks to handle them.