Fixed asyncio CancelScope not recognizing its own cancellation exce…

…ption

...if it comes wrapped in an exception group.
Fixes #634.

docs/versionhistory.rst

@@ -12,6 +12,8 @@ This library adheres to `Semantic Versioning 2.0 <http://semver.org/>`_.
   - Removed a checkpoint when exiting a task group
 - Bumped minimum version of trio to v0.23
 - Exposed the ``ResourceGuard`` class in the public API
+- Fixed discrepancy between ``asyncio`` and ``trio`` where reraising a cancellation
+  exception in an ``except*`` block would incorrectly bubble out of its cancel scope
 
 **4.0.0**
 

pyproject.toml

@@ -44,6 +44,7 @@ trio = ["trio >= 0.23"]
 test = [
     "anyio[trio]",
     "coverage[toml] >= 7",
+    "exceptiongroup >= 1.2.0",
     "hypothesis >= 4.0",
     "psutil >= 5.9",
     "pytest >= 7.0",

src/anyio/_backends/_asyncio.py

@@ -418,8 +418,18 @@ def __exit__(
         if self._shield:
             self._deliver_cancellation_to_parent()
 
-        if isinstance(exc_val, CancelledError) and self._cancel_called:
-            self._cancelled_caught = self._uncancel(exc_val)
+        if self._cancel_called:
+            if isinstance(exc_val, CancelledError):
+                self._cancelled_caught = self._uncancel(exc_val)
+            elif isinstance(exc_val, BaseExceptionGroup) and (
+                excgrp := exc_val.split(CancelledError)[0]
+            ):
+                for exc in excgrp.exceptions:
+                    if isinstance(exc, CancelledError):
+                        self._cancelled_caught = self._uncancel(exc)
+                        if self._cancelled_caught:
+                            break
+
             return self._cancelled_caught
 
         return None

tests/test_taskgroups.py

@@ -8,6 +8,7 @@
 from typing import Any, NoReturn, cast
 
 import pytest
+from exceptiongroup import catch
 
 import anyio
 from anyio import (
@@ -1282,6 +1283,16 @@ async def test_cancel_before_entering_task_group() -> None:
             pytest.fail("This should not raise a cancellation exception")
 
 
+async def test_reraise_cancelled_in_excgroup() -> None:
+    def handler(excgrp: BaseExceptionGroup) -> None:
+        raise
+
+    with CancelScope() as scope:
+        scope.cancel()
+        with catch({get_cancelled_exc_class(): handler}):
+            await anyio.sleep_forever()
+
+
 class TestTaskStatusTyping:
     """
     These tests do not do anything at run time, but since the test suite is also checked