Add a test

Lib/test/test_asyncio/test_tasks.py

@@ -634,6 +634,32 @@ async def coro():
                 self.assertEqual(actual,
                     (asyncio.CancelledError, expected_args, 0))
 
+    def test_cancellation_exception_context(self):
+        loop = asyncio.new_event_loop()
+        self.set_event_loop(loop)
+
+        async def sleep():
+            await asyncio.sleep(10)
+
+        async def coro():
+            inner_task = self.new_task(loop, sleep())
+            await asyncio.sleep(0)
+            loop.call_soon(inner_task.cancel, 'msg')
+            try:
+                await inner_task
+            except asyncio.CancelledError as ex:
+                raise ValueError("cancelled") from ex
+
+        task = self.new_task(loop, coro())
+        with self.assertRaises(ValueError) as cm:
+            loop.run_until_complete(task)
+        exc = cm.exception
+        self.assertEqual(exc.args, ('cancelled',))
+
+        actual = get_innermost_context(exc)
+        self.assertEqual(actual,
+            (asyncio.CancelledError, ('msg',), 1))
+
     def test_cancel_with_message_before_starting_task(self):
         loop = asyncio.new_event_loop()
         self.set_event_loop(loop)