Occasional incorrect chaining of CancelledError when calling 'cancel' on result of 'asyncio.gather'

import asyncio

async def f1():
    await asyncio.sleep(1)

async def f2():
    return 42

async def main():
    # case 1
    gfut = asyncio.gather(f1(), f2(), return_exceptions=True)
    # case 2
    #gfut = asyncio.gather(f2(), f1(), return_exceptions=True)
    await asyncio.sleep(0.1)
    gfut.cancel("my message")
    await gfut

asyncio.run(main())

# case 1

#Traceback (most recent call last):
#  File "/Users/vladima/Sources/gather.py", line 14, in main
#    print(await gfut)
#asyncio.exceptions.CancelledError
#
#During handling of the above exception, another exception occurred:
#
#Traceback (most recent call last):
#  File "/Users/vladima/Sources/gather.py", line 16, in <module>
#    asyncio.run(main())
#  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/asyncio/runners.py", line 44, in run
#    return loop.run_until_complete(main)
#  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/asyncio/base_events.py", line 646, in run_until_complete
#    return future.result()
#asyncio.exceptions.CancelledError

# case 2
#Traceback (most recent call last):
#  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/asyncio/tasks.py", line 605, in sleep
#    return await future
#asyncio.exceptions.CancelledError: my message

#During handling of the above exception, another exception occurred:

#Traceback (most recent call last):
#  File "/Users/vladima/Sources/gather.py", line 14, in main
#    print(await gfut)
#asyncio.exceptions.CancelledError
#
#During handling of the above exception, another exception occurred:
#
#Traceback (most recent call last):
#  File "/Users/vladima/Sources/gather.py", line 16, in <module>
#    asyncio.run(main())
#  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/asyncio/runners.py", line 44, in run
#    return loop.run_until_complete(main)
#  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/asyncio/base_events.py", line 646, in run_until_complete
#    return future.result()
#asyncio.exceptions.CancelledError

Currently traceback and description parts of CancelledError depend upon the order of arguments that were passed into asyncio.gather. The reason I believe is this: here fut would be bound to the last item in children which might or might not correspond to the cancelled task and instead it should probably be one of tasks that were actually cancelled (which is still not ideal since there might be many of them but I guess with gather it is as good as it can be).

Linked PRs

• gh-133243

Start by running the two asyncio.gather cases in the issue, then inspect Lib/asyncio/tasks.py around line 812, where the issue identifies the ordering-dependent future selection. Done means cancellation traceback and description behavior no longer depend on the order of gather arguments; review linked PR gh-133243 before starting.