AttributeError: 'TaskGroup' object has no attribute '_exceptions' #1109

Things to check first

• I have searched the existing issues and didn't find my bug already reported there

• I have checked that my bug is still present in the latest release

AnyIO version

4.13.0

Python version

3.13.3

What happened?

Entering the context of a task group multiple times raises an AttributeError

Traceback (most recent call last):
  File "...\.venv\Lib\site-packages\anyio\_backends\_asyncio.py", line 794, in __aexit__
    if self._exceptions:
       ^^^^^^^^^^^^^^^^
AttributeError: 'TaskGroup' object has no attribute '_exceptions'

I suggest to raise a different exception if this behavior is not intended

How can we reproduce the bug?

async def reproduce():
    tg = anyio.create_task_group()
    async with tg:
        print(1)
    async with tg:
        print(2)
    async with tg:
        print(3)


anyio.run(reproduce)

outputs

1
2
Traceback (most recent call last): ...