GH-146096: Fix base exception group repr

[sergey-miryanov]

Fix segfault when calling repr for BaseExceptionGroup.

cc @dr-carlos

• Issue: Out-of-bounds tuple access in BaseExceptionGroup_repr #146096

[dr-carlos]

Thanks for the fix @sergey-miryanov! All looks good to me.
My apologies for not considering that args was mutable in the first implementation!

It's a pity that we don't preserve the list-like repr (ExceptionGroup('msg', [ValueError()])) in this case. But mutating the args to something without an exceptions list in it seems like a weird enough use case that we can just do this?
If this is something that people actually do (why?), we could just set the self->excs_str in all cases, but that can be quite inefficient and I'd really prefer not to.

[iritkatriel]

Do we need to check that it's a Tuple first?

[dr-carlos]

No, args is always converted to a tuple in its setter:
https://github.com/sergey-miryanov/cpython/blob/4f9a6ba7ba98a83d5594aacadcddb90696ffd5d7/Objects/exceptions.c#L351-L364

[iritkatriel]

Could add an assert just so it reads like it makes sense.

[sergey-miryanov]

Done

[sergey-miryanov]

I'm not sure that failed test related to change.

[sergey-miryanov]

Thanks all!