Crash report

What happened?

The following code causes an assertion in a cpython debug build

name_3=5name_2=2name_1=0whilename_3<=name_2>name_1:try:raiseexcept:passfinally:pass

output (Python 3.13.0a5+):

python3:Python/flowgraph.c:994:remove_unreachable:Assertion `target->b_predecessors==0||target==b->b_next'failed.

The bug is in the current 3.13 branch.
I bisected the problem down to04697bc
@iritkatriel I think this is again one for you.

Little side note. I found this bug after fuzzing for ~3 days. I want to start to test the cpython main branch in the future and I hope to find bugs like this earlier.

CPython versions tested on:

3.13

Operating systems tested on:

No response

Output from running 'python -VV' on the command line:

No response

Linked PRs

• gh-124871: fix 'visited' tracking in compiler's reachability analysis #124952
• [3.13] gh-124871: fix 'visited' tracking in compiler's reachability analysis (GH-124952) #124977