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JIT (correctness): Different `UnboundLocalError` for same code with JIT on or off

devdanzin

Bug report

Bug description:

The following code results in two different UnboundLocalError exceptions depending on whether it's run with the JIT on or off:

def f():

    if 0:
        class Class:
            pass

    if 0:
        instance = Class()

    for _ in range(5000):
        try:
            _ = instance.f()
        except Exception:
            pass
    Class.__bases__ = (object,)

f()

With JIT on, the UnboundLocalError points to 'instance', while with JIT off it points to 'Class'.

Traceback with JIT on:

Traceback (most recent call last):
  File "/home/danzin/divergences/divergence_child_6010_3_1.py", line 17, in <module>
    f()
    ~^^
  File "/home/danzin/divergences/divergence_child_6010_3_1.py", line 12, in f
    _ = instance.f()
        ^^^^^^^^
UnboundLocalError: cannot access local variable 'instance' where it is not associated with a value

Traceback with JIT off:

Traceback (most recent call last):
  File "/home/danzin/divergences/divergence_child_6010_3_1.py", line 17, in <module>
    f()
    ~^^
  File "/home/danzin/divergences/divergence_child_6010_3_1.py", line 15, in f
    Class.__bases__ = (object,)
    ^^^^^
UnboundLocalError: cannot access local variable 'Class' where it is not associated with a value

Output for PYTHON_LLTRACE=4:
divergence_01_lltrace.txt

Output for PYTHON_OPT_DEBUG=4:
divergence_01_opt_debug.txt

Please let me know whether you're able to reproduce this issue.

Found using lafleur.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs