PEP 709: Crash with lambda + nested scope

[JelleZijlstra]

This crashes on main:

def a():
    def a():
        [(lambda : b) for b in [a]]
        print(b)

The original reproducer (found with my fork of https://github.com/carljm/compgenerator) was

class a:
    def a():
        class a:
            [(lambda : (a := a[(a := 2)])[b]) for b in (lambda b, a: 7)[a]]
            [][2] = b
            (1)[lambda a: a] = 4
            (2)[2] = b = a
        (4)[lambda b, a: b] = a = lambda : 1

Found in #104528 (comment) but turned out be unrelated to that PR.

Linked PRs

• gh-104602: ensure all cellvars are known up front #104603
• gh-104602: Add additional test for listcomp with lambda #104639

[JelleZijlstra]

Further fuzzing found this case that was also fixed by #104603:

class __a:
    __a[1] = [(1) for __b in [] for __a in [lambda : __a]]

This one failed as follows:

>>> class __a:
...     __a[1] = [(1) for __b in [] for __a in [lambda : __a]]
... 
SystemError: compiler_lookup_arg(name='_a__a') with reftype=5 failed in __a; freevars of code <lambda>: ('_a__a',)

which is different from the original repro case, which segfaulted. I'll see if I can distill a useful test case out of it.