gh-158072: Fix JIT crash with omitted default arguments

[lgeiger]

This fixes a JIT crash when a class is called without passing an optional __init__ argument. The optimizer treated that argument as uninitialized, even though Python fills it from a default.

I've verified that change fixes #158072 on my machine. I'm not a JIT expert and this fix was developed with significant LLM help so review with care 😇

• Issue: 3.15 JIT crash in starlette test suite, when brotli is installed #158072

[mgorny]

Thanks. I can confirm that it fixes my original crash as well.

[cocolato]

If __init__.__code__ changes, this can still produce incorrect results. So we can limit arg_len to co->co_argcount before this loop.

Suggested change

	arg_len = Py_MIN(arg_len, co->co_argcount);
	for (int i = 0; i < arg_len; i++) {

And we need a new test for this (or implement it in other PR)

[lgeiger]

Oh, I didn't think about that! Done in 689e8e7

[cocolato]

LGTM, and let's wait a core dev to review this

[markshannon]

Looks good.

Thanks @lgeiger for implementing this, and @cocolato for the review.

[lgeiger]

Great, thanks for the fast review and merge! Should we also backport this to 3.15, since the original report was for 3.15 or would you rather only keep it for 3.16?

[mgorny]

Yes, please. 3.16 is still long way ahead, and 3.15 is the primary target I'm testing JIT on.

[miss-islington-app]

Thanks @lgeiger for the PR, and @markshannon for merging it 🌮🎉.. I'm working now to backport this PR to: 3.15.
🐍🍒⛏🤖

[bedevere-app]

GH-158543 is a backport of this pull request to the 3.15 branch.