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Not sure why lint tests are failing, I ran prettier on everything I changed...

It's erroring because export var structured_clone = structuredClone prevents Svelte from being tree-shaken (I'm not exactly sure why).

Regardless, this seems like a very bad idea — it will mean that code suddenly breaks in production. If we were to monkey-patch structuredClone in dev, the only thing it should be doing is replacing the inevitable error with a more descriptive one.

Should it throw when attempting to clone a $state proxy instead?

I mean, it does, the error is just a bit unclear. The easy solution would be to wrap the underlying call to structuredClone in a try-catch block that inspects the error to see if it's a DataCloneError, and prints a message accordingly, but that would have false positives if you did e.g. structuredClone(new Proxy({}, {})) and the message referred to Svelte state.

The more complete solution would basically be to reimplement structuredClone in userland, including handling cycles etc.

The more complete solution would basically be to reimplement structuredClone in userland, including handling cycles etc.

Would this also clone state proxies, or would it throw an error if attempted?

It would throw an error. The behaviour would need to be identical to structuredClone except for the very slightly more helpful error message

@Rich-Harris I've added the more complete option (not completely finished yet) and the basic option. Most of the concerns I have are in the comments in clone.js.