[v5 Runes] Re-assigning NaN to an already NaN variable triggers update

[Malix-Labs]

Describe the bug

1. In v5 Runes, re-assigning NaN to an already NaN variable triggers update
2. Inconsistency between v5 Runes and v5 Classic:
   In v5 Classic (and v4), re-assigning NaN to an already NaN variable doesn't trigger update

Reproduction

https://svelte-5-preview.vercel.app/#H4sIAAAAAAAACsWQTWvDMAyG_4owhSYQkkNvbhIYu_ew6zJo6iqrmWMbWxmMkP8-4nw07NCx046SXj3iUc8aqdAz_tozXbfIOHuyliWMvuxY-E9UhCxh3nROjJ3cCyctlZWuSLbWOIJnVXsvBTTOtLBPs7lOp-X9JvnSafRrLlT3VJ7d0Tq_HcqZk2e3Q1npfDmTrYEAWMYTexyyhLXmKhuJV8bJdTgkq90MeWwYWtxYkkZ7cCO46JtaeRyW65svKCTQXXtBdxxHFe04CKO9UZgq8x6dA4DDrp9Swzk-_rS9dERGg9FCSfFR9FEMRTlToYBTfRrCrW1nZEx7vziHz_zZ-KEpFLDzVBNG8SIttbco6L9034Zvw2X5Z8wCAAA=

Logs

No response

System Info

not needed

Severity

annoyance

[Malix-Labs]

About the first issue:

• Comment from @Malix-off:

  • Issue Description from @Rich-Harris:

    ⋯ I'm pretty sure that's a small enough edge case that we needn't worry about it

    I disagree and think the performance tradeoff is better than the deprecated support for NaN checking before update tradeoff

About the second issue:

I think having a logic inconsistency between v5 Classic and v5 Runes could be misleading, and thus is not desirable.

[Rich-Harris]

I stand by my assessment — it would be very wasteful to check for NaN every time. Candidly, if you have NaN somewhere in your state, it's very likely an opportunity for improvement.

[dummdidumm]

Instead of using === for comparison we could use Object.is which handles this case correctly. I'm not what that would mean for performance though

[Rich-Harris]

Nothing, according to this microbenchmark, though I'm not sure I trust it — it violates my expectations quite deeply.

[Malix-Labs]

Quite unexpected for me too

if you have NaN somewhere in your state, it's very likely an opportunity for improvement

By the way, I do agree falling back to NaN is bugs and weird behavior prone, but it is unfortunately the default value for some HTML input elements state (valueAsNumber and sometimes value)