Child $effect of $derived is not destroyed in conditional expression when not called

[bdpartridge]

Describe the bug

When a child $effect is conditionally called by a $derived, the effect cleanup is deferred until right before the effect is called again or the component is destroyed. See the linked reproduction for a simple example. Notice how the effect cleanup isn't run until right before the getValue function is called again; try toggling the checkbox.

Since the getValue function effectively becomes unused by the component when the condition becomes false, it seems like the effect should be destroyed. If the effect were being used to track a subscription, for example, this could be problematic because there'd be no immediate signal when the component had lost interest in the results of getValue.

I'm not sure if this is a bug or expected behavior, so apologies for the noise if this is working as expected.

Reproduction

https://svelte-5-preview.vercel.app/#H4sIAAAAAAAAE22RTW7CMBCFrzKasggSSvYhUPUMrbqpKxHiMZgaO4ongSrK3Ss7gSCVlT32--bv9ai0IY_5V4-2PBPm-FbXuEL-rUPgOzJMuELv2qYKL4WvGl3zVljB-ly7hqE_EH-WpqUBVOPOIDDNuhCnI56evMC1sAExxOCP7hL1sIGF55Ip4aal5fom6G6fkhrdkUxm4hVuxRKByjmBS8ihtZKUtiRDjiKbW7SF1F3stTDlnky8Ci60rVuGMORGYHWk6mfvrgJhr63MY0xy09_LDpBN5PvRXcb-YtLslrXIpkK2f9FqHnGIsnqiP440Dac99PE2pGOiIOkzrQZhcYVnJ7XSJDEPmxlWd3ce9zrbdPKPFtE12qJaW7F2dt5YhJfQh4oLUooqTpIlbLbjk-DKWe8MpcYdkt0oyGExNbobDRLcELeNhZF8gkBlqLRt_Q8dxmPi49da2KcTfw9_25uWMpsCAAA=

Logs

No response

System Info

Svelte 5.0.0-next.269

Severity

annoyance

[7nik]

This seems to be an expected behaviour and result of $derived's laziness: when showValue is false, the value isn't used anywhere, so the derived doesn't update. Later, when showValue changes to true, the derived updates because its dependency had hanged.

Adding {value} outside the #if "fixes" the problem.

[trueadm]

This isn't a bug – this is working as intended. The derived was created in the component – not inside the template, which means the inner effect inside the derived will also be associated with the component too. So the only way to inner effect can re-run is if the derived itself re-runs and as explained by @7nik this is done lazily – so the behaviour you see is expected.

[bdpartridge]

Makes sense. Thanks for the explanation. My example was a bit contrived anyway. It'd be more realistic to use value inside the {#if ...} expression, and that does work nicely.