Reactive statement not fired

[null-dev]

Describe the bug
In some strange cases, the reactive statement that depends on a variable doesn't get fired when the variable changes.

To Reproduce
See this REPL:
https://svelte.dev/repl/4c2b3a63f4ed4b718561ba6ca7a62629?version=3.22.2

In the REPL, there is a reactive statement that will log the value of a to the console every time it changes (line 11). There is another reactive statement that will change the value of a whenever the value of b changes (line 13). Notice that when we change the value of b by clicking the button, the value of a does change, yet the reactive statement that depends on a (line 11) is not called!

I'm currently running Chrome 81 on Linux.

Workaround:
We can make the above REPL work again by moving line 13 to line 4.

Severity
Medium, since we can still workaround it.

[Conduitry]

This is the expected behavior, although not particularly documented at the moment.

When compiling a component, Svelte looks at which reactive blocks depend on other ones - based on what variables are used and what variables are assigned within the reactive block - and sorts them accordingly, resolving ties according to the order that they're original in the component source. During each invalidation/update phase, the reactive blocks are run through in this order. Invalidations happening in one of them can cause later blocks to also be run - but once a block has been passed, no validations this cycle can make it be run again.

In the case of your component, the compiler can't see that it would be helpful to put $: if(b > 0) changeA(); before $: console.log("Detected A value change: ", a); because it can't see that it might invalidate a.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[dummdidumm]

This is fixed in Svelte 5 with tracking dependencies at runtime.