fix: don't set state withing with_parent in proxy

[paoloricciuti]

Closes #16164

We can't set everywhere within with_parent otherwise if it's the first time we are reading a derived it could look like we are setting state in a derived (which you are not).

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Preview: https://svelte-dev-git-preview-svelte-16176-svelte.vercel.app/

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pnpm add https://pkg.pr.new/svelte@16176

[7nik]

shouldn't {my_derived.example} update when data.example changes?

All the changes are wiped by

$effect(() => {
		my_derived.example = 'Bar'
	});

that confirms $inspect:

update { example: "Examp" }
update { example: "Bar" }
update { example: "Exam" }
update { example: "Bar" }
update { example: "Examp" }
update { example: "Bar" }

[paoloricciuti]

Duh obviously...let me grab a coffe 😄

[dummdidumm]

with_parent in general is flawed, don't think we can keep its current version. Also see #15870 (comment)

[paoloricciuti]

Mh that's interesting...I'll try to see what it would take to "fix it"

[dummdidumm]

I think the real fix - and I've talked with Rich/Elliott about this and they agree - is to revert (part of) #15553 and instead fix the specific issue it was closing another way. With the possiblity to declare state fields with $state etc in the constructor much of the need for it is gone anyway.

[dummdidumm]

Will merge this anyway in the meantime since this is valid 👍