Svelte 5: default values should be readonly

[Rich-Harris]

Describe the bug

As of #9739, props are readonly (unless used with bind:). By extension, default values should also be readonly (enforced at dev time).

At present, that doesn't happen — default values can be mutated, but it's not reflected anywhere

Reproduction

https://svelte-5-preview.vercel.app/#H4sIAAAAAAAAE42P0WrDMAxFf0WYQVNWkvXVrQNjn7HsYXWczZ1jGVsZDON_n-1AKbQPRS--8tXRVWSTNiow_h6Z_ZwV4-zVObZj9OeKCL_KkMo64OJl6RyD9NpRP1gAPTv0BG_f2owweZxh03ZVtevg5jDYY3eZyGL1dn1GzjjqSauRcfKLSrtLgup5LMNARhFEwNNZSQKRnxIXSxxeIOUS8OQ8utBsb5KcFiK0gFYaLX9EbLYgeoiFudLaSoJnAftD6Uq0AY1qDX41146CTqmGqajAIV7_p7J53Xbn7I_0D3kxjdKEAQAA

Logs

No response

System Info

next

Severity

annoyance

[j]

Wanted to see if this works and...

https://svelte-5-preview.vercel.app/#H4sIAAAAAAAAE41QzWrEIBB-lUEKm9Alaa9uEih9jKaHXWOorXFEJ4UivnvVQA7ZHoqX-Wa-PwxsVlp6xt8CM9dFMs5erGVnRj82A_8tNcmEPa5O5E3nhVOWhtEAqMWiI3j9UHqC2eECp6YtqNmEp8tounZXJLBx2yFZLjipWcmJcXKrjOe9QeH8r8NIWhIEwNunFAR9GgWuhjg8QUyvhwfr0PqqvuT4kQQaT-DpSjIfy1Dt-lgf-95WIjSARmglvvpQ1dAPELJV0Tabsimp8NjDcwqKcfibwCHcL2PmHliHe9duPZLt_ce9x18BDzdCxgEAAA==

<script>
	let { object = { count: 0 } } = $props();

	const state = $state({ object })
</script>

<button onclick={() => {
	state.object.count += 1;
}}>
	state.object.count: {state.object.count}
	object.count: {object.count}
</button>

[brunnerh]

Due to how render updates are currently batched (#9768) that is not a valid test of reactivity being triggered (in case that was the point here).
If you use an $effect to test for that, only the state signals:

<script>
	let { object = { count: 0 } } = $props();

	const state = $state({ object })

	$effect(() => {
		console.log('in state', state.object.count);
	});
	$effect(() => {
		console.log('in object', object.count);
	});
</script>

<button onclick={() => state.object.count += 1}>
	+1
</button>

[j]

Due to how render updates are currently batched (#9768) that is not a valid test of reactivity being triggered (in case that was the point here). If you use an $effect to test for that, only the state signals:

I get that. I just found it interesting that in the example @Rich-Harris posted, object.count is not reactive but using it within $state then referencing the original object has the proxied version it is.

I guess I thought $state would deep copy a POJO's scalar/supported values and not mutate.