fix: make form fields.value() reflect input_value passed to .as()

[DrUse1]

Fixes #15707 :

<form {...test_form}>
	// not displaying 'default text'
	<p>{test_form.fields.name.value()}</p>

	// displays 'default text'
	<input {...test_form.fields.name.as('text', 'default text')} />
</form>

When calling fields.name.as('text', 'default'), the input_value was used as a display fallback in the returned props but was never accessible to fields.name.value(), which returned undefined until the user interacted with the input.

When .as() is called with an input_value, it now writes it to the cache via set_input, so get_value() can immediately return it.

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Preview: https://svelte-dev-git-preview-kit-15690-svelte.vercel.app/

[paoloricciuti]

I think this could lead to out of sync values if the value changes in an event, and then it's read synchronously the line after

[paoloricciuti]

Also this would not work if the code in the repro changes to this

<form {...test_form}>
	// not displaying 'default text'
	<p>{test_form.fields.name.value()}</p>
	// displays 'default text'
	<input {...test_form.fields.name.as('text', 'default text')} />
</form>

[DrUse1]

I tried puting the display before the input and it still works

[DrUse1]

I will put it like this in the test to be sure

[paoloricciuti]

I think this only works by chance and the fact that default is not stateful makes it even more weird. What should happen in this case for example?

<form {...test_form}>
	// not displaying 'default text'
	<p>{test_form.fields.name.value()}</p>
	// displays 'default text'
	{#if show}
		<input {...test_form.fields.name.as('text', 'default text')} />
	{/if}
</form>

when show become true? What about when it become false?

I'll check with the rest of the team but I think we should probably just document this behaviour considering how weird it is.

[DrUse1]

Works for me, I’ll let you take it from here. I don't want to mess this up :)