$$slots with slot forwarding

Describe the bug If you forward a slot (ex. <slot name="label" slot="label" />) the slot is always considered passed to the parent even if not passed to the child. This breaks any conditional logic based on passed slots within the parent.

To Reproduce Here is a REPL demonstrating the issue: https://svelte.dev/repl/d0cd92776d534349acf8e3e2deb074fa?version=3.35.0 (see console output and presence of red description <div>).

Expected behavior Referencing the REPL, I would expect $$slots.description to not be present as a passed slot in the parent Field component, just like $$slots.description within TextField.

Severity The only workaround is to duplicate code across components and not leverage composition with slot forward, which isn’t terrible nor optimal.

[techniq]

One slight workaround for some use cases is to use :empty to apply display: none on the block (or empty:hidden class for tailwind). REPL

This doesn’t solve a lot of other uses cases, and also falls apart if you have any markup within it (basically more than anything but a simple slot). Even a {#if false}{/if} causes a single space to render which breaks :empty until Selectors Level 4 is implemented by browsers.

[techniq]

Conditional Slots RFC by @tanhauhau would be able to handle this by checking if the slot is declared

<Field>
   {#if $$slots.label}
	<slot name="label" slot="label" />
   {/if}
   {#if $$slots.description}
	<slot name="description" slot="description" />
   {/if}
</Field>