fix: <Route> received an unexpected slot "default"

[bfanger]

Previously:

<Route>
  {#if subroute}
    <Subroute />
  {/if}
</Route>

if the subroute is falsy an empty default slot is passed which causes the warning. #981

This change renders:

{#if subroute}
  <Route>
    <Subroute />
  </Route>
{:else}
  <Route />
{/if}

Caveat:.
Svelte as has no VirtualDOM, so when a route toggles between having a subroute and not having subroute the component will be remounted and internal state is lost. This is could be a non-issue, as I was not able to produce that scenario.

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Latest commit: a75eda8

The changes in this PR will be included in the next version bump.

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[bluwy]

I used to do this trick in my other projects, but a caveat I found is that this would also rerender the component if only the child components change, which is sometimes undesirable, especially for route transitions. And so, I think we should fix the warning in Svelte side, and leave the code as is for now to prevent a potential breaking change.

[bfanger]

When does that happen? Are there situations where the upper level component stays the same but the underlying level changes to falsy?

[Rich-Harris]

I'm also curious about this — if we can fix it here rather than in Svelte, that would be preferable I think, since the warning is useful.

The components 'branch' is an array of nodes, where every node except the leaf is a layout. Layouts always have a slotted child, leaves never do. Which means that I don't think there's ever a scenario in which a component could move between the consequent and the alternate — layouts will always be in the consequent, leaves will always be in the alternate.

In other words this is a brilliantly simple (and in hindsight, obvious!) fix for this issue as far as I can see — thank you @bfanger! But I'd love to better understand @bluwy's concern before merging.

[bluwy]

Ah thanks for the explanation. I wasn't sure what the components array would contain, but if the quirk I mention never happens. This looks good to me!