Detect when dynamic components are in components and not data

[Rich-Harris]

This is incorrect, but it's not immediately obvious why (it took me a while to figure out).

The component constructors need to be in data, not components — like this. Is there a way to detect that mistake and warn on it?

[ekhaled]

Do they have to be in data?
I mean, can we not find a way around of leaving them in components.

I suppose this might mean components keeping track of all their children, but that might be better in the long run when HMR is introduced (maybe)

[Rich-Harris]

can we not find a way around of leaving them in components

The short answer is 'no', it's not possible. The longer answer: if you had something like this...

<VirtualList component={{Item}} height=500/>

...then inside the <VirtualList> component you'd have something like this:

{{#each visibleItems as item}}
  <:Component {component} :item/>
{{/each}}

The parent component would have no knowledge that component was a special property and that Item should therefore come from components and not data, and the child component wouldn't have access to Item other than as data.

This is one of those things where something that initially seems like it would reduce confusion would end up creating a lot more. It's better if we just have one concept for this, and try and come up with ways to guide people towards the correct understanding — such as a dev-time warning that lets you know that Item was on Svelte's internal expectedProperties list and is present on components (where it is unused) but not on the initial state object created from data.