feat: better type checking for bindings in Svelte 5 #2477
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dummdidumm
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This reverts commit 8c080cf.
This was referenced Sep 19, 2024
dummdidumm
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Sep 19, 2024
This reverts commit 8c080cf. This reverts #2477. Sadly, the idea didn't work out, as shown by two opened bug reports: - #2506: A type union can be narrowed on the input, but not on the way back out - #2494: A generic nested within a bound value is not properly resolved and not falling back to `any` (in #2477 we thought of the generic case, but only for when the generic is the whole value type, not when it's nested) For these reasons I don't see a way to properly implement #1392 at the moment.
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#1392
This adds enhanced type checking for bindings in Svelte 5: We're not only passing the variable in, we're also assigning the value of the component property back to the variable. That way, we can catch type errors like the child binding having a wider type as the input we give it.
It's done for Svelte 5 only because it's a) easier there b) doesn't break as much code (people who upgrade can then also upgrade the types, or use type assertions in the template, which is only possible in Svelte 5).
There's one limitation: Because of how the transformation works, we cannot infer generics. In other words, we will not catch type errors for bindings that rely on a generic type. The combination of generics + bindings is probably rare enough that this is okay, and we can revisit this later to try to find a way to make it work, if it comes up.
Also note that this does not affect DOM element bindings like
<input bind:value={...} />, this is only about component bindings.