export const breaks in svelte 5

[KTibow]

Describe the bug

export const no longer allows you to export a fixed value once you migrate to svelte 5

Reproduction

svelte 4 (observe that it uses svelte 4 syntax, and a 2 shows up, exported from Thing): https://svelte.dev/playground/1c7f6e4cd62c4788866295f2ee325706?version=5.20.2
svelte 5 (observe that it uses svelte 5 syntax, and since v is undefined, nothing shows up): https://svelte.dev/playground/000e19b8335847938feffe83a200675a?version=5.20.2

Logs

System Info

n/a (only repl used)

Severity

blocking an upgrade

[paoloricciuti]

This is expected and a documented breaking change

https://svelte.dev/docs/svelte/v5-migration-guide#Breaking-changes-in-runes-mode-Bindings-to-component-exports-are-not-allowed

Thanks for reporting tho 😊

[KTibow]

@paoloricciuti why doesn't it error/warn then

[Thiagolino8]

Ensuring the shape of an object (like the props object) is a job for TypeScript

[KTibow]

very well, I was in the REPL and wasn't using TS and wasted a lot of time as the result

[WillsterJohnson]

Use of export let provides an error, as it isn't valid in runes mode.
Shouldn't export const (or any use of export) do the same? Afaik export is banned in runes mode.

Can this be reopened to track this missing error, or is it a duplicate of another issue?

[KTibow]

@WillsterJohnson no you can still do it with bind:this={thing} then thing.property (which imo is really weird but technically works)

[paoloricciuti]

@WillsterJohnson no you can still do it with bind:this={thing} then thing.property (which imo is really weird but technically works)

Correct

[Thiagolino8]

export let is prohibited because you are most likely wanting to create a prop with the svelte 3/4 syntax in mind than wanting to create an accessor
Eventually when the legacy syntax is removed the use of export let should be allowed again.
#10310 (comment)