Seems like auto-import is broken in js/non-ts-svelte files importing export default from ts/ts-svelte files. And also declare function like onMount in svelte 3.

Is this reproducible without Svelte? i.e. import a export default of a .ts file inside a .js file doesn't work? If so, we should open a bug report in the TypeScript repo.

Yeah. it also happens in js files. Also found that the problem with onMount in svelte 3 isn't because it has a declare modifier. It's because of the barrel export. Barrel export seems like a way bigger problem than the default export. Upstream issue microsoft/TypeScript#57740

Let's wait until VSCode releases with TypeScript 5.4 to see if enough people complain about it and see if the fix can made into a 5.4 patch release. If not, we can probably either

1. Adjust the test and add a dedicated test for auto-import reexport/default export. Skipped it and enabled it when the fix was released.
2. Pin the version in 5.3 but release the fixes with binding control flow and transition type-check.

The bug was put into the backlog, it doesn't look like it has any priority. We may have to dig into the code base ourselves and make a TS pull request. Would you be willing to try that?
In the meantime doing option 2 sounds good (in fact I already pinned it because I had to release a fix)

It looks like the TypeScript version in the svelte-check package still needs to be updated. I'm getting errors when running svelte-check that don't appear in VS Code, where I'm running the latest TypeScript.