`svelte:boundary` fails with async `onMount`

[Serator]

Describe the bug

// App.svelte

<script>
  import Test from './Test.svelte'
</script>

<svelte:boundary>
  <Test />

  {#snippet failed()}
    Error occurred!
  {/snippet}
</svelte:boundary>

// Test.svelte

<script>
  import {onMount} from 'svelte'

  onMount(async () => {// <<< The `async` keyword here prevents the message in `snippet failed()` above.
    throw Error()
  })
</script>

The message Error occurred! does not appear if onMount is async. If you remove the async keyword from the code above, everything works as expected.

Reproduction

v5.19.5

Logs

System Info

-

Severity

annoyance

[dummdidumm]

That's expected, because this error is happening outside the rendering process (we should make this a bit clearer in the docs)

[Serator]

@dummdidumm Thanks for the explanation. It would be valuable to also have an example of a recommended workaround for this limitation.

[dummdidumm]

I don't know what that would look like - your reproduction is minimal (which is good, for a reproduction!) but lacks a real use case, which makes it hard to formulate something.
In general: Make sure that you try-catch or .catch anything that you know likely throws yourself that is outside of Svelte's rendering.

[Serator]

My case is to create a wrapper component with svelte:boundary that will replace its content with an error message in case there is an error in the content. In onMount there will be a query to the database (it can actually be any asynchronous code) which may fail. Actually, I want to replace try-catch inside onMount (this is not in reproduction) with a solution with svelte:boundary, but the described constraint prevents this.

[dummdidumm]

That sounds like something you cannot solve with an error boundary today - better to do this through a state variable with pending/failed etc., or use {#await}

[Serator]

Got it, thanks!

[Serator]

https://svelte.dev/playground/d731ac65d54a4c3bade64b4f6cf7a157?version=5.19.5 - an ugly workaround using a third-party $effect. An inbuilt solution would come in handy!