onMount type allows returning function when async

[sidharthv96]

Describe the bug

The documentation for providing a return function from onMount states

This behaviour will only work when the function passed to onMount synchronously returns a value. async functions always return a Promise, and as such cannot synchronously return a function.

But, the generated type in lifecycle.d.ts is

export declare function onMount(fn: () => any): void;

Which accepts return functions, even when async. This causes a silent failure, which is impossible to detect without reading the documentation. The error can happen when someone converts an existing onMount with a destroy callback to async.

Strictly typing it like below makes TS throw an error when a function is returned wrapped in a promise.

export declare function onMount(fn: () => void | Promise<void> | (() => void)): void;

Error after changing type.

Reproduction

<script lang="ts">
  import { onMount } from 'svelte';

  onMount(async () => {
  // ^ No error

    // .... await someThing

    return () => {
      // Destroyed will not be call as the function is async.
    };
  });
</script>

Logs

No response

System Info

"svelte": "^3.59.1"

Severity

annoyance

[Conduitry]

This is already fixed in 4.0.0 via #8136. It was not included in v3 as it would be a breaking change.

[sidharthv96]

Just saw that when checking the git blame of internals.
Thank you.