Update src/runtime/internal/lifecycle.ts

src/runtime/internal/lifecycle.ts

@@ -33,7 +33,7 @@ export function beforeUpdate(fn: () => any) {
  * 
  * https://svelte.dev/docs#run-time-svelte-onmount
  */
-export function onMount<T>(fn: () => T extends Promise<() => any> ? never : T): void {
+export function onMount<T>(fn: () => T extends Promise<() => any> ? "Returning a function asynchronously from onMount won't call that function on destroy" : T): void {
 	get_current_component().$$.on_mount.push(fn);
 }