error if missing experimental flag

documentation/docs/98-reference/.generated/client-errors.md

@@ -130,6 +130,12 @@ $effect(() => {
 
 Often when encountering this issue, the value in question shouldn't be state (for example, if you are pushing to a `logs` array in an effect, make `logs` a normal array rather than `$state([])`). In the rare cases where you really _do_ need to write to state in an effect — [which you should avoid]($effect#When-not-to-use-$effect) — you can read the state with [untrack](svelte#untrack) to avoid adding it as a dependency.
 
+### experimental_async_fork
+
+```
+Cannot use `fork(...)` unless the `experimental.async` compiler option is `true`
+```
+
 ### flush_sync_in_effect
 
 ```

packages/svelte/messages/client-errors/errors.md

@@ -100,6 +100,10 @@ $effect(() => {
 
 Often when encountering this issue, the value in question shouldn't be state (for example, if you are pushing to a `logs` array in an effect, make `logs` a normal array rather than `$state([])`). In the rare cases where you really _do_ need to write to state in an effect — [which you should avoid]($effect#When-not-to-use-$effect) — you can read the state with [untrack](svelte#untrack) to avoid adding it as a dependency.
 
+## experimental_async_fork
+
+> Cannot use `fork(...)` unless the `experimental.async` compiler option is `true`
+
 ## flush_sync_in_effect
 
 > Cannot use `flushSync` inside an effect

packages/svelte/src/internal/client/errors.js

@@ -439,4 +439,20 @@ export function svelte_boundary_reset_onerror() {
 	} else {
 		throw new Error(`https://svelte.dev/e/svelte_boundary_reset_onerror`);
 	}
+}
+
+/**
+ * Cannot use `fork(...)` unless the `experimental.async` compiler option is `true`
+ * @returns {never}
+ */
+export function experimental_async_fork() {
+	if (DEV) {
+		const error = new Error(`experimental_async_fork\nCannot use \`fork(...)\` unless the \`experimental.async\` compiler option is \`true\`\nhttps://svelte.dev/e/experimental_async_fork`);
+
+		error.name = 'Svelte error';
+
+		throw error;
+	} else {
+		throw new Error(`https://svelte.dev/e/experimental_async_fork`);
+	}
 }

packages/svelte/src/internal/client/reactivity/batch.js

@@ -877,6 +877,10 @@ export function eager(fn) {
  * @returns {{ commit: () => void, discard: () => void }}
  */
 export function fork(fn) {
+	if (!async_mode_flag) {
+		e.experimental_async_fork();
+	}
+
 	if (current_batch !== null) {
 		throw new Error('cannot fork here'); // TODO better error
 	}