add a fork_timing error, regenerate

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

@@ -146,6 +146,12 @@ The `flushSync()` function can be used to flush any pending effects synchronousl
 
 This restriction only applies when using the `experimental.async` option, which will be active by default in Svelte 6.
 
+### fork_timing
+
+```
+Cannot create a fork inside an effect or when state changes are pending
+```
+
 ### get_abort_signal_outside_reaction
 
 ```

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

@@ -112,6 +112,10 @@ The `flushSync()` function can be used to flush any pending effects synchronousl
 
 This restriction only applies when using the `experimental.async` option, which will be active by default in Svelte 6.
 
+## fork_timing
+
+> Cannot create a fork inside an effect or when state changes are pending
+
 ## get_abort_signal_outside_reaction
 
 > `getAbortSignal()` can only be called inside an effect or derived

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

@@ -456,3 +456,19 @@ export function svelte_boundary_reset_onerror() {
 		throw new Error(`https://svelte.dev/e/svelte_boundary_reset_onerror`);
 	}
 }
+
+/**
+ * Cannot create a fork inside an effect or when state changes are pending
+ * @returns {never}
+ */
+export function fork_timing() {
+	if (DEV) {
+		const error = new Error(`fork_timing\nCannot create a fork inside an effect or when state changes are pending\nhttps://svelte.dev/e/fork_timing`);
+
+		error.name = 'Svelte error';
+
+		throw error;
+	} else {
+		throw new Error(`https://svelte.dev/e/fork_timing`);
+	}
+}

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

@@ -895,7 +895,7 @@ export function fork(fn) {
 	}
 
 	if (current_batch !== null) {
-		throw new Error('cannot fork here'); // TODO better error
+		e.fork_timing();
 	}
 
 	const batch = Batch.ensure();

packages/svelte/types/index.d.ts

@@ -448,7 +448,19 @@ declare module 'svelte' {
 	 * Returns void if no callback is provided, otherwise returns the result of calling the callback.
 	 * */
 	export function flushSync<T = void>(fn?: (() => T) | undefined): T;
-
+	/**
+	 * Creates a 'fork', in which state changes are evaluated but not applied to the DOM.
+	 * This is useful for speculatively loading data (for example) when you suspect that
+	 * the user is about to take some action.
+	 *
+	 * Frameworks like SvelteKit can use this to preload data when the user touches or
+	 * hovers over a link, making any subsequent navigation feel instantaneous.
+	 *
+	 * The `fn` parameter is a synchronous function that modifies some state. The
+	 * state changes will be reverted after the fork is initialised, then reapplied
+	 * if and when the fork is eventually committed.
+	 *
+	 * */
 	export function fork(fn: () => void): {
 		commit: () => void;
 		discard: () => void;