chore: add error for derived self referencing (#12746)

* chore: add warning for derived self referencin

* update build

* address feedback

* address feedback

* build

* messages shouldn't end with a period

* simplify test

* regenerate

* newlines are free

* no need to export this, we can move it closer to where it's used

* fix double negative

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/new-cooks-roll.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+chore: add error for derived self referencing

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

@@ -18,6 +18,10 @@
 
 > Attempted to instantiate %component% with `new %name%`, which is no longer valid in Svelte 5. If this component is not under your control, set the `compatibility.componentApi` compiler option to `4` to keep it working. See https://svelte-5-preview.vercel.app/docs/breaking-changes#components-are-no-longer-classes for more information
 
+## derived_references_self
+
+> A derived value cannot reference itself recursively
+
 ## each_key_duplicate
 
 > Keyed each block has duplicate key at indexes %a% and %b%

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

@@ -93,6 +93,22 @@ export function component_api_invalid_new(component, name) {
 	}
 }
 
+/**
+ * A derived value cannot reference itself recursively
+ * @returns {never}
+ */
+export function derived_references_self() {
+	if (DEV) {
+		const error = new Error(`derived_references_self\nA derived value cannot reference itself recursively`);
+
+		error.name = 'Svelte error';
+		throw error;
+	} else {
+		// TODO print a link to the documentation
+		throw new Error("derived_references_self");
+	}
+}
+
 /**
  * Keyed each block has duplicate key `%value%` at indexes %a% and %b%
  * @param {string} a

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

@@ -1,4 +1,5 @@
 /** @import { Derived } from '#client' */
+import { DEV } from 'esm-env';
 import { CLEAN, DERIVED, DESTROYED, DIRTY, MAYBE_DIRTY, UNOWNED } from '../constants.js';
 import {
 	current_reaction,
@@ -11,6 +12,7 @@ import {
 	increment_version
 } from '../runtime.js';
 import { equals, safe_equals } from './equality.js';
+import * as e from '../errors.js';
 
 export let updating_derived = false;
 
@@ -79,17 +81,36 @@ function destroy_derived_children(derived) {
 	}
 }
 
+/**
+ * The currently updating deriveds, used to detect infinite recursion
+ * in dev mode and provide a nicer error than 'too much recursion'
+ * @type {Derived[]}
+ */
+let stack = [];
+
 /**
  * @param {Derived} derived
  * @returns {void}
  */
 export function update_derived(derived) {
+	if (DEV) {
+		if (stack.includes(derived)) {
+			e.derived_references_self();
+		}
+
+		stack.push(derived);
+	}
+
 	var previous_updating_derived = updating_derived;
 	updating_derived = true;
 	destroy_derived_children(derived);
 	var value = update_reaction(derived);
 	updating_derived = previous_updating_derived;
 
+	if (DEV) {
+		stack.pop();
+	}
+
 	var status =
 		(current_skip_reaction || (derived.f & UNOWNED) !== 0) && derived.deps !== null
 			? MAYBE_DIRTY

packages/svelte/tests/runtime-runes/samples/derived-fn-recursive/_config.js

@@ -0,0 +1,21 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	html: `<button>increment</button>\n0`,
+
+	mode: ['client'],
+
+	test({ assert, target }) {
+		const btn = target.querySelector('button');
+
+		btn?.click();
+
+		assert.throws(
+			flushSync,
+			'derived_references_self\nA derived value cannot reference itself recursively'
+		);
+
+		assert.htmlEqual(target.innerHTML, `<button>increment</button>\n0`);
+	}
+});

packages/svelte/tests/runtime-runes/samples/derived-fn-recursive/main.svelte

@@ -0,0 +1,11 @@
+<script>
+	let count = $state(0);
+	let even = $derived.by(() => {
+		return count > 0 ? even : 0;
+	})
+
+</script>
+
+<button onclick={() => count++}>increment</button>
+
+{even}