feat: allow deriveds to reference their own values

.changeset/honest-dingos-bake.md

@@ -0,0 +1,5 @@
+---
+'svelte': minor
+---
+
+feat: allow deriveds to reference their own values

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

@@ -283,13 +283,6 @@ export function destroy_derived_effects(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 {Effect | null}
@@ -313,40 +306,17 @@ function get_derived_parent_effect(derived) {
  * @returns {T}
  */
 export function execute_derived(derived) {
-	var value;
 	var prev_active_effect = active_effect;
 
 	set_active_effect(get_derived_parent_effect(derived));
 
-	if (DEV) {
-		let prev_eager_effects = eager_effects;
-		set_eager_effects(new Set());
-		try {
-			if (includes.call(stack, derived)) {
-				e.derived_references_self();
-			}
-
-			stack.push(derived);
-
-			derived.f &= ~WAS_MARKED;
-			destroy_derived_effects(derived);
-			value = update_reaction(derived);
-		} finally {
-			set_active_effect(prev_active_effect);
-			set_eager_effects(prev_eager_effects);
-			stack.pop();
-		}
-	} else {
-		try {
-			derived.f &= ~WAS_MARKED;
-			destroy_derived_effects(derived);
-			value = update_reaction(derived);
-		} finally {
-			set_active_effect(prev_active_effect);
-		}
+	try {
+		derived.f &= ~WAS_MARKED;
+		destroy_derived_effects(derived);
+		return update_reaction(derived);
+	} finally {
+		set_active_effect(prev_active_effect);
 	}
-
-	return value;
 }
 
 /**

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

@@ -618,6 +618,10 @@ export function get(signal) {
 	if (is_derived) {
 		var derived = /** @type {Derived} */ (signal);
 
+		if ((derived.f & REACTION_IS_UPDATING) !== 0) {
+			return derived.v === UNINITIALIZED ? undefined : derived.v;
+		}
+
 		if (is_destroying_effect) {
 			var value = derived.v;
 

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

@@ -1,21 +0,0 @@
-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

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