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

@@ -291,13 +291,6 @@ 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}
@@ -321,40 +314,17 @@ 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

@@ -170,7 +170,10 @@ export function is_dirty(reaction) {
 		for (var i = 0; i < length; i++) {
 			var dependency = dependencies[i];
 
-			if (is_dirty(/** @type {Derived} */ (dependency))) {
+			if (
+				(dependency.f & REACTION_IS_UPDATING) === 0 &&
+				is_dirty(/** @type {Derived} */ (dependency))
+			) {
 				update_derived(/** @type {Derived} */ (dependency));
 			}
 
@@ -624,6 +627,14 @@ export function get(signal) {
 	if (is_derived) {
 		var derived = /** @type {Derived} */ (signal);
 
+		if ((derived.f & REACTION_IS_UPDATING) !== 0) {
+			return derived.v === UNINITIALIZED
+				? /** @type {V} */ (undefined)
+				: batch_values?.has(derived)
+					? batch_values.get(derived)
+					: derived.v;
+		}
+
 		if (is_destroying_effect) {
 			var value = derived.v;