From 536df75300b736d1f6a7e67bc40ae4b7aaa0867c Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Wed, 18 Jun 2025 17:50:37 -0400
Subject: [PATCH] update indirectly-affected bindings on mutation

---
 .../2-analyze/visitors/RegularElement.js      | 29 +++++++++
 .../client/visitors/AssignmentExpression.js   | 23 ++++++-
 .../client/visitors/RegularElement.js         | 60 -------------------
 packages/svelte/src/compiler/phases/scope.js  |  6 ++
 4 files changed, 56 insertions(+), 62 deletions(-)

diff --git a/packages/svelte/src/compiler/phases/2-analyze/visitors/RegularElement.js b/packages/svelte/src/compiler/phases/2-analyze/visitors/RegularElement.js
index d5689e5d5592..2b8077f6abf5 100644
--- a/packages/svelte/src/compiler/phases/2-analyze/visitors/RegularElement.js
+++ b/packages/svelte/src/compiler/phases/2-analyze/visitors/RegularElement.js
@@ -12,6 +12,7 @@ import { regex_starts_with_newline } from '../../patterns.js';
 import { check_element } from './shared/a11y.js';
 import { validate_element } from './shared/element.js';
 import { mark_subtree_dynamic } from './shared/fragment.js';
+import { object } from '../../../utils/ast.js';
 
 /**
  * @param {AST.RegularElement} node
@@ -59,6 +60,34 @@ export function RegularElement(node, context) {
 		}
 	}
 
+	// Special case: `<select bind:value={foo}><option>{bar}</option>`
+	// means we need to invalidate `bar` whenever `foo` is mutated
+	if (node.name === 'select') {
+		for (const attribute of node.attributes) {
+			if (
+				attribute.type === 'BindDirective' &&
+				attribute.name === 'value' &&
+				attribute.expression.type !== 'SequenceExpression'
+			) {
+				const identifier = object(attribute.expression);
+				const binding = identifier && context.state.scope.get(identifier.name);
+
+				if (binding) {
+					for (const name of context.state.scope.references.keys()) {
+						if (name === binding.node.name) continue;
+						const indirect = context.state.scope.get(name);
+
+						if (indirect) {
+							binding.legacy_indirect_bindings.add(indirect);
+						}
+					}
+				}
+
+				break;
+			}
+		}
+	}
+
 	// Special case: single expression tag child of option element -> add "fake" attribute
 	// to ensure that value types are the same (else for example numbers would be strings)
 	if (
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/AssignmentExpression.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/AssignmentExpression.js
index ce190814f8d8..41069272f228 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/AssignmentExpression.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/AssignmentExpression.js
@@ -8,7 +8,7 @@ import {
 	is_event_attribute
 } from '../../../../utils/ast.js';
 import { dev, locate_node } from '../../../../state.js';
-import { should_proxy } from '../utils.js';
+import { build_getter, should_proxy } from '../utils.js';
 import { visit_assignment_expression } from '../../shared/assignments.js';
 import { validate_mutation } from './shared/utils.js';
 import { get_rune } from '../../../scope.js';
@@ -146,7 +146,7 @@ function build_assignment(operator, left, right, context) {
 
 	// mutation
 	if (transform?.mutate) {
-		return transform.mutate(
+		let mutation = transform.mutate(
 			object,
 			b.assignment(
 				operator,
@@ -154,6 +154,25 @@ function build_assignment(operator, left, right, context) {
 				/** @type {Expression} */ (context.visit(right))
 			)
 		);
+
+		if (binding.legacy_indirect_bindings.size > 0) {
+			mutation = b.sequence([
+				mutation,
+				b.call(
+					'$.invalidate_inner_signals',
+					b.arrow(
+						[],
+						b.block(
+							Array.from(binding.legacy_indirect_bindings).map((binding) =>
+								b.stmt(build_getter({ ...binding.node }, context.state))
+							)
+						)
+					)
+				)
+			]);
+		}
+
+		return mutation;
 	}
 
 	// in cases like `(object.items ??= []).push(value)`, we may need to warn
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/RegularElement.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/RegularElement.js
index 1aefff0db08e..2bf1bfce419a 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/RegularElement.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/RegularElement.js
@@ -191,10 +191,6 @@ export function RegularElement(node, context) {
 		}
 	}
 
-	if (node.name === 'select' && bindings.has('value')) {
-		setup_select_synchronization(/** @type {AST.BindDirective} */ (bindings.get('value')), context);
-	}
-
 	// Let bindings first, they can be used on attributes
 	context.state.init.push(...lets);
 
@@ -395,62 +391,6 @@ export function RegularElement(node, context) {
 	context.state.template.pop_element();
 }
 
-/**
- * Special case: if we have a value binding on a select element, we need to set up synchronization
- * between the value binding and inner signals, for indirect updates
- * @param {AST.BindDirective} value_binding
- * @param {ComponentContext} context
- */
-function setup_select_synchronization(value_binding, context) {
-	if (context.state.analysis.runes) return;
-
-	let bound = value_binding.expression;
-
-	if (bound.type === 'SequenceExpression') {
-		return;
-	}
-
-	while (bound.type === 'MemberExpression') {
-		bound = /** @type {Identifier | MemberExpression} */ (bound.object);
-	}
-
-	/** @type {string[]} */
-	const names = [];
-
-	for (const [name, refs] of context.state.scope.references) {
-		if (
-			refs.length > 0 &&
-			// prevent infinite loop
-			name !== bound.name
-		) {
-			names.push(name);
-		}
-	}
-
-	const invalidator = b.call(
-		'$.invalidate_inner_signals',
-		b.thunk(
-			b.block(
-				names.map((name) => {
-					const serialized = build_getter(b.id(name), context.state);
-					return b.stmt(serialized);
-				})
-			)
-		)
-	);
-
-	context.state.init.push(
-		b.stmt(
-			b.call(
-				'$.template_effect',
-				b.thunk(
-					b.block([b.stmt(/** @type {Expression} */ (context.visit(bound))), b.stmt(invalidator)])
-				)
-			)
-		)
-	);
-}
-
 /**
  * @param {AST.ClassDirective[]} class_directives
  * @param {Expression[]} expressions
diff --git a/packages/svelte/src/compiler/phases/scope.js b/packages/svelte/src/compiler/phases/scope.js
index 8a2cc39ba735..deef083f7439 100644
--- a/packages/svelte/src/compiler/phases/scope.js
+++ b/packages/svelte/src/compiler/phases/scope.js
@@ -114,6 +114,12 @@ export class Binding {
 	 */
 	legacy_dependencies = [];
 
+	/**
+	 * Bindings that should be invalidated when this binding is invalidated
+	 * @type {Set<Binding>}
+	 */
+	legacy_indirect_bindings = new Set();
+
 	/**
 	 * Legacy props: the `class` in `{ export klass as class}`. $props(): The `class` in { class: klass } = $props()
 	 * @type {string | null}