fix: catch error on @const tag in svelte:boundary in DEV mode

.changeset/sixty-cats-search.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: catch error on @const tag in svelte:boundary in DEV mode

packages/svelte/src/compiler/phases/3-transform/client/visitors/SvelteBoundary.js

@@ -1,6 +1,7 @@
 /** @import { BlockStatement, Statement, Expression } from 'estree' */
 /** @import { AST } from '#compiler' */
 /** @import { ComponentContext } from '../types' */
+import { dev } from '../../../../state.js';
 import * as b from '../../../../utils/builders.js';
 
 /**
@@ -35,6 +36,9 @@ export function SvelteBoundary(node, context) {
 	/** @type {Statement[]} */
 	const external_statements = [];
 
+	/** @type {Statement[]} */
+	const internal_statements = [];
+
 	const snippets_visits = [];
 
 	// Capture the `failed` implicit snippet prop
@@ -53,7 +57,20 @@ export function SvelteBoundary(node, context) {
 			/** @type {Statement[]} */
 			const init = [];
 			context.visit(child, { ...context.state, init });
-			external_statements.push(...init);
+
+			if (dev) {
+				// In dev we must separate the declarations from the code
+				// that eagerly evaluate the expression...
+				for (const statement of init) {
+					if (statement.type === 'VariableDeclaration') {
+						external_statements.push(statement);
+					} else {
+						internal_statements.push(statement);
+					}
+				}
+			} else {
+				external_statements.push(...init);
+			}
 		} else {
 			nodes.push(child);
 		}
@@ -63,6 +80,10 @@ export function SvelteBoundary(node, context) {
 
 	const block = /** @type {BlockStatement} */ (context.visit({ ...node.fragment, nodes }));
 
+	if (dev && internal_statements.length) {
+		block.body.unshift(...internal_statements);
+	}
+
 	const boundary = b.stmt(
 		b.call('$.boundary', context.state.node, props, b.arrow([b.id('$$anchor')], block))
 	);

packages/svelte/tests/runtime-runes/samples/svelte-boundary-dev-const/_config.js

@@ -0,0 +1,33 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+// https://github.com/sveltejs/svelte/issues/15368
+export default test({
+	compileOptions: {
+		dev: true
+	},
+
+	mode: ['client'],
+
+	html: `
+		<p>BOOM</p>
+		<p>BOOM</p>
+		<div>OK</div>
+		<div>OK</div>
+	`,
+
+	async test({ target, assert, component }) {
+		component.ok = false;
+		flushSync();
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+		<p>BOOM</p>
+		<p>BOOM</p>
+		<p>BOOM</p>
+		<p>BOOM</p>
+		`
+		);
+	}
+});

packages/svelte/tests/runtime-runes/samples/svelte-boundary-dev-const/main.svelte

@@ -0,0 +1,49 @@
+<script>
+	let { ok = true } = $props();
+
+	function throwError() {
+	  throw new Error();
+	}
+
+	function throwErrorOnUpdate() {
+		if (ok) {
+			return "OK";
+		} else {
+			throwError();
+		}
+	}
+</script>
+
+<svelte:boundary>
+	<div>{throwError()}</div>
+
+	{#snippet failed()}
+		<p>BOOM</p>
+	{/snippet}
+</svelte:boundary>
+
+<svelte:boundary>
+	{@const result = throwError()}
+	<div>{result}</div>
+
+	{#snippet failed()}
+		<p>BOOM</p>
+	{/snippet}
+</svelte:boundary>
+
+<svelte:boundary>
+	<div>{throwErrorOnUpdate()}</div>
+
+	{#snippet failed()}
+		<p>BOOM</p>
+	{/snippet}
+</svelte:boundary>
+
+<svelte:boundary>
+	{@const result = throwErrorOnUpdate()}
+	<div>{result}</div>
+
+	{#snippet failed()}
+		<p>BOOM</p>
+	{/snippet}
+</svelte:boundary>