fix: allow binding to const with spread in legacy mode (#13849)

.changeset/nine-pigs-approve.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: allow binding to const with spread in legacy mode

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

@@ -177,6 +177,19 @@ const spread_props_handler = {
 			if (typeof p === 'object' && p !== null && key in p) return p[key];
 		}
 	},
+	set(target, key, value) {
+		let i = target.props.length;
+		while (i--) {
+			let p = target.props[i];
+			if (is_function(p)) p = p();
+			const desc = get_descriptor(p, key);
+			if (desc && desc.set) {
+				desc.set(value);
+				return true;
+			}
+		}
+		return false;
+	},
 	getOwnPropertyDescriptor(target, key) {
 		let i = target.props.length;
 		while (i--) {

packages/svelte/src/internal/server/index.js

@@ -270,7 +270,12 @@ export function spread_props(props) {
 	for (let i = 0; i < props.length; i++) {
 		const obj = props[i];
 		for (key in obj) {
-			merged_props[key] = obj[key];
+			const desc = Object.getOwnPropertyDescriptor(obj, key);
+			if (desc) {
+				Object.defineProperty(merged_props, key, desc);
+			} else {
+				merged_props[key] = obj[key];
+			}
 		}
 	}
 	return merged_props;

packages/svelte/tests/runtime-legacy/samples/bind-export-const-with-spread/Test.svelte

@@ -0,0 +1,3 @@
+<script>
+	export const x = 42;
+</script>

packages/svelte/tests/runtime-legacy/samples/bind-export-const-with-spread/_config.js

@@ -0,0 +1,9 @@
+import { test } from '../../test';
+
+export default test({
+	html: `<p>42</p>`,
+	async test({ target, assert }) {
+		const p = target.querySelector('p');
+		assert.equal(p?.innerHTML, '42');
+	}
+});

packages/svelte/tests/runtime-legacy/samples/bind-export-const-with-spread/main.svelte

@@ -0,0 +1,11 @@
+<script>
+	import Test from "./Test.svelte";
+
+	let x;
+</script>
+
+<Test
+	bind:x
+	{...{}}
+/>
+<p>{x}</p>