From 5dc98303db89bcb1e2d1d925a2de54aee18459c8 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 5 Dec 2023 14:52:02 -0500
Subject: [PATCH 1/8] WIP

---
 packages/svelte/src/internal/client/runtime.js |  5 +++++
 packages/svelte/tests/runtime-legacy/shared.ts |  5 +++++
 .../proxy-prop-default-readonly/Counter.svelte |  8 ++++++++
 .../proxy-prop-default-readonly/_config.js     | 18 ++++++++++++++++++
 .../proxy-prop-default-readonly/main.svelte    |  5 +++++
 5 files changed, 41 insertions(+)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly/Counter.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly/main.svelte

diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 462372f83f97..4c3f82c66745 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -3,6 +3,7 @@ import { subscribe_to_store } from '../../store/utils.js';
 import { EMPTY_FUNC, run_all } from '../common.js';
 import { get_descriptor, get_descriptors, is_array } from './utils.js';
 import { PROPS_CALL_DEFAULT_VALUE, PROPS_IS_IMMUTABLE, PROPS_IS_RUNES } from '../../constants.js';
+import { readonly } from './proxy/readonly.js';
 
 export const SOURCE = 1;
 export const DERIVED = 1 << 1;
@@ -1437,6 +1438,10 @@ export function prop_source(props_obj, key, flags, default_value) {
 		value =
 			// @ts-expect-error would need a cumbersome method overload to type this
 			call_default_value ? default_value() : default_value;
+
+		if (DEV && immutable) {
+			value = readonly(/** @type {any} */ (value));
+		}
 	}
 
 	const source_signal = immutable ? source(value) : mutable_source(value);
diff --git a/packages/svelte/tests/runtime-legacy/shared.ts b/packages/svelte/tests/runtime-legacy/shared.ts
index a1944d47e5f3..63c4ca10c9bd 100644
--- a/packages/svelte/tests/runtime-legacy/shared.ts
+++ b/packages/svelte/tests/runtime-legacy/shared.ts
@@ -305,6 +305,11 @@ async function run_test_variant(
 						compileOptions
 					});
 				}
+
+				if (config.runtime_error) {
+					unintended_error = true;
+					assert.fail('Expected a runtime error');
+				}
 			} finally {
 				instance.$destroy();
 				assert_html_equal(
diff --git a/packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly/Counter.svelte b/packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly/Counter.svelte
new file mode 100644
index 000000000000..91ccc2af8188
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly/Counter.svelte
@@ -0,0 +1,8 @@
+<script>
+	/** @type {{ object?: { count: number }}} */
+	let { object = { count: 0 } } = $props();
+</script>
+
+<button onclick={() => object.count += 1}>
+	clicks: {object.count}
+</button>
diff --git a/packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly/_config.js b/packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly/_config.js
new file mode 100644
index 000000000000..5e0eb0e2da60
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly/_config.js
@@ -0,0 +1,18 @@
+import { test } from '../../test';
+
+export default test({
+	html: `<button>clicks: 0</button>`,
+
+	compileOptions: {
+		dev: true
+	},
+
+	async test({ assert, target }) {
+		const btn = target.querySelector('button');
+		await btn?.click();
+
+		assert.htmlEqual(target.innerHTML, `<button>clicks: 0</button>`);
+	},
+
+	runtime_error: 'Props are read-only, unless used with `bind:`'
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly/main.svelte b/packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly/main.svelte
new file mode 100644
index 000000000000..391afc4b232e
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly/main.svelte
@@ -0,0 +1,5 @@
+<script>
+	import Counter from './Counter.svelte';
+</script>
+
+<Counter />

From c263119408521a68170c56f098057f354758474f Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 5 Dec 2023 15:16:52 -0500
Subject: [PATCH 2/8] update tests

---
 .../class-state-derived-unowned/_config.js        | 15 ++++++++++-----
 .../samples/class-state-derived-unowned/log.js    |  1 +
 .../class-state-derived-unowned/main.svelte       | 14 +++++++-------
 .../samples/class-state-init-eager-2/_config.js   |  9 +++++++--
 .../samples/class-state-init-eager-2/log.js       |  1 +
 .../samples/class-state-init-eager-2/main.svelte  |  2 +-
 .../samples/class-state-init-eager-3/_config.js   |  9 +++++++--
 .../samples/class-state-init-eager-3/log.js       |  1 +
 .../samples/class-state-init-eager-3/main.svelte  |  2 +-
 .../samples/class-state-init-eager/_config.js     |  9 +++++++--
 .../samples/class-state-init-eager/log.js         |  1 +
 .../samples/class-state-init-eager/main.svelte    |  2 +-
 .../samples/event-attribute-template/_config.js   |  9 +++++++--
 .../samples/event-attribute-template/log.js       |  1 +
 .../samples/event-attribute-template/main.svelte  |  2 +-
 15 files changed, 54 insertions(+), 24 deletions(-)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/class-state-derived-unowned/log.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/class-state-init-eager-2/log.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/class-state-init-eager-3/log.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/class-state-init-eager/log.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/event-attribute-template/log.js

diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-derived-unowned/_config.js b/packages/svelte/tests/runtime-runes/samples/class-state-derived-unowned/_config.js
index d73f3351a30c..e0f3a58f3ff2 100644
--- a/packages/svelte/tests/runtime-runes/samples/class-state-derived-unowned/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-derived-unowned/_config.js
@@ -1,36 +1,41 @@
 import { flushSync } from 'svelte';
 import { test } from '../../test';
+import { log } from './log.js';
 
 export default test({
 	// The component context class instance gets shared between tests, strangely, causing hydration to fail?
 	skip_if_hydrate: 'permanent',
 
-	async test({ assert, target, component }) {
+	before_test() {
+		log.length = 0;
+	},
+
+	async test({ assert, target }) {
 		const btn = target.querySelector('button');
 
 		flushSync(() => {
 			btn?.click();
 		});
 
-		assert.deepEqual(component.log, [0, 'class trigger false', 'local trigger false', 1]);
+		assert.deepEqual(log, [0, 'class trigger false', 'local trigger false', 1]);
 
 		flushSync(() => {
 			btn?.click();
 		});
 
-		assert.deepEqual(component.log, [0, 'class trigger false', 'local trigger false', 1, 2]);
+		assert.deepEqual(log, [0, 'class trigger false', 'local trigger false', 1, 2]);
 
 		flushSync(() => {
 			btn?.click();
 		});
 
-		assert.deepEqual(component.log, [0, 'class trigger false', 'local trigger false', 1, 2, 3]);
+		assert.deepEqual(log, [0, 'class trigger false', 'local trigger false', 1, 2, 3]);
 
 		flushSync(() => {
 			btn?.click();
 		});
 
-		assert.deepEqual(component.log, [
+		assert.deepEqual(log, [
 			0,
 			'class trigger false',
 			'local trigger false',
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-derived-unowned/log.js b/packages/svelte/tests/runtime-runes/samples/class-state-derived-unowned/log.js
new file mode 100644
index 000000000000..0abf1fa9159e
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-derived-unowned/log.js
@@ -0,0 +1 @@
+export const log = [];
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-derived-unowned/main.svelte b/packages/svelte/tests/runtime-runes/samples/class-state-derived-unowned/main.svelte
index 58b96457dbb6..6f1c26db048e 100644
--- a/packages/svelte/tests/runtime-runes/samples/class-state-derived-unowned/main.svelte
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-derived-unowned/main.svelte
@@ -1,17 +1,17 @@
 <script context="module">
-		class SomeLogic {
-			someValue = $state(0);
-			isAboveThree = $derived(this.someValue > 3);
-			trigger() {
-				this.someValue++;
-			}
+	class SomeLogic {
+		someValue = $state(0);
+		isAboveThree = $derived(this.someValue > 3);
+		trigger() {
+			this.someValue++;
 		}
+	}
 
 	const someLogic = new SomeLogic();
 </script>
 
 <script>
-	const {log = []} = $props();
+	import { log } from './log.js';
 
 	function increment() {
 		someLogic.trigger();
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-2/_config.js b/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-2/_config.js
index 81f5456ad344..e11107a95cb8 100644
--- a/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-2/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-2/_config.js
@@ -1,9 +1,14 @@
 import { test } from '../../test';
+import { log } from './log.js';
 
 export default test({
 	html: `<button>0</button>`,
 
-	async test({ assert, target, component }) {
+	before_test() {
+		log.length = 0;
+	},
+
+	async test({ assert, target }) {
 		const btn = target.querySelector('button');
 
 		await btn?.click();
@@ -12,6 +17,6 @@ export default test({
 		await btn?.click();
 		assert.htmlEqual(target.innerHTML, `<button>2</button>`);
 
-		assert.deepEqual(component.log, [undefined]);
+		assert.deepEqual(log, [undefined]);
 	}
 });
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-2/log.js b/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-2/log.js
new file mode 100644
index 000000000000..0abf1fa9159e
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-2/log.js
@@ -0,0 +1 @@
+export const log = [];
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-2/main.svelte b/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-2/main.svelte
index a8420548ccea..5a0d7b0de245 100644
--- a/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-2/main.svelte
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-2/main.svelte
@@ -1,5 +1,5 @@
 <script>
-	const {log = []} = $props();
+	import { log } from './log.js';
 
 	const logger = (obj) => {
 		log.push(obj.count)
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-3/_config.js b/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-3/_config.js
index 81f5456ad344..e11107a95cb8 100644
--- a/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-3/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-3/_config.js
@@ -1,9 +1,14 @@
 import { test } from '../../test';
+import { log } from './log.js';
 
 export default test({
 	html: `<button>0</button>`,
 
-	async test({ assert, target, component }) {
+	before_test() {
+		log.length = 0;
+	},
+
+	async test({ assert, target }) {
 		const btn = target.querySelector('button');
 
 		await btn?.click();
@@ -12,6 +17,6 @@ export default test({
 		await btn?.click();
 		assert.htmlEqual(target.innerHTML, `<button>2</button>`);
 
-		assert.deepEqual(component.log, [undefined]);
+		assert.deepEqual(log, [undefined]);
 	}
 });
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-3/log.js b/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-3/log.js
new file mode 100644
index 000000000000..0abf1fa9159e
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-3/log.js
@@ -0,0 +1 @@
+export const log = [];
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-3/main.svelte b/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-3/main.svelte
index 289a3beebd62..b4e63089b4be 100644
--- a/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-3/main.svelte
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-3/main.svelte
@@ -1,5 +1,5 @@
 <script>
-	const {log = []} = $props();
+	import { log } from './log.js';
 
 	class Counter {
 		count = $state();
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-init-eager/_config.js b/packages/svelte/tests/runtime-runes/samples/class-state-init-eager/_config.js
index 597626f9a7d4..e6f52509c452 100644
--- a/packages/svelte/tests/runtime-runes/samples/class-state-init-eager/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-init-eager/_config.js
@@ -1,9 +1,14 @@
 import { test } from '../../test';
+import { log } from './log.js';
 
 export default test({
 	html: `<button>0</button>`,
 
-	async test({ assert, target, component }) {
+	before_test() {
+		log.length = 0;
+	},
+
+	async test({ assert, target }) {
 		const btn = target.querySelector('button');
 
 		await btn?.click();
@@ -12,6 +17,6 @@ export default test({
 		await btn?.click();
 		assert.htmlEqual(target.innerHTML, `<button>2</button>`);
 
-		assert.deepEqual(component.log, [100]);
+		assert.deepEqual(log, [100]);
 	}
 });
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-init-eager/log.js b/packages/svelte/tests/runtime-runes/samples/class-state-init-eager/log.js
new file mode 100644
index 000000000000..0abf1fa9159e
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-init-eager/log.js
@@ -0,0 +1 @@
+export const log = [];
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-init-eager/main.svelte b/packages/svelte/tests/runtime-runes/samples/class-state-init-eager/main.svelte
index 1411755df916..1fff8c1fece5 100644
--- a/packages/svelte/tests/runtime-runes/samples/class-state-init-eager/main.svelte
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-init-eager/main.svelte
@@ -1,5 +1,5 @@
 <script>
-	const {log = []} = $props();
+	import { log } from './log.js';
 
 	class Counter {
 		count = $state(100);
diff --git a/packages/svelte/tests/runtime-runes/samples/event-attribute-template/_config.js b/packages/svelte/tests/runtime-runes/samples/event-attribute-template/_config.js
index 340b0ff4c984..c8d797b1a697 100644
--- a/packages/svelte/tests/runtime-runes/samples/event-attribute-template/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/event-attribute-template/_config.js
@@ -1,8 +1,13 @@
 import { flushSync } from 'svelte';
 import { test } from '../../test';
+import { log } from './log.js';
 
 export default test({
-	async test({ assert, target, component }) {
+	before_test() {
+		log.length = 0;
+	},
+
+	async test({ assert, target }) {
 		const [b1] = target.querySelectorAll('button');
 
 		flushSync(() => {
@@ -10,6 +15,6 @@ export default test({
 		});
 
 		await Promise.resolve();
-		assert.deepEqual(component.log, ['onclick']);
+		assert.deepEqual(log, ['onclick']);
 	}
 });
diff --git a/packages/svelte/tests/runtime-runes/samples/event-attribute-template/log.js b/packages/svelte/tests/runtime-runes/samples/event-attribute-template/log.js
new file mode 100644
index 000000000000..0abf1fa9159e
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/event-attribute-template/log.js
@@ -0,0 +1 @@
+export const log = [];
diff --git a/packages/svelte/tests/runtime-runes/samples/event-attribute-template/main.svelte b/packages/svelte/tests/runtime-runes/samples/event-attribute-template/main.svelte
index 66db0b6013fc..5976ab15f88a 100644
--- a/packages/svelte/tests/runtime-runes/samples/event-attribute-template/main.svelte
+++ b/packages/svelte/tests/runtime-runes/samples/event-attribute-template/main.svelte
@@ -1,5 +1,5 @@
 <script>
-	const {log = []} = $props();
+	import { log } from './log.js';
 
 	function send() {
 		log.push("onclick")

From 5fdafb1acc2baafa3eb592b09157df60fda046da Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 5 Dec 2023 15:26:48 -0500
Subject: [PATCH 3/8] only make readonly in runes mode

---
 packages/svelte/src/internal/client/runtime.js | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 4c3f82c66745..e9eeace7f0c8 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -1423,13 +1423,14 @@ export function is_store(val) {
 export function prop_source(props_obj, key, flags, default_value) {
 	const call_default_value = (flags & PROPS_CALL_DEFAULT_VALUE) !== 0;
 	const immutable = (flags & PROPS_IS_IMMUTABLE) !== 0;
+	const runes = (flags & PROPS_IS_RUNES) !== 0;
 
 	const props = is_signal(props_obj) ? get(props_obj) : props_obj;
 	const update_bound_prop = get_descriptor(props, key)?.set;
 	let value = props[key];
 	const should_set_default_value = value === undefined && default_value !== undefined;
 
-	if (update_bound_prop && default_value !== undefined && (flags & PROPS_IS_RUNES) !== 0) {
+	if (update_bound_prop && runes && default_value !== undefined) {
 		// TODO consolidate all these random runtime errors
 		throw new Error('Cannot use fallback values with bind:');
 	}
@@ -1439,7 +1440,7 @@ export function prop_source(props_obj, key, flags, default_value) {
 			// @ts-expect-error would need a cumbersome method overload to type this
 			call_default_value ? default_value() : default_value;
 
-		if (DEV && immutable) {
+		if (DEV && runes) {
 			value = readonly(/** @type {any} */ (value));
 		}
 	}

From ddbc5a9c92a38f9fec614aefc19d76994f4611a8 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 5 Dec 2023 15:30:11 -0500
Subject: [PATCH 4/8] remove this for now

---
 packages/svelte/tests/runtime-legacy/shared.ts | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/packages/svelte/tests/runtime-legacy/shared.ts b/packages/svelte/tests/runtime-legacy/shared.ts
index 63c4ca10c9bd..a1944d47e5f3 100644
--- a/packages/svelte/tests/runtime-legacy/shared.ts
+++ b/packages/svelte/tests/runtime-legacy/shared.ts
@@ -305,11 +305,6 @@ async function run_test_variant(
 						compileOptions
 					});
 				}
-
-				if (config.runtime_error) {
-					unintended_error = true;
-					assert.fail('Expected a runtime error');
-				}
 			} finally {
 				instance.$destroy();
 				assert_html_equal(

From 7f97a9570c5a27d117acf146053b66d7267b7759 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 5 Dec 2023 15:30:31 -0500
Subject: [PATCH 5/8] changeset

---
 .changeset/lazy-months-knock.md | 5 +++++
 1 file changed, 5 insertions(+)
 create mode 100644 .changeset/lazy-months-knock.md

diff --git a/.changeset/lazy-months-knock.md b/.changeset/lazy-months-knock.md
new file mode 100644
index 000000000000..193e628fcfcd
--- /dev/null
+++ b/.changeset/lazy-months-knock.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+feat: make fallback prop values readonly

From d8848122e9b5b7910145db59642baa6243d9c95d Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 5 Dec 2023 15:40:42 -0500
Subject: [PATCH 6/8] ugh

---
 .../runtime-runes/samples/class-state-derived-unowned/log.js     | 1 +
 .../tests/runtime-runes/samples/class-state-init-eager-2/log.js  | 1 +
 .../tests/runtime-runes/samples/class-state-init-eager-3/log.js  | 1 +
 .../tests/runtime-runes/samples/class-state-init-eager/log.js    | 1 +
 .../tests/runtime-runes/samples/event-attribute-template/log.js  | 1 +
 5 files changed, 5 insertions(+)

diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-derived-unowned/log.js b/packages/svelte/tests/runtime-runes/samples/class-state-derived-unowned/log.js
index 0abf1fa9159e..d3df521f4da7 100644
--- a/packages/svelte/tests/runtime-runes/samples/class-state-derived-unowned/log.js
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-derived-unowned/log.js
@@ -1 +1,2 @@
+/** @type {any[]} */
 export const log = [];
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-2/log.js b/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-2/log.js
index 0abf1fa9159e..d3df521f4da7 100644
--- a/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-2/log.js
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-2/log.js
@@ -1 +1,2 @@
+/** @type {any[]} */
 export const log = [];
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-3/log.js b/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-3/log.js
index 0abf1fa9159e..d3df521f4da7 100644
--- a/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-3/log.js
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-init-eager-3/log.js
@@ -1 +1,2 @@
+/** @type {any[]} */
 export const log = [];
diff --git a/packages/svelte/tests/runtime-runes/samples/class-state-init-eager/log.js b/packages/svelte/tests/runtime-runes/samples/class-state-init-eager/log.js
index 0abf1fa9159e..d3df521f4da7 100644
--- a/packages/svelte/tests/runtime-runes/samples/class-state-init-eager/log.js
+++ b/packages/svelte/tests/runtime-runes/samples/class-state-init-eager/log.js
@@ -1 +1,2 @@
+/** @type {any[]} */
 export const log = [];
diff --git a/packages/svelte/tests/runtime-runes/samples/event-attribute-template/log.js b/packages/svelte/tests/runtime-runes/samples/event-attribute-template/log.js
index 0abf1fa9159e..d3df521f4da7 100644
--- a/packages/svelte/tests/runtime-runes/samples/event-attribute-template/log.js
+++ b/packages/svelte/tests/runtime-runes/samples/event-attribute-template/log.js
@@ -1 +1,2 @@
+/** @type {any[]} */
 export const log = [];

From fce95f0d58a59930f4ef64377e20bbfffafe687a Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 5 Dec 2023 15:53:13 -0500
Subject: [PATCH 7/8] add reassignment test

---
 .../Counter.svelte                            |  8 ++++++++
 .../_config.js                                | 19 +++++++++++++++++++
 .../main.svelte                               |  5 +++++
 3 files changed, 32 insertions(+)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly-reassigned/Counter.svelte
 create mode 100644 packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly-reassigned/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly-reassigned/main.svelte

diff --git a/packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly-reassigned/Counter.svelte b/packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly-reassigned/Counter.svelte
new file mode 100644
index 000000000000..1097b68f6f30
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly-reassigned/Counter.svelte
@@ -0,0 +1,8 @@
+<script>
+	/** @type {{ object?: { count: number }}} */
+	let { object = { count: 0 } } = $props();
+</script>
+
+<button onclick={() => object = { count: object.count + 1 } }>
+	clicks: {object.count}
+</button>
diff --git a/packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly-reassigned/_config.js b/packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly-reassigned/_config.js
new file mode 100644
index 000000000000..9787b1ca9271
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly-reassigned/_config.js
@@ -0,0 +1,19 @@
+import { test } from '../../test';
+
+export default test({
+	html: `<button>clicks: 0</button>`,
+
+	compileOptions: {
+		dev: true
+	},
+
+	async test({ assert, target }) {
+		const btn = target.querySelector('button');
+
+		await btn?.click();
+		assert.htmlEqual(target.innerHTML, `<button>clicks: 1</button>`);
+
+		await btn?.click();
+		assert.htmlEqual(target.innerHTML, `<button>clicks: 2</button>`);
+	}
+});
diff --git a/packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly-reassigned/main.svelte b/packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly-reassigned/main.svelte
new file mode 100644
index 000000000000..391afc4b232e
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly-reassigned/main.svelte
@@ -0,0 +1,5 @@
+<script>
+	import Counter from './Counter.svelte';
+</script>
+
+<Counter />

From 23064b1d21fba923905163881f107719f084b784 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Tue, 5 Dec 2023 15:55:38 -0500
Subject: [PATCH 8/8] tweak message

---
 packages/svelte/src/internal/client/proxy/readonly.js           | 2 +-
 .../samples/proxy-prop-default-readonly/_config.js              | 2 +-
 .../tests/runtime-runes/samples/proxy-prop-readonly/_config.js  | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/packages/svelte/src/internal/client/proxy/readonly.js b/packages/svelte/src/internal/client/proxy/readonly.js
index fa698c3b0830..f0dbea76a194 100644
--- a/packages/svelte/src/internal/client/proxy/readonly.js
+++ b/packages/svelte/src/internal/client/proxy/readonly.js
@@ -44,7 +44,7 @@ export function readonly(value) {
 
 /** @returns {never} */
 const readonly_error = () => {
-	throw new Error(`Props are read-only, unless used with \`bind:\``);
+	throw new Error(`Props cannot be mutated, unless used with \`bind:\``);
 };
 
 /** @type {ProxyHandler<StateObject<any>>} */
diff --git a/packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly/_config.js b/packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly/_config.js
index 5e0eb0e2da60..65d89b5f46c7 100644
--- a/packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/proxy-prop-default-readonly/_config.js
@@ -14,5 +14,5 @@ export default test({
 		assert.htmlEqual(target.innerHTML, `<button>clicks: 0</button>`);
 	},
 
-	runtime_error: 'Props are read-only, unless used with `bind:`'
+	runtime_error: 'Props cannot be mutated, unless used with `bind:`'
 });
diff --git a/packages/svelte/tests/runtime-runes/samples/proxy-prop-readonly/_config.js b/packages/svelte/tests/runtime-runes/samples/proxy-prop-readonly/_config.js
index 5e0eb0e2da60..65d89b5f46c7 100644
--- a/packages/svelte/tests/runtime-runes/samples/proxy-prop-readonly/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/proxy-prop-readonly/_config.js
@@ -14,5 +14,5 @@ export default test({
 		assert.htmlEqual(target.innerHTML, `<button>clicks: 0</button>`);
 	},
 
-	runtime_error: 'Props are read-only, unless used with `bind:`'
+	runtime_error: 'Props cannot be mutated, unless used with `bind:`'
 });