fix: prevent ownership warnings if the fallback of a bindable is used

.changeset/wise-turkeys-yell.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: prevent ownership warnings if the fallback of a bindable is used

packages/svelte/src/internal/client/dev/ownership.js

@@ -27,7 +27,7 @@ export function create_ownership_validator(props) {
 		 */
 		mutation: (prop, path, result, line, column) => {
 			const name = path[0];
-			if (is_bound(props, name) || !parent) {
+			if (is_bound_or_unset(props, name) || !parent) {
 				return result;
 			}
 
@@ -52,7 +52,7 @@ export function create_ownership_validator(props) {
 		 * @param {() => any} value
 		 */
 		binding: (key, child_component, value) => {
-			if (!is_bound(props, key) && parent && value()?.[STATE_SYMBOL]) {
+			if (!is_bound_or_unset(props, key) && parent && value()?.[STATE_SYMBOL]) {
 				w.ownership_invalid_binding(
 					component[FILENAME],
 					key,
@@ -68,9 +68,13 @@ export function create_ownership_validator(props) {
  * @param {Record<string, any>} props
  * @param {string} prop_name
  */
-function is_bound(props, prop_name) {
+function is_bound_or_unset(props, prop_name) {
 	// Can be the case when someone does `mount(Component, props)` with `let props = $state({...})`
 	// or `createClassComponent(Component, props)`
 	const is_entry_props = STATE_SYMBOL in props || LEGACY_PROPS in props;
-	return !!get_descriptor(props, prop_name)?.set || (is_entry_props && prop_name in props);
+	return (
+		!!get_descriptor(props, prop_name)?.set ||
+		(is_entry_props && prop_name in props) ||
+		!(prop_name in props)
+	);
 }

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

@@ -7,7 +7,7 @@ import {
 	PROPS_IS_RUNES,
 	PROPS_IS_UPDATED
 } from '../../../constants.js';
-import { get_descriptor, is_function } from '../../shared/utils.js';
+import { define_property, get_descriptor, is_function } from '../../shared/utils.js';
 import { mutable_source, set, source, update } from './sources.js';
 import { derived, derived_safe_equal } from './deriveds.js';
 import { get, captured_signals, untrack } from '../runtime.js';

packages/svelte/tests/runtime-runes/samples/ownership-invalid-binding-bindable-fallback/Child.svelte

@@ -0,0 +1,5 @@
+<script>
+	const { test = $bindable() } = $props();
+</script>
+
+{test}

packages/svelte/tests/runtime-runes/samples/ownership-invalid-binding-bindable-fallback/Parent.svelte

@@ -0,0 +1,7 @@
+<script>
+	import Child from './Child.svelte';
+
+	let { test = $bindable({}) } = $props();
+</script>
+
+<Child bind:test />

packages/svelte/tests/runtime-runes/samples/ownership-invalid-binding-bindable-fallback/_config.js

@@ -0,0 +1,11 @@
+import { test } from '../../test';
+
+export default test({
+	mode: ['client'],
+	compileOptions: {
+		dev: true
+	},
+	async test({ warnings, assert }) {
+		assert.deepEqual(warnings, []);
+	}
+});

packages/svelte/tests/runtime-runes/samples/ownership-invalid-binding-bindable-fallback/main.svelte

@@ -0,0 +1,5 @@
+<script>
+	import Parent from './Parent.svelte';
+</script>
+
+<Parent />

packages/svelte/tests/runtime-runes/samples/ownership-invalid-mutation-bindable-fallback/Parent.svelte

@@ -0,0 +1,8 @@
+<script>
+	let { test = $bindable({}) } = $props();
+</script>
+
+<button onclick={()=>test = {}}></button>
+<button onclick={()=>test.test = {}}></button>
+
+{test}

packages/svelte/tests/runtime-runes/samples/ownership-invalid-mutation-bindable-fallback/_config.js

@@ -0,0 +1,23 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	mode: ['client'],
+	compileOptions: {
+		dev: true
+	},
+	async test({ warnings, assert, target }) {
+		const [btn, btn2] = target.querySelectorAll('button');
+		flushSync(() => {
+			btn2.click();
+		});
+		assert.deepEqual(warnings, []);
+		flushSync(() => {
+			btn.click();
+		});
+		flushSync(() => {
+			btn2.click();
+		});
+		assert.deepEqual(warnings, []);
+	}
+});

packages/svelte/tests/runtime-runes/samples/ownership-invalid-mutation-bindable-fallback/main.svelte

@@ -0,0 +1,5 @@
+<script>
+	import Parent from './Parent.svelte';
+</script>
+
+<Parent />