fix: value/checked not correctly set using spread (#15239)

* set value/checked by JS

* test

* changeset

* fix test form-default-value-spread

.changeset/fuzzy-zoos-repeat.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: value/checked not correctly set using spread

packages/svelte/src/internal/client/dom/elements/attributes.js

@@ -399,15 +399,18 @@ export function set_attributes(
 				if (name === 'value' || name === 'checked') {
 					// removing value/checked also removes defaultValue/defaultChecked — preserve
 					let input = /** @type {HTMLInputElement} */ (element);
-
+					const use_default = prev === undefined;
 					if (name === 'value') {
-						let prev = input.defaultValue;
+						let previous = input.defaultValue;
 						input.removeAttribute(name);
-						input.defaultValue = prev;
+						input.defaultValue = previous;
+						// @ts-ignore
+						input.value = input.__value = use_default ? previous : null;
 					} else {
-						let prev = input.defaultChecked;
+						let previous = input.defaultChecked;
 						input.removeAttribute(name);
-						input.defaultChecked = prev;
+						input.defaultChecked = previous;
+						input.checked = use_default ? previous : false;
 					}
 				} else {
 					element.removeAttribute(key);

packages/svelte/tests/runtime-runes/samples/attribute-spread-input/_config.js

@@ -0,0 +1,33 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ target, assert }) {
+		// Test for https://github.com/sveltejs/svelte/issues/15237
+		const [setValues, clearValue] = target.querySelectorAll('button');
+		const [text1, text2, check1, check2] = target.querySelectorAll('input');
+
+		assert.equal(text1.value, '');
+		assert.equal(text2.value, '');
+		assert.equal(check1.checked, false);
+		assert.equal(check2.checked, false);
+
+		flushSync(() => {
+			setValues.click();
+		});
+
+		assert.equal(text1.value, 'message');
+		assert.equal(text2.value, 'message');
+		assert.equal(check1.checked, true);
+		assert.equal(check2.checked, true);
+
+		flushSync(() => {
+			clearValue.click();
+		});
+
+		assert.equal(text1.value, '');
+		assert.equal(text2.value, '');
+		assert.equal(check1.checked, false);
+		assert.equal(check2.checked, false);
+	}
+});

packages/svelte/tests/runtime-runes/samples/attribute-spread-input/main.svelte

@@ -0,0 +1,22 @@
+<script>
+	let value = $state();
+	let checked = $state(false);
+
+	function setValues() {
+		value = 'message';
+		checked = true;
+	}
+	function clearValues() {
+		value = null;
+		checked = null;
+	}
+</script>
+
+<button onclick={setValues}>setValues</button>
+<button onclick={clearValues}>clearValues</button>
+
+<input type="text" {value} />
+<input type="text" {value} {...{}} />
+
+<input type="checkbox" {checked} />
+<input type="checkbox" {checked} {...{}} />