fix: allow deletion of $$restProps properties (#12736)

.changeset/loud-news-deliver.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: allow deletion of $$restProps properties

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

@@ -126,6 +126,13 @@ const legacy_rest_props_handler = {
 			};
 		}
 	},
+	deleteProperty(target, key) {
+		// Svelte 4 allowed for deletions on $$restProps
+		if (target.exclude.includes(key)) return false;
+		target.exclude.push(key);
+		update(target.version);
+		return true;
+	},
 	has(target, key) {
 		if (target.exclude.includes(key)) return false;
 		return key in target.props;

packages/svelte/tests/runtime-legacy/samples/rest-props-delete/App.svelte

@@ -0,0 +1,10 @@
+<script>
+	function prune() {
+		$$restProps.a
+		delete $$restProps.a
+	}
+</script>
+
+<button on:click={prune}>delete a</button>
+
+{JSON.stringify($$restProps)}

packages/svelte/tests/runtime-legacy/samples/rest-props-delete/_config.js

@@ -0,0 +1,15 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	html: `<button>delete a</button>\n{"a":1,"b":2}`,
+
+	test({ assert, target }) {
+		const [btn1] = target.querySelectorAll('button');
+
+		btn1.click();
+		flushSync();
+
+		assert.htmlEqual(target.innerHTML, `<button>delete a</button>\n{"b":2}`);
+	}
+});

packages/svelte/tests/runtime-legacy/samples/rest-props-delete/main.svelte

@@ -0,0 +1,5 @@
+<script>
+	import App from "./App.svelte";
+</script>
+
+<App a={1} b={2} />