fix: don't set state withing with_parent in proxy (#16176)

Closes #16164

We can't set everywhere within with_parent otherwise if it's the first time we are reading a derived it could look like we are setting state in a derived (which you are not).

Author: paoloricciuti

.changeset/beige-plants-laugh.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: don't set state withing `with_parent` in proxy

packages/svelte/src/internal/client/proxy.js

@@ -93,21 +93,19 @@ export function proxy(value) {
 				// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/Proxy/getOwnPropertyDescriptor#invariants
 				e.state_descriptors_fixed();
 			}
-
-			with_parent(() => {
-				var s = sources.get(prop);
-
-				if (s === undefined) {
-					s = source(descriptor.value, stack);
+			var s = sources.get(prop);
+			if (s === undefined) {
+				s = with_parent(() => {
+					var s = source(descriptor.value, stack);
 					sources.set(prop, s);
-
 					if (DEV && typeof prop === 'string') {
 						tag(s, get_label(path, prop));
 					}
-				} else {
-					set(s, descriptor.value, true);
-				}
-			});
+					return s;
+				});
+			} else {
+				set(s, descriptor.value, true);
+			}
 
 			return true;
 		},
@@ -268,11 +266,8 @@ export function proxy(value) {
 			// object property before writing to that property.
 			if (s === undefined) {
 				if (!has || get_descriptor(target, prop)?.writable) {
-					s = with_parent(() => {
-						var s = source(undefined, stack);
-						set(s, proxy(value));
-						return s;
-					});
+					s = with_parent(() => source(undefined, stack));
+					set(s, proxy(value));
 
 					sources.set(prop, s);
 

packages/svelte/tests/runtime-runes/samples/proxy-set-with-parent/_config.js

@@ -0,0 +1,5 @@
+import { test } from '../../test';
+
+export default test({
+	async test() {}
+});

packages/svelte/tests/runtime-runes/samples/proxy-set-with-parent/main.svelte

@@ -0,0 +1,15 @@
+<script>
+	function with_writes(initialState) {
+	  const derive = $state(initialState)
+	  return derive
+	}
+
+	let data = $state({ example: 'Example' })
+	let my_derived = $derived(with_writes({ example: data.example }))
+
+	$effect(() => {
+		my_derived.example = 'Bar'
+	});
+</script>
+
+<input bind:value={data.example} />