allow writing to public deriveds on server

packages/svelte/src/compiler/phases/3-transform/server/visitors/ClassBody.js

@@ -36,7 +36,8 @@ export function ClassBody(node, context) {
 
 			body.push(
 				b.prop_def(field.key, null),
-				b.method('get', b.key(name), [], [b.return(b.call(member))])
+				b.method('get', b.key(name), [], [b.return(b.call(member))]),
+				b.method('set', b.key(name), [b.id('$$value')], [b.return(b.call(member, b.id('$$value')))])
 			);
 		}
 	}
@@ -61,6 +62,7 @@ export function ClassBody(node, context) {
 		if (name[0] === '#' || field.type === '$state' || field.type === '$state.raw') {
 			body.push(/** @type {PropertyDefinition} */ (context.visit(definition, child_state)));
 		} else if (field.node === definition) {
+			// $derived / $derived.by
 			const member = b.member(b.this, field.key);
 
 			body.push(
@@ -69,7 +71,8 @@ export function ClassBody(node, context) {
 					/** @type {CallExpression} */ (context.visit(field.value, child_state))
 				),
 
-				b.method('get', definition.key, [], [b.return(b.call(member))])
+				b.method('get', definition.key, [], [b.return(b.call(member))]),
+				b.method('set', b.key(name), [b.id('$$value')], [b.return(b.call(member, b.id('$$value')))])
 			);
 		}
 	}

packages/svelte/tests/runtime-runes/samples/writable-derived-3/_config.js

@@ -0,0 +1,5 @@
+import { test } from '../../test';
+
+export default test({
+	html: `3 3 3 3`
+});

packages/svelte/tests/runtime-runes/samples/writable-derived-3/main.svelte

@@ -0,0 +1,29 @@
+<script>
+	class X {
+    x = $state(1);
+    on_class = $derived(this.x * 2);
+    #on_class_private = $derived(this.x * 2);
+    #in_constructor_private
+
+    constructor() {
+      this.#in_constructor_private = $derived(this.x * 2);
+      this.in_constructor = $derived(this.x * 2);
+      this.#on_class_private = 3;
+      this.#in_constructor_private = 3;
+    }
+
+    get on_class_private() {
+      return this.#on_class_private;
+    }
+
+    get in_constructor_private() {
+      return this.#in_constructor_private;
+    }
+  }
+
+  const x = new X();
+  x.on_class = 3;
+  x.in_constructor = 3;
+</script>
+
+{x.on_class} {x.in_constructor} {x.on_class_private} {x.in_constructor_private}