fix: make deriveds on the server lazy again

Fixes a regression introduced in #15820: deriveds need to be lazily called on the server, too, since they can close over variables only later defined

Fixes #15960

.changeset/khaki-carrots-destroy.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: make deriveds on the server lazy again

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

@@ -23,6 +23,7 @@ import { Identifier } from './visitors/Identifier.js';
 import { IfBlock } from './visitors/IfBlock.js';
 import { KeyBlock } from './visitors/KeyBlock.js';
 import { LabeledStatement } from './visitors/LabeledStatement.js';
+import { MemberExpression } from './visitors/MemberExpression.js';
 import { PropertyDefinition } from './visitors/PropertyDefinition.js';
 import { RegularElement } from './visitors/RegularElement.js';
 import { RenderTag } from './visitors/RenderTag.js';
@@ -48,6 +49,7 @@ const global_visitors = {
 	ExpressionStatement,
 	Identifier,
 	LabeledStatement,
+	MemberExpression,
 	PropertyDefinition,
 	UpdateExpression,
 	VariableDeclaration

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

@@ -22,10 +22,6 @@ export function ClassBody(node, context) {
 	const child_state = { ...context.state, state_fields };
 
 	for (const [name, field] of state_fields) {
-		if (name[0] === '#') {
-			continue;
-		}
-
 		// insert backing fields for stuff declared in the constructor
 		if (
 			field &&

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

@@ -0,0 +1,23 @@
+/** @import { ClassBody, MemberExpression } from 'estree' */
+/** @import { Context } from '../types.js' */
+import * as b from '#compiler/builders';
+
+/**
+ * @param {MemberExpression} node
+ * @param {Context} context
+ */
+export function MemberExpression(node, context) {
+	if (
+		context.state.analysis.runes &&
+		node.object.type === 'ThisExpression' &&
+		node.property.type === 'PrivateIdentifier'
+	) {
+		const field = context.state.state_fields?.get(`#${node.property.name}`);
+
+		if (field?.type === '$derived' || field?.type === '$derived.by') {
+			return b.call(node);
+		}
+	}
+
+	context.next();
+}

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

@@ -11,7 +11,7 @@ export function PropertyDefinition(node, context) {
 	if (context.state.analysis.runes && node.value != null && node.value.type === 'CallExpression') {
 		const rune = get_rune(node.value, context.state.scope);
 
-		if (rune === '$state' || rune === '$state.raw' || rune === '$derived') {
+		if (rune === '$state' || rune === '$state.raw') {
 			return {
 				...node,
 				value:
@@ -21,13 +21,14 @@ export function PropertyDefinition(node, context) {
 			};
 		}
 
-		if (rune === '$derived.by') {
+		if (rune === '$derived.by' || rune === '$derived') {
+			const fn = /** @type {Expression} */ (context.visit(node.value.arguments[0]));
 			return {
 				...node,
 				value:
 					node.value.arguments.length === 0
 						? null
-						: b.call(/** @type {Expression} */ (context.visit(node.value.arguments[0])))
+						: b.call('$.once', rune === '$derived' ? b.thunk(fn) : fn)
 			};
 		}
 	}

packages/svelte/tests/runtime-runes/samples/class-state-derived-private/_config.js

@@ -5,6 +5,7 @@ export default test({
 	html: `
 		<button>0</button>
 		<p>doubled: 0</p>
+		<p>trippled: 0</p>
 	`,
 
 	test({ assert, target }) {
@@ -17,6 +18,7 @@ export default test({
 			`
 				<button>1</button>
 				<p>doubled: 2</p>
+				<p>trippled: 3</p>
 			`
 		);
 
@@ -27,6 +29,7 @@ export default test({
 			`
 				<button>2</button>
 				<p>doubled: 4</p>
+				<p>trippled: 9</p>
 			`
 		);
 	}

packages/svelte/tests/runtime-runes/samples/class-state-derived-private/main.svelte

@@ -2,14 +2,24 @@
 	class Counter {
 		count = $state(0);
 		#doubled = $derived(this.count * 2);
+		#trippled = $derived.by(() => this.count * this.by);
 
-		get embiggened() {
+		constructor(by) {
+			this.by = by;
+		}
+
+		get embiggened1() {
 			return this.#doubled;
 		}
+
+		get embiggened2() {
+			return this.#trippled;
+		}
 	}
 
-	const counter = new Counter();
+	const counter = new Counter(3);
 </script>
 
 <button on:click={() => counter.count++}>{counter.count}</button>
-<p>doubled: {counter.embiggened}</p>
+<p>doubled: {counter.embiggened1}</p>
+<p>trippled: {counter.embiggened2}</p>