From 97e0fd356ff34be2c06f475cbb05c85a28f108cf Mon Sep 17 00:00:00 2001
From: Copilot <198982749+Copilot@users.noreply.github.com>
Date: Tue, 2 Sep 2025 20:24:42 +0800
Subject: [PATCH] Merge pull request #1 from
 ZiuChen/copilot/fix-fb61cdb4-ea0a-43f9-8c78-3cf2eb834cfb

Fix: Show helpful error when using `new App()` syntax in Svelte 5
---
 .../3-transform/client/transform-client.js    |  3 +-
 .../samples/new-component-error/_config.js    | 28 +++++++++++++++++++
 .../samples/new-component-error/main.svelte   |  6 ++++
 3 files changed, 36 insertions(+), 1 deletion(-)
 create mode 100644 packages/svelte/tests/runtime-runes/samples/new-component-error/_config.js
 create mode 100644 packages/svelte/tests/runtime-runes/samples/new-component-error/main.svelte

diff --git a/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js b/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
index bdd7eb3f17..54e33110e2 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
@@ -592,7 +592,8 @@ export function client_component(analysis, options) {
 				)
 			)
 		);
-	} else if (dev) {
+	} else {
+		// Always check for invalid constructor usage, not just in dev mode
 		component_block.body.unshift(b.stmt(b.call('$.check_target', b.id('new.target'))));
 	}
 
diff --git a/packages/svelte/tests/runtime-runes/samples/new-component-error/_config.js b/packages/svelte/tests/runtime-runes/samples/new-component-error/_config.js
new file mode 100644
index 0000000000..5d5ca61cdc
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/new-component-error/_config.js
@@ -0,0 +1,28 @@
+import { test } from '../../test';
+
+export default test({
+	mode: ['client'],
+	compileOptions: {
+		dev: false  // Test the fix works in production mode (where the issue was most severe)
+	},
+	async test({ mod, assert }) {
+		// Try to instantiate the component using the old `new Component()` syntax
+		// This should now throw a helpful error message instead of the cryptic "nodes_start" error
+		try {
+			const ComponentClass = mod.default;
+			const app = new ComponentClass();
+			assert.fail('Expected error when calling new ComponentClass()');
+		} catch (error) {
+			// The main fix: should NOT get the cryptic nodes_start error anymore
+			assert.ok(!error.message.includes('nodes_start'), 'Should not get cryptic nodes_start error');
+			
+			// Should get a helpful error message instead
+			const isHelpfulError = error.message.includes('no longer valid in Svelte 5') ||
+								  error.message.includes('https://svelte.dev/e/component_api_invalid_new');
+			assert.ok(isHelpfulError, 'Should get a helpful error message or URL');
+			
+			// Should be a proper Svelte error
+			assert.equal(error.name, 'Svelte error', 'Should be a Svelte error');
+		}
+	}
+});
\ No newline at end of file
diff --git a/packages/svelte/tests/runtime-runes/samples/new-component-error/main.svelte b/packages/svelte/tests/runtime-runes/samples/new-component-error/main.svelte
new file mode 100644
index 0000000000..d60de47844
--- /dev/null
+++ b/packages/svelte/tests/runtime-runes/samples/new-component-error/main.svelte
@@ -0,0 +1,6 @@
+<script>
+	let count = 0;
+</script>
+
+<h1>Hello {count}</h1>
+<button on:click={() => count++}>Click me</button>
\ No newline at end of file