From bbde0799959a44a836615ae793c97bf7c48bd98e Mon Sep 17 00:00:00 2001
From: Simon Holthausen <simon.holthausen@vercel.com>
Date: Sun, 28 Apr 2024 14:54:47 +0200
Subject: [PATCH] fix: disallow sequence expressions in `@const` tags

closes #11349
---
 .changeset/orange-masks-exercise.md                       | 5 +++++
 packages/svelte/messages/compile-errors/template.md       | 2 +-
 packages/svelte/src/compiler/errors.js                    | 2 +-
 packages/svelte/src/compiler/phases/1-parse/state/tag.js  | 3 +++
 .../compiler-errors/samples/const-tag-sequence/_config.js | 8 ++++++++
 .../samples/const-tag-sequence/main.svelte                | 3 +++
 6 files changed, 21 insertions(+), 2 deletions(-)
 create mode 100644 .changeset/orange-masks-exercise.md
 create mode 100644 packages/svelte/tests/compiler-errors/samples/const-tag-sequence/_config.js
 create mode 100644 packages/svelte/tests/compiler-errors/samples/const-tag-sequence/main.svelte

diff --git a/.changeset/orange-masks-exercise.md b/.changeset/orange-masks-exercise.md
new file mode 100644
index 000000000000..76fdc4aab815
--- /dev/null
+++ b/.changeset/orange-masks-exercise.md
@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: disallow sequence expressions in `@const` tags
diff --git a/packages/svelte/messages/compile-errors/template.md b/packages/svelte/messages/compile-errors/template.md
index c1b8c5f3c3be..d9365a51fbac 100644
--- a/packages/svelte/messages/compile-errors/template.md
+++ b/packages/svelte/messages/compile-errors/template.md
@@ -94,7 +94,7 @@
 
 ## const_tag_invalid_expression
 
-> {@const ...} must be an assignment
+> {@const ...} must consist of a single variable declaration
 
 ## const_tag_invalid_placement
 
diff --git a/packages/svelte/src/compiler/errors.js b/packages/svelte/src/compiler/errors.js
index a00a2cd593b6..f071cbe6fd05 100644
--- a/packages/svelte/src/compiler/errors.js
+++ b/packages/svelte/src/compiler/errors.js
@@ -734,7 +734,7 @@ export function component_invalid_directive(node) {
  * @returns {never}
  */
 export function const_tag_invalid_expression(node) {
-	e(node, "const_tag_invalid_expression", "{@const ...} must be an assignment");
+	e(node, "const_tag_invalid_expression", "{@const ...} must consist of a single variable declaration");
 }
 
 /**
diff --git a/packages/svelte/src/compiler/phases/1-parse/state/tag.js b/packages/svelte/src/compiler/phases/1-parse/state/tag.js
index bf6adccccfcf..86e713b522a0 100644
--- a/packages/svelte/src/compiler/phases/1-parse/state/tag.js
+++ b/packages/svelte/src/compiler/phases/1-parse/state/tag.js
@@ -552,6 +552,9 @@ function special(parser) {
 		parser.allow_whitespace();
 
 		const init = read_expression(parser);
+		if (init.type === 'SequenceExpression') {
+			e.const_tag_invalid_expression(init);
+		}
 		parser.allow_whitespace();
 
 		parser.eat('}', true);
diff --git a/packages/svelte/tests/compiler-errors/samples/const-tag-sequence/_config.js b/packages/svelte/tests/compiler-errors/samples/const-tag-sequence/_config.js
new file mode 100644
index 000000000000..eac1302fdc2f
--- /dev/null
+++ b/packages/svelte/tests/compiler-errors/samples/const-tag-sequence/_config.js
@@ -0,0 +1,8 @@
+import { test } from '../../test';
+
+export default test({
+	error: {
+		code: 'const_tag_invalid_expression',
+		message: '{@const ...} must consist of a single variable declaration'
+	}
+});
diff --git a/packages/svelte/tests/compiler-errors/samples/const-tag-sequence/main.svelte b/packages/svelte/tests/compiler-errors/samples/const-tag-sequence/main.svelte
new file mode 100644
index 000000000000..5d66ea33eb92
--- /dev/null
+++ b/packages/svelte/tests/compiler-errors/samples/const-tag-sequence/main.svelte
@@ -0,0 +1,3 @@
+{#if true}
+	{@const foo = 'foo', bar = 'bar'}
+{/if}
\ No newline at end of file