From 5e0845fe3ef5c2fade444a17828ef977fee3e78d Mon Sep 17 00:00:00 2001
From: Simon H <5968653+dummdidumm@users.noreply.github.com>
Date: Mon, 29 Apr 2024 22:48:40 +0200
Subject: [PATCH] fix: disallow sequence expressions in `@const` tags (#11357)

* fix: disallow sequence expressions in `@const` tags

closes #11349

* allow parenthesized sequence expression
---
 .changeset/orange-masks-exercise.md                      | 5 +++++
 packages/svelte/messages/compile-errors/template.md      | 2 +-
 packages/svelte/src/compiler/errors.js                   | 4 ++--
 packages/svelte/src/compiler/phases/1-parse/state/tag.js | 8 ++++++++
 .../samples/const-tag-sequence/_config.js                | 9 +++++++++
 .../samples/const-tag-sequence/main.svelte               | 7 +++++++
 6 files changed, 32 insertions(+), 3 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 0000000000..76fdc4aab8
--- /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 293d4845cd..3460d8800b 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 d2834d03d4..dd979170a7 100644
--- a/packages/svelte/src/compiler/errors.js
+++ b/packages/svelte/src/compiler/errors.js
@@ -729,12 +729,12 @@ export function component_invalid_directive(node) {
 }
 
 /**
- * {@const ...} must be an assignment
+ * {@const ...} must consist of a single variable declaration
  * @param {null | number | NodeLike} 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 a41c5d09a0..9e797f4c57 100644
--- a/packages/svelte/src/compiler/phases/1-parse/state/tag.js
+++ b/packages/svelte/src/compiler/phases/1-parse/state/tag.js
@@ -551,7 +551,15 @@ function special(parser) {
 		parser.eat('=', true);
 		parser.allow_whitespace();
 
+		const expression_start = parser.index;
 		const init = read_expression(parser);
+		if (
+			init.type === 'SequenceExpression' &&
+			!parser.template.substring(expression_start, init.start).includes('(')
+		) {
+			// const a = (b, c) is allowed but a = b, c = d is not;
+			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 0000000000..a9e6ee1250
--- /dev/null
+++ b/packages/svelte/tests/compiler-errors/samples/const-tag-sequence/_config.js
@@ -0,0 +1,9 @@
+import { test } from '../../test';
+
+export default test({
+	error: {
+		code: 'const_tag_invalid_expression',
+		message: '{@const ...} must consist of a single variable declaration',
+		position: [75, 93]
+	}
+});
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 0000000000..7e446e6a89
--- /dev/null
+++ b/packages/svelte/tests/compiler-errors/samples/const-tag-sequence/main.svelte
@@ -0,0 +1,7 @@
+{#if true}
+	{@const foo = ('bar', 'baz')}
+{/if}
+
+{#if true}
+	{@const foo = 'foo', bar = 'bar'}
+{/if}
\ No newline at end of file