fix: disallow sequence expressions in @const tags

closes #11349

.changeset/orange-masks-exercise.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: disallow sequence expressions in `@const` tags

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
 

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");
 }
 
 /**

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);

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'
+	}
+});

packages/svelte/tests/compiler-errors/samples/const-tag-sequence/main.svelte

@@ -0,0 +1,3 @@
+{#if true}
+	{@const foo = 'foo', bar = 'bar'}
+{/if}