the parser says what was in parens: the four scans back through the source for a ( go

packages/svelte/src/compiler/phases/1-parse/index.js

@@ -98,6 +98,7 @@ export class Parser {
 			locations: true,
 			scopes: true,
 			errorRecovery: loose,
+			parenthesized: true,
 			// a script may export what the component declares elsewhere
 			allowUndeclaredExports: true
 		});

packages/svelte/src/compiler/phases/1-parse/state/tag.js

@@ -604,14 +604,10 @@ function special(parser) {
 		parser.eat('=', true);
 		parser.allow_whitespace();
 
-		const expression_start = parser.index;
 		const init = read_expression(parser);
 		// parser is past wrapping parens, but `init.end` is not — use the parser position
 		const declarator_end = parser.index;
-		if (
-			init.type === 'SequenceExpression' &&
-			!parser.template.substring(expression_start, init.start).includes('(')
-		) {
+		if (init.type === 'SequenceExpression' && !init.parenthesized) {
 			// const a = (b, c) is allowed but a = b, c = d is not;
 			e.const_tag_invalid_expression(init);
 		}

packages/svelte/src/compiler/phases/2-analyze/visitors/BindDirective.js

@@ -135,23 +135,9 @@ export function BindDirective(node, context) {
 			e.bind_group_invalid_expression(node);
 		}
 
-		let i = /** @type {number} */ (node.expression.start);
-		let leading_comments_start = /**@type {any}*/ (node.expression.leadingComments?.at(0))?.start;
-		let leading_comments_end = /**@type {any}*/ (node.expression.leadingComments?.at(-1))?.end;
-		while (context.state.analysis.source[--i] !== '{') {
-			if (
-				context.state.analysis.source[i] === '(' &&
-				// if the parenthesis is in a leading comment we don't need to throw the error
-				!(
-					leading_comments_start &&
-					leading_comments_end &&
-					i <= leading_comments_end &&
-					i >= leading_comments_start
-				)
-			) {
+		if (node.expression.parenthesized) {
 			e.bind_invalid_parens(node, node.name);
 		}
-		}
 
 		if (node.expression.expressions.length !== 2) {
 			e.bind_invalid_expression(node);

packages/svelte/src/compiler/phases/2-analyze/visitors/shared/component.js

@@ -93,10 +93,7 @@ export function visit_component(node, context) {
 				validate_attribute(attribute, node);
 
 				if (is_expression_attribute(attribute)) {
-					disallow_unparenthesized_sequences(
-						get_attribute_expression(attribute),
-						context.state.analysis.source
-					);
+					disallow_unparenthesized_sequences(get_attribute_expression(attribute));
 				}
 			}
 
@@ -112,7 +109,7 @@ export function visit_component(node, context) {
 		}
 
 		if (attribute.type === 'AttachTag') {
-			disallow_unparenthesized_sequences(attribute.expression, context.state.analysis.source);
+			disallow_unparenthesized_sequences(attribute.expression);
 		}
 	}
 
@@ -163,15 +160,9 @@ export function visit_component(node, context) {
 
 /**
  * @param {Expression} expression
- * @param {string} source
  */
-function disallow_unparenthesized_sequences(expression, source) {
-	if (expression.type === 'SequenceExpression') {
-		let i = /** @type {number} */ (expression.start);
-		while (--i > 0) {
-			const char = source[i];
-			if (char === '(') break; // parenthesized sequence expressions are ok
-			if (char === '{') e.attribute_invalid_sequence_expression(expression);
-		}
+function disallow_unparenthesized_sequences(expression) {
+	if (expression.type === 'SequenceExpression' && !expression.parenthesized) {
+		e.attribute_invalid_sequence_expression(expression);
 	}
 }

packages/svelte/src/compiler/phases/2-analyze/visitors/shared/element.js

@@ -45,13 +45,8 @@ export function validate_element(node, context) {
 
 				if (is_expression) {
 					const expression = get_attribute_expression(attribute);
-					if (expression.type === 'SequenceExpression') {
-						let i = /** @type {number} */ (expression.start);
-						while (--i > 0) {
-							const char = context.state.analysis.source[i];
-							if (char === '(') break; // parenthesized sequence expressions are ok
-							if (char === '{') e.attribute_invalid_sequence_expression(expression);
-						}
+					if (expression.type === 'SequenceExpression' && !expression.parenthesized) {
+						e.attribute_invalid_sequence_expression(expression);
 					}
 				}
 			}

packages/svelte/src/compiler/types/template.d.ts

@@ -665,5 +665,7 @@ declare module 'estree' {
 		end?: number;
 		/** Added by acorn-typescript */
 		typeAnnotation?: any;
+		/** The source wraps the node in parens, from the parser; absent otherwise */
+		parenthesized?: true;
 	}
 }

packages/svelte/tests/parser-modern/samples/const-tag-parenthesized-init/output.json

@@ -79,6 +79,7 @@
 													"column": 24
 												}
 											},
+											"parenthesized": true,
 											"object": {
 												"type": "Identifier",
 												"start": 36,
@@ -171,6 +172,7 @@
 													"column": 24
 												}
 											},
+											"parenthesized": true,
 											"operator": "=",
 											"left": {
 												"type": "Identifier",
@@ -295,6 +297,7 @@
 													"column": 27
 												}
 											},
+											"parenthesized": true,
 											"properties": [
 												{
 													"type": "Property",
@@ -523,5 +526,6 @@
 			}
 		]
 	},
-	"options": null
+	"options": null,
+	"comments": []
 }

packages/svelte/tests/parser-modern/samples/each-block-object-pattern/output.json

@@ -1005,6 +1005,7 @@
 												"column": 60
 											}
 										},
+										"parenthesized": true,
 										"id": null,
 										"expression": false,
 										"generator": false,

packages/svelte/tests/parser-modern/samples/parens/output.json

@@ -25,6 +25,7 @@
 							"column": 9
 						}
 					},
+					"parenthesized": true,
 					"leadingComments": [
 						{
 							"type": "Block",

packages/svelte/tests/parser-modern/test.ts

@@ -76,6 +76,9 @@ function clean(ast: AST.SvelteNode) {
 			delete node.leadingComments;
 			// @ts-ignore
 			delete node.trailingComments;
+			// the printer drops parens it does not need
+			// @ts-ignore
+			delete node.parenthesized;
 
 			context.next();
 		},