fix: correctly parse each with loose parser

.changeset/famous-scissors-begin.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: correctly parse `each` with loose parser

packages/svelte/src/compiler/phases/1-parse/read/expression.js

@@ -8,9 +8,31 @@ import { find_matching_bracket } from '../utils/bracket.js';
 /**
  * @param {Parser} parser
  * @param {string} [opening_token]
+ * @returns {Expression | undefined}
+ */
+export function get_loose_identifier(parser, opening_token) {
+	// Find the next } and treat it as the end of the expression
+	const end = find_matching_bracket(parser.template, parser.index, opening_token ?? '{');
+	if (end) {
+		const start = parser.index;
+		parser.index = end;
+		// We don't know what the expression is and signal this by returning an empty identifier
+		return {
+			type: 'Identifier',
+			start,
+			end,
+			name: ''
+		};
+	}
+}
+
+/**
+ * @param {Parser} parser
+ * @param {string} [opening_token]
+ * @param {boolean} [disallow_loose]
  * @returns {Expression}
  */
-export default function read_expression(parser, opening_token) {
+export default function read_expression(parser, opening_token, disallow_loose) {
 	try {
 		const node = parse_expression_at(parser.template, parser.ts, parser.index);
 
@@ -41,19 +63,12 @@ export default function read_expression(parser, opening_token) {
 
 		return /** @type {Expression} */ (node);
 	} catch (err) {
-		if (parser.loose) {
-			// Find the next } and treat it as the end of the expression
-			const end = find_matching_bracket(parser.template, parser.index, opening_token ?? '{');
-			if (end) {
-				const start = parser.index;
-				parser.index = end;
-				// We don't know what the expression is and signal this by returning an empty identifier
-				return {
-					type: 'Identifier',
-					start,
-					end,
-					name: ''
-				};
+		// If we are in an each loop we need the error to be thrown in cases like
+		// `as { y = z }` so we still throw and handle the error there
+		if (parser.loose && !disallow_loose) {
+			const expression = get_loose_identifier(parser, opening_token);
+			if (expression) {
+				return expression;
 			}
 		}
 

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

@@ -1,13 +1,13 @@
 /** @import { ArrowFunctionExpression, Expression, Identifier, Pattern } from 'estree' */
 /** @import { AST } from '#compiler' */
 /** @import { Parser } from '../index.js' */
-import read_pattern from '../read/context.js';
-import read_expression from '../read/expression.js';
-import * as e from '../../../errors.js';
-import { create_fragment } from '../utils/create.js';
 import { walk } from 'zimmerframe';
-import { parse_expression_at } from '../acorn.js';
+import * as e from '../../../errors.js';
 import { create_expression_metadata } from '../../nodes.js';
+import { parse_expression_at } from '../acorn.js';
+import read_pattern from '../read/context.js';
+import read_expression, { get_loose_identifier } from '../read/expression.js';
+import { create_fragment } from '../utils/create.js';
 
 const regex_whitespace_with_closing_curly_brace = /^\s*}/;
 
@@ -87,15 +87,23 @@ function open(parser) {
 		// we get a valid expression
 		while (!expression) {
 			try {
-				expression = read_expression(parser);
+				expression = read_expression(parser, undefined, true);
 			} catch (err) {
 				end = /** @type {any} */ (err).position[0] - 2;
 
 				while (end > start && parser.template.slice(end, end + 2) !== 'as') {
 					end -= 1;
 				}
 
-				if (end <= start) throw err;
+				if (end <= start) {
+					if (parser.loose) {
+						expression = get_loose_identifier(parser);
+						if (expression) {
+							break;
+						}
+					}
+					throw err;
+				}
 
 				// @ts-expect-error parser.template is meant to be readonly, this is a special case
 				parser.template = template.slice(0, end);

packages/svelte/tests/parser-modern/samples/loose-valid-each-as/input.svelte

@@ -0,0 +1,7 @@
+<script lang="ts">
+	let arr = [];
+</script>
+
+{#each arr as [key, value = 'default']}
+	<div>{key}: {value}</div>
+{/each}