fix: adapt lang="ts" hint to simplified parser architecture

Resolve merge issues with #18075 and #18077:
- Fix inverted typescript flag in handle_parse_error
- Remove stale parser.acorn_error() call in tag.js
- Move TS syntax check inside try block in read_expression

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

@@ -61,7 +61,7 @@ export function parse(source, comments, typescript, is_script) {
 	} catch (err) {
 		// TODO the `return` in necessary for TS<7 due to a bug; otherwise
 		// the `finally` block is regarded as unreachable
-		return handle_parse_error(err, source, typescript);
+		return handle_parse_error(err);
 	} finally {
 		if (is_script) {
 			// @ts-expect-error
@@ -102,15 +102,15 @@ const regex_position_indicator = / \(\d+:\d+\)$/;
 
 /**
  * @param {any} err
- * @param {string} source
+ * @param {string} [source]
- * @param {boolean} typescript
+ * @param {boolean} [typescript]
  * @returns {never}
  */
 function handle_parse_error(err, source, typescript) {
 	let message = /** @type {string} */ (err.message).replace(regex_position_indicator, '');
 	let pos = /** @type {number} */ (err.pos);
 
-	if (typescript && source[pos] === ':') {
+	if (!typescript && source && source[pos] === ':') {
 		message += ` (did you forget to add \`lang="ts"\`?)`;
 	}
 

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

@@ -33,11 +33,8 @@ export function get_loose_identifier(parser, opening_token) {
  * @returns {Expression}
  */
 export default function read_expression(parser, opening_token, disallow_loose) {
-	/** @type {ReturnType<typeof parse_expression_at>} */
-	let node;
-
 	try {
-		node = parse_expression_at(parser, parser.template, parser.index);
+		const node = parse_expression_at(parser, parser.template, parser.index);
 
 		let index = /** @type {number} */ (node.end);
 
@@ -45,6 +42,24 @@ export default function read_expression(parser, opening_token, disallow_loose) {
 		if (last_comment && last_comment.end > index) index = last_comment.end;
 
 		parser.index = index;
+
+		if (!parser.ts) {
+			let j = parser.index;
+			while (j < parser.template.length && regex_whitespace.test(parser.template[j])) j++;
+
+			const remaining = parser.template.slice(j);
+
+			if (
+				remaining[0] === ':' ||
+				/^satisfies[\s(]/.test(remaining) ||
+				(remaining[0] === '!' && remaining[1] !== '=') ||
+				(/^as[\s(]/.test(remaining) && !disallow_loose)
+			) {
+				e.js_parse_error(j, `Unexpected token (did you forget to add \`lang="ts"\`?)`);
+			}
+		}
+
+		return /** @type {Expression} */ (remove_parens(node));
 	} catch (err) {
 		// 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
@@ -57,22 +72,4 @@ export default function read_expression(parser, opening_token, disallow_loose) {
 
 		throw err;
 	}
-
-	if (!parser.ts) {
-		let j = parser.index;
-		while (j < parser.template.length && regex_whitespace.test(parser.template[j])) j++;
-
-		const remaining = parser.template.slice(j);
-
-		if (
-			remaining[0] === ':' ||
-			/^satisfies[\s(]/.test(remaining) ||
-			(remaining[0] === '!' && remaining[1] !== '=') ||
-			(/^as[\s(]/.test(remaining) && !disallow_loose)
-		) {
-			e.js_parse_error(j, `Unexpected token (did you forget to add \`lang="ts"\`?)`);
-		}
-	}
-
-	return /** @type {Expression} */ (remove_parens(node));
 }

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

@@ -390,19 +390,11 @@ function open(parser) {
 		const prelude = parser.template.slice(0, params_start).replace(/\S/g, ' ');
 		const params = parser.template.slice(params_start, parser.index);
 
-		let function_expression;
+		let function_expression = matched
-
+			? /** @type {ArrowFunctionExpression} */ (
-		if (matched) {
-			try {
-				function_expression = /** @type {ArrowFunctionExpression} */ (
 					parse_expression_at(parser, prelude + `${params} => {}`, params_start)
-				);
+				)
-			} catch (/** @type {any} */ err) {
+			: { params: [] };
-				parser.acorn_error(err);
-			}
-		} else {
-			function_expression = { params: [] };
-		}
 
 		parser.allow_whitespace();
 		parser.eat('}', true);