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 {boolean} typescript
+ * @param {string} [source]
+ * @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);
 
@@ -45,18 +42,6 @@ export default function read_expression(parser, opening_token, disallow_loose) {
 		if (last_comment && last_comment.end > index) index = last_comment.end;
 
 		parser.index = index;
-	} 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
-		if (parser.loose && !disallow_loose) {
-			const expression = get_loose_identifier(parser, opening_token);
-			if (expression) {
-				return expression;
-			}
-		}
-
-		throw err;
-	}
 
 		if (!parser.ts) {
 			let j = parser.index;
@@ -75,4 +60,16 @@ export default function read_expression(parser, opening_token, disallow_loose) {
 		}
 
 		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
+		if (parser.loose && !disallow_loose) {
+			const expression = get_loose_identifier(parser, opening_token);
+			if (expression) {
+				return expression;
+			}
+		}
+
+		throw err;
+	}
 }

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;
-
-		if (matched) {
-			try {
-				function_expression = /** @type {ArrowFunctionExpression} */ (
+		let function_expression = matched
+			? /** @type {ArrowFunctionExpression} */ (
 					parse_expression_at(parser, prelude + `${params} => {}`, params_start)
-				);
-			} catch (/** @type {any} */ err) {
-				parser.acorn_error(err);
-			}
-		} else {
-			function_expression = { params: [] };
-		}
+				)
+			: { params: [] };
 
 		parser.allow_whitespace();
 		parser.eat('}', true);