fix: improve error message for TS-like snippet annotations without lang="ts"

.changeset/lang-ts-error.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: improve error message for TS-like snippet annotations without lang="ts"

packages/svelte/.gitignore

@@ -13,3 +13,5 @@
 /transition.d.ts
 
 /scripts/_bundle.js
+*.Zone.Identifier
+

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

@@ -45,6 +45,17 @@
 	const pattern_string = parser.template.slice(start, i);
 
 	try {
+		// Check for TypeScript parameter annotations in the pattern string
+		if (!parser.ts) {
+			const parameterRegex = /\w+\s*:\s*\w+/;
+			if (parameterRegex.test(pattern_string)) {
+				e.typescript_invalid_feature(
+					start,
+					'type annotations in snippets. Did you forget to add lang="ts" to your script tag?'
+				);
+			}
+		}
+
 		// the length of the `space_with_newline` has to be start - 1
 		// because we added a `(` in front of the pattern_string,
 		// which shifted the entire string to right by 1
@@ -67,7 +78,9 @@
 			)
 		).left;
 
-		expression.typeAnnotation = read_type_annotation(parser);
+		const annotation = read_type_annotation(parser);
+
+		expression.typeAnnotation = annotation;
 		if (expression.typeAnnotation) {
 			expression.end = expression.typeAnnotation.end;
 		}

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

@@ -396,6 +396,14 @@ function open(parser) {
 		const prelude = parser.template.slice(0, params_start).replace(/\S/g, ' ');
 		const params = parser.template.slice(params_start, parser.index);
 
+		// Check for TypeScript parameter annotations without lang="ts"
+		if (!parser.ts && matched && /\w+\s*:\s*\w+/.test(params)) {
+			e.typescript_invalid_feature(
+				parser.index,
+				'type annotations in snippets. Did you forget to add lang="ts" to your script tag?'
+			);
+		}
+
 		let function_expression = matched
 			? /** @type {ArrowFunctionExpression} */ (
 					parse_expression_at(