fix: issue with implicit public modifier causing undefined properties (#14153)

closes #14152

.changeset/four-countries-taste.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: error on TypeScript's `readonly` modifier

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

@@ -94,7 +94,7 @@ const visitors = {
 		e.typescript_invalid_feature(node, 'enums');
 	},
 	TSParameterProperty(node, context) {
-		if (node.accessibility && context.path.at(-2)?.kind === 'constructor') {
+		if ((node.readonly || node.accessibility) && context.path.at(-2)?.kind === 'constructor') {
 			e.typescript_invalid_feature(node, 'accessibility modifiers on constructor parameters');
 		}
 		return node.parameter;

packages/svelte/tests/runtime-runes/samples/typescript/main.svelte

@@ -9,7 +9,10 @@
 	}
 
 	class Foo {
-		constructor(readonly name: string) {}
+		public name: string;
+		constructor(name: string) {
+			this.name = name;
+		}
 	}
 
 	declare const declared_const: number;

packages/svelte/tests/validator/samples/ts-unsupported-modifier-readonly/errors.json

@@ -0,0 +1,14 @@
+[
+	{
+		"code": "typescript_invalid_feature",
+		"message": "TypeScript language features like accessibility modifiers on constructor parameters are not natively supported, and their use is generally discouraged. Outside of `<script>` tags, these features are not supported. For use within `<script>` tags, you will need to use a preprocessor to convert it to JavaScript before it gets passed to the Svelte compiler. If you are using `vitePreprocess`, make sure to specifically enable preprocessing script tags (`vitePreprocess({ script: true })`)",
+		"start": {
+			"line": 3,
+			"column": 14
+		},
+		"end": {
+			"line": 3,
+			"column": 32
+		}
+	}
+]

packages/svelte/tests/validator/samples/ts-unsupported-modifier-readonly/input.svelte

@@ -0,0 +1,5 @@
+<script lang="ts">
+	class Foo {
+		constructor(readonly x: number) {}
+	}
+</script>