handle module declarations and namespaces

packages/svelte/messages/compile-errors/script.md

@@ -162,4 +162,4 @@ Using a `$` prefix to refer to the value of a store is only possible inside `.sv
 
 ## typescript_invalid_feature
 
-> TypeScript language feature like %feature% are not natively supported, and their use is generally discouraged. You will need to use a preprocessor to convert it to JavaScript before it gets passed to the Svelte compiler
+> TypeScript language features like %feature% are not natively supported, and their use is generally discouraged. You will need to use a preprocessor to convert it to JavaScript before it gets passed to the Svelte compiler

packages/svelte/src/compiler/errors.js

@@ -435,13 +435,13 @@ export function store_invalid_subscription_module(node) {
 }
 
 /**
- * TypeScript language feature like %feature% are not natively supported, and their use is generally discouraged. You will need to use a preprocessor to convert it to JavaScript before it gets passed to the Svelte compiler
+ * TypeScript language features like %feature% are not natively supported, and their use is generally discouraged. You will need to use a preprocessor to convert it to JavaScript before it gets passed to the Svelte compiler
  * @param {null | number | NodeLike} node
  * @param {string} feature
  * @returns {never}
  */
 export function typescript_invalid_feature(node, feature) {
-	e(node, "typescript_invalid_feature", `TypeScript language feature like ${feature} are not natively supported, and their use is generally discouraged. You will need to use a preprocessor to convert it to JavaScript before it gets passed to the Svelte compiler`);
+	e(node, "typescript_invalid_feature", `TypeScript language features like ${feature} are not natively supported, and their use is generally discouraged. You will need to use a preprocessor to convert it to JavaScript before it gets passed to the Svelte compiler`);
 }
 
 /**

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

@@ -89,7 +89,13 @@ const visitors = {
 		return node;
 	},
 	FunctionExpression: remove_this_param,
-	FunctionDeclaration: remove_this_param
+	FunctionDeclaration: remove_this_param,
+	TSModuleDeclaration(node, context) {
+		if (!node.body) return b.empty;
+		// namespaces can contain non-type nodes
+		const cleaned = context.visit(node.body.body);
+		if (cleaned.length === 0) return b.empty;
+	}
 };
 
 /**

packages/svelte/src/compiler/phases/2-analyze/visitors/TSModuleDeclaration.js

@@ -0,0 +1,10 @@
+/** @import { Context } from '../types' */
+import * as e from '../../../errors.js';
+
+/**
+ * @param {any} node
+ * @param {Context} context
+ */
+export function TSModuleDeclaration(node, context) {
+	e.typescript_invalid_feature(node, 'namespaces with non-type nodes');
+}