fix: error at compile time on unsupported TypeScript language features

Author: Rich-Harris

.changeset/perfect-cooks-shop.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: error at compile time on unsupported TypeScript language features

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

@@ -159,3 +159,7 @@
 > Cannot reference store value outside a `.svelte` file
 
 Using a `$` prefix to refer to the value of a store is only possible inside `.svelte` files, where Svelte can automatically create subscriptions when a component is mounted and unsubscribe when the component is unmounted. Consider migrating to runes instead.
+
+## 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

packages/svelte/src/compiler/errors.js

@@ -434,6 +434,16 @@ export function store_invalid_subscription_module(node) {
 	e(node, "store_invalid_subscription_module", "Cannot reference store value outside a `.svelte` file");
 }
 
+/**
+ * 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
+ * @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`);
+}
+
 /**
  * Declaration cannot be empty
  * @param {null | number | NodeLike} node

packages/svelte/src/compiler/phases/2-analyze/index.js

@@ -60,6 +60,7 @@ import { SvelteSelf } from './visitors/SvelteSelf.js';
 import { TaggedTemplateExpression } from './visitors/TaggedTemplateExpression.js';
 import { Text } from './visitors/Text.js';
 import { TitleElement } from './visitors/TitleElement.js';
+import { TSEnumDeclaration } from './visitors/TSEnumDeclaration.js';
 import { UpdateExpression } from './visitors/UpdateExpression.js';
 import { UseDirective } from './visitors/UseDirective.js';
 import { VariableDeclarator } from './visitors/VariableDeclarator.js';
@@ -168,7 +169,10 @@ const visitors = {
 	TitleElement,
 	UpdateExpression,
 	UseDirective,
-	VariableDeclarator
+	VariableDeclarator,
+
+	// @ts-expect-error
+	TSEnumDeclaration
 };
 
 /**

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

@@ -0,0 +1,10 @@
+/** @import { Context } from '../types' */
+import * as e from '../../../errors.js';
+
+/**
+ * @param {any} node
+ * @param {Context} context
+ */
+export function TSEnumDeclaration(node, context) {
+	e.typescript_invalid_feature(node, 'enums');
+}