fix: disallow `export { foo as default }` in `<script module>` (#16447)

* fix: disallow `export { foo as default }` in `<script module>`

* add test

* fix test

.changeset/curly-buttons-burn.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: disallow `export { foo as default }` in `<script module>`

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

@@ -11,6 +11,17 @@ export function ExportNamedDeclaration(node, context) {
 	// visit children, so bindings are correctly initialised
 	context.next();
 
+	if (
+		context.state.ast_type &&
+		node.specifiers.some((specifier) =>
+			specifier.exported.type === 'Identifier'
+				? specifier.exported.name === 'default'
+				: specifier.exported.value === 'default'
+		)
+	) {
+		e.module_illegal_default_export(node);
+	}
+
 	if (node.declaration?.type === 'VariableDeclaration') {
 		// in runes mode, forbid `export let`
 		if (

packages/svelte/tests/validator/samples/default-export-indirect/errors.json

@@ -0,0 +1,14 @@
+[
+	{
+		"code": "module_illegal_default_export",
+		"message": "A component cannot have a default export",
+		"start": {
+			"line": 3,
+			"column": 4
+		},
+		"end": {
+			"line": 3,
+			"column": 32
+		}
+	}
+]

packages/svelte/tests/validator/samples/default-export-indirect/input.svelte

@@ -0,0 +1,4 @@
+<script module>
+    let answer = 42;
+    export { answer as default};
+</script>