fix: report `derived_invalid_export` for `export let x = $derived(...)` (#18692)

Fixes #15842

Run the more targeted validations first

---------

Co-authored-by: Simon Holthausen <simon.holthausen@vercel.com>

.changeset/heavy-pumas-report.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: report `derived_invalid_export` for `export let x = $derived(...)` in runes mode

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

@@ -1,4 +1,4 @@
-/** @import { ExportNamedDeclaration, Identifier } from 'estree' */
+/** @import { ExportNamedDeclaration, Identifier, VariableDeclaration } from 'estree' */
 /** @import { Context } from '../types' */
 import * as e from '../../../errors.js';
 import { extract_identifiers } from '../../../utils/ast.js';
@@ -23,15 +23,6 @@ export function ExportNamedDeclaration(node, context) {
 	}
 
 	if (node.declaration?.type === 'VariableDeclaration') {
-		// in runes mode, forbid `export let`
-		if (
-			context.state.analysis.runes &&
-			context.state.ast_type === 'instance' &&
-			node.declaration.kind === 'let'
-		) {
-			e.legacy_export_invalid(node);
-		}
-
 		for (const declarator of node.declaration.declarations) {
 			for (const id of extract_identifiers(declarator.id)) {
 				const binding = context.state.scope.get(id.name);
@@ -46,6 +37,15 @@ export function ExportNamedDeclaration(node, context) {
 				}
 			}
 		}
+
+		// in runes mode, forbid `export let`
+		if (
+			context.state.analysis.runes &&
+			context.state.ast_type === 'instance' &&
+			node.declaration.kind === 'let'
+		) {
+			e.legacy_export_invalid(node);
+		}
 	}
 
 	if (context.state.analysis.runes) {

packages/svelte/tests/compiler-errors/samples/runes-export-let-derived/_config.js

@@ -0,0 +1,9 @@
+import { test } from '../../test';
+
+export default test({
+	error: {
+		code: 'derived_invalid_export',
+		message:
+			'Cannot export derived state from a module. To expose the current derived value, export a function returning its value'
+	}
+});

packages/svelte/tests/compiler-errors/samples/runes-export-let-derived/main.svelte

@@ -0,0 +1,4 @@
+<script>
+	let count = $state(0);
+	export let double = $derived(count * 2);
+</script>