feat: ban export default function via no-restricted-syntax

.changeset/no-default-exported-function.md

@@ -0,0 +1,7 @@
+---
+"@gaia-react/lint": patch
+---
+
+Close `prefer-arrow-functions` upstream gap for `export default function NamedFn(){}` via `no-restricted-syntax` selector.
+
+The `eslint-plugin-prefer-arrow-functions` plugin has a hardcoded exemption (`guard.js:hasNameAndIsExportedAsDefaultExport`) that silently passes named default-exported declarations regardless of `allowNamedFunctions` setting. This release adds a `no-restricted-syntax` rule with selector `ExportDefaultDeclaration > FunctionDeclaration` to flag the pattern. Convert to `const Name = () => {}; export default Name;` instead. Ignored on `**/*.d.ts` since ambient declarations have no body to convert.

src/configs/base.ts

@@ -257,6 +257,26 @@ const preferArrowFunctionsConfig: Linter.Config[] = [
       'prefer-arrow-functions/prefer-arrow-functions': 'error',
     },
   },
+  {
+    // `prefer-arrow-functions` has a hardcoded exemption for named
+    // default-exported declarations (`guard.js:hasNameAndIsExportedAsDefaultExport`),
+    // so `export default function Foo() {}` slips through. This selector closes
+    // that gap. Convert to `const Foo = () => {}; export default Foo;` instead.
+    // `.d.ts` is ignored because ambient `export default function …(): T;`
+    // declarations have no body to convert.
+    ignores: ['**/*.d.ts'],
+    name: 'prefer-arrow/no-default-exported-function',
+    rules: {
+      'no-restricted-syntax': [
+        'error',
+        {
+          message:
+            'Use `const Name = () => {}; export default Name;` instead. The prefer-arrow-functions plugin exempts named default-exported declarations upstream.',
+          selector: 'ExportDefaultDeclaration > FunctionDeclaration',
+        },
+      ],
+    },
+  },
   {
     files: ['**/*.d.ts'],
     name: 'ts-definition-files/prefer-arrow-off',