Tighten type on identity function

Author: pbugnion

packages/controls/src/widget_upload.ts

@@ -27,7 +27,7 @@ export class FileUploadModel extends CoreDOMWidgetModel {
     static serializers = {
         ...CoreDOMWidgetModel.serializers,
         // use a dummy serializer for value to circumvent the default serializer.
-        value: { serialize: (x: any) => x },
+        value: { serialize: <T>(x: T): T => x },
     };
 }