extension_declares_constructor

Extensions can't declare constructors.

The analyzer produces this diagnostic when a constructor declaration is found in an extension. It isn't valid to define a constructor because extensions aren't classes, and it isn't possible to create an instance of an extension.

The following code produces this diagnostic because there is a constructor declaration in E:

extension E on String {
  E() : super();
}

Remove the constructor or replace it with a static method.

Unless stated otherwise, the documentation on this site reflects Dart 3.8.1. Page last updated on 2025-05-08. View source or report an issue.