extension_override_argument_not_assignable

The type of the argument to the extension override '{0}' isn't assignable to the extended type '{1}'.

The analyzer produces this diagnostic when the argument to an extension override isn't assignable to the type being extended by the extension.

The following code produces this diagnostic because 3 isn't a String:

extension E on String {
  void method() {}
}

void f() {
  E(3).method();
}

If you're using the correct extension, then update the argument to have the correct type:

extension E on String {
  void method() {}
}

void f() {
  E(3.toString()).method();
}

If there's a different extension that's valid for the type of the argument, then either replace the name of the extension or unwrap the argument so that the correct extension is found.

Unless stated otherwise, the documentation on this site reflects Dart 3.8.0. Page last updated on 2025-05-08. View source or report an issue.