extension_type_declares_instance_field

Extension types can't declare instance fields.

The analyzer produces this diagnostic when there's a field declaration in the body of an extension type declaration.

The following code produces this diagnostic because the extension type E declares a field named f:

extension type E(int i) {
  final int f = 0;
}

If you don't need the field, then remove it or replace it with a getter and/or setter:

extension type E(int i) {
  int get f => 0;
}

If you need the field, then convert the extension type into a class:

class E {
  final int i;

  final int f = 0;

  E(this.i);
}