invalid_factory_method_decl

Factory method '{0}' must have a return type.

The analyzer produces this diagnostic when a method that is annotated with the factory annotation has a return type of void.

The following code produces this diagnostic because the method createC is annotated with the factory annotation but doesn't return any value:

import 'package:meta/meta.dart';

class Factory {
  @factory
  void createC() {}
}

class C {}

Change the return type to something other than void:

import 'package:meta/meta.dart';

class Factory {
  @factory
  C createC() => C();
}

class C {}