Error with Multiple Generations for the Same Class When Using strict-casts

[fernandoptr]

Description

The new feature of Multiple Generations for the Same Class #124 introduces an issue when the strict-casts language rule is enabled in the analysis_options.yaml. This causes a type error when generating code using build_runner.

Steps to Reproduce

Create app_env_fields.dart:

abstract interface class AppEnvFields {
  abstract final String apiUrl;
}

Create envs.env.dart:

import 'package:envied/envied.dart';
import 'app_env_fields.dart';

part `'envs.env.g.dart'`;

@Envied(path: '.env.production', name: 'Prod')
@Envied(path: '.env.staging', name: 'Staging')
final class Envs implements AppEnvFields {
  factory Envs() => _instance;

  static const bool kDebugMode = true;

  static final dynamic _instance = kDebugMode ? _Staging() : _Prod();

  @override
  @EnviedField(varName: 'REST_API_URL')
  final String apiUrl = _instance.apiUrl;
}

Add strict-casts to analysis_options.yaml:

analyzer:
  language:
    strict-casts: true

Run build_runner:

flutter pub run build_runner build

Expected Behavior

The code should generate successfully, and the Envs class should work without any linting or type errors.

Actual Behavior

The following error occurs:

A value of type 'dynamic' can't be returned from the constructor 'Envs' because it has a return type of 'Envs'.dart(return_of_invalid_type)

The issue is caused by the line:

factory Envs() => _instance;