Symfony迁移Flex时when@dev配置加载错误求助

迁移到Symfony Flex并按照Symfony4升级教程操作时，执行php bin/console触发错误：无法加载/Users/mattias/Documents/www/webtools/config/packages/monolog.yaml中的when@dev配置，提示未找到对应扩展命名空间；移除when@dev后，when@test也出现相同错误。怀疑获取到了适配Symfony5的Monolog配置，现提供项目composer.json内容：

{
    "name": "a/b",
    "license": "proprietary",
    "type": "project",
    "autoload": {
        "psr-4": {
            "AppBundle\\": "src/AppBundle",
            "App\\": "src/"
        },
        "classmap": [ "app/AppKernel.php", "app/AppCache.php" ]
    },
    "autoload-dev": {
        "psr-4": {
            "Tests\\AppBundle\\": "tests/AppBundle",
            "App\\Tests\\": "tests/"
        }
    },
    "repositories": [
        {
            "type": "package",
            "package": {
                "name": "jquery/jquery",
                "version": "1.11.1",
                "dist": {
                    "url": "https://code.jquery.com/jquery-1.11.1.js",
                    "type": "file"
                }
            }
        }
    ],
    "require": {
        "php": "^7.1.3",
        "doctrine/doctrine-bundle": "^1.6",
        "doctrine/doctrine-cache-bundle": "^1.2",
        "doctrine/orm": "^2.5",
        "friendsofsymfony/user-bundle": "~2.1",
        "sensio/framework-extra-bundle": "^5.0.0",
        "symfony/console": "^4.0",
        "symfony/flex": "^1.19",
        "symfony/framework-bundle": "^4.0",
        "symfony/lts": "^4@dev",
        "symfony/monolog-bundle": "^3.1.0",
        "symfony/polyfill-apcu": "^1.0",
        "symfony/swiftmailer-bundle": "^3.1",
        "symfony/yaml": "^4.0",
        "twig/twig": "^1.0||^2.0"
    },
    "require-dev": {
        "symfony/dotenv": "^4.0",
        "symfony/phpunit-bridge": "^4.0"
    },
    "config": {
        "preferred-install": {
            "*": "dist"
        },
        "sort-packages": true,
        "allow-plugins": {
            "symfony/flex": true
        }
    },
    "scripts": {
        "auto-scripts": {
            "cache:clear": "symfony-cmd",
            "assets:install %PUBLIC_DIR%": "symfony-cmd"
        },
        "post-install-cmd": [
            "@auto-scripts"
        ],
        "post-update-cmd": [
            "@auto-scripts"
        ]
    },
    "conflict": {
        "symfony/symfony": "*"
    },
    "extra": {
        "symfony": {
            "id": "",
            "allow-contrib": false
        }
    }
}

核心原因 #

when@dev/when@test这种内联环境配置语法是Symfony 4.3及以上版本才支持的特性，而你的项目当前依赖的Symfony版本可能低于4.3，同时symfony/monolog-bundle版本范围过宽，拉取了兼容Symfony5的配置文件，导致语法不兼容。

1. 锁定Monolog Bundle适配Symfony4的版本 #

修改composer.json中symfony/monolog-bundle的版本约束，确保只安装Symfony4兼容的分支：

"symfony/monolog-bundle": "^3.5"

^3.5是Symfony4兼容的最高稳定分支，不会引入Symfony5专属的配置格式。

2. 调整配置文件语法（若保留Symfony<4.3版本） #

如果不想升级Symfony核心版本到4.3+，需要将内联的环境配置拆分到对应环境目录：

• 将monolog.yaml中when@dev区块的内容，移动到config/packages/dev/monolog.yaml文件
• 将when@test区块的内容，移动到config/packages/test/monolog.yaml文件
• 主config/packages/monolog.yaml仅保留所有环境通用的配置项

3. 更新依赖 #

修改composer.json后，执行以下命令更新依赖并清理缓存：

composer update
php bin/console cache:clear

可选：锁定Symfony Flex版本 #

若Flex自动拉取高版本配置文件，可将其版本限制为Symfony4适配范围：

"symfony/flex": "^1.10"

内容的提问来源于stack exchange，提问作者Matt Welander