Deploying Symfony on Heroku

I'm trying to deploy a Symfony 5.2 on Heroku, I've already configured the Procfile and the required Config Vars such APP_ENV (prod), APP_SECRET and others related to the project.

Procfile:

web: vendor/bin/heroku-php-apache2 public/

composer.json:

{
"type": "project",
"license": "proprietary",
"require": {
    "php": "^7.2.5",
    "ext-ctype": "*",
    "ext-iconv": "*",
    "knplabs/knp-paginator-bundle": "^5.2",
    "lexik/jwt-authentication-bundle": "^2.8",
    "nelmio/cors-bundle": "^2.0",
    "sensio/framework-extra-bundle": "^5.5",
    "symfony/asset": "5.1.*",
    "symfony/console": "5.1.*",
    "symfony/dotenv": "5.1.*",
    "symfony/flex": "^1.3.1",
    "symfony/framework-bundle": "5.1.*",
    "symfony/mailer": "5.1.*",
    "symfony/orm-pack": "^1.0",
    "symfony/security-bundle": "5.1.*",
    "symfony/serializer": "5.1.*",
    "symfony/yaml": "5.1.*",
    "twig/twig": "^3.0"        
},
"require-dev": {
    "doctrine/doctrine-fixtures-bundle": "^3.3",
    "symfony/maker-bundle": "^1.19",
    "symfony/profiler-pack": "^1.0"
},
"config": {
    "optimize-autoloader": true,
    "preferred-install": {
        "*": "dist"
    },
    "sort-packages": true
},
"autoload": {
    "psr-4": {
        "App\\": "src/"
    }
},
"autoload-dev": {
    "psr-4": {
        "App\\Tests\\": "tests/"
    }
},
"replace": {
    "paragonie/random_compat": "2.*",
    "symfony/polyfill-ctype": "*",
    "symfony/polyfill-iconv": "*",
    "symfony/polyfill-php72": "*",
    "symfony/polyfill-php71": "*",
    "symfony/polyfill-php70": "*",
    "symfony/polyfill-php56": "*"
},
"scripts": {
    "auto-scripts": {
        "cache:clear": "symfony-cmd",
        "assets:install %PUBLIC_DIR%": "symfony-cmd"
    },
    "post-install-cmd": [
        "@auto-scripts"
    ],
    "post-update-cmd": [
        "@auto-scripts"
    ],
    "compile": [
        "php bin/console doctrine:migrations:migrate"
    ]
},
"conflict": {
    "symfony/symfony": "*"
},
"extra": {
    "symfony": {
        "allow-contrib": false,
        "require": "5.1.*"
    }
}

}

No matter what I do I always end up with an error related to something missing such as:

PHP Fatal error:  Uncaught Error: Class 'Symfony\Bundle\TwigBundle\TwigBundle' not found in /tmp/build_271a300c_/vendor/symfony/framework-bundle/Kernel/MicroKernelTrait.php:74

My guess is the problem is related with everything ending up in this /tmp/build_271a300c_ folder and when the project is deployed it lost the refereed folder. But I have no idea what to do with this information I appreciate any help.

Solution

Solved the problem with, what I think, is nothing more than a workaround.

The deploy process was complaining always about the require-dev dependencies, so I did:

Moved all require-dev to require;
Run composer update to refresh the composer.lock file;
Pushed and tried the deploy again with success.

This solution was take for many people in other issues I read, I'm just describing step-by-step what was done to fix it. Still, I don't think it's the correct fix for this, after all the dev-dependencies shouldn't being installed on prod environment.