Thank you @weaverryan.

Is there any solution out there?
I've posted my environment over here symfony/flex#346 (comment)

@jeroendesloovere looks totally unrelated... If you found a bug, please open a separate issue.

Are you sure?
I have src/DataFixtures/* classes which inherit from DoctrineFixturesBundle which is only loaded in in test and dev environments.

So when I try to deploy to Heroku (which uses prod environment), these DoctrineFixturesBundle classes can't be autoloaded and so everything throws an error.

//...
use Doctrine\Bundle\FixturesBundle\Fixture;
use Doctrine\Common\DataFixtures\OrderedFixtureInterface;
use Doctrine\Common\Persistence\ObjectManager;

class LoadUserData extends Fixture implements OrderedFixtureInterface
{
    public function load(ObjectManager $manager)
    {
        // ...
    }

    public function getOrder()
    {
        return 10;
    }
}

Solution

Using composer require doctrine-fixtures-bundle --dev was so wrong.
This has to be composer require doctrine-fixtures-bundle.

Since in my case, this bundle is used in tests.

Wrong composer.json

    "require": {
        // ...
    },
    "require-dev": {
        // ...
        "doctrine/doctrine-fixtures-bundle": "^3.0",
    },

Correct composer.json

    "require": {
        // ...
        "doctrine/doctrine-fixtures-bundle": "^3.0",
    },
    "require-dev": {
        // ...
    },

Other things I tried but failed

# services.yaml
services:

    # ignore datafixtures in production environment,
    # because it usages DoctrineFixturesBundle classes which are only loaded in for "dev" and "test"  environments
    BabyGuesser\DataFixtures\:
        resource: '../src/DataFixtures/*'
        autowire: false
        autoconfigure: false
        public: false

Local everything is perfect, but when pushing to heroku the cache:clear failed every time.

Hello guys,

I had the same error with the DoctrineFixtureBundle, to fix this problem, I added this in my service.yaml

services:

    App\:
        resource: '../src/*'
        exclude: '../src/{DependencyInjection,Entity,Migrations,Tests,DataFixtures,Kernel.php}'

The DateFixtures directory is exclude and I don't have any more this error using prod with this bundle in my composer-dev.

I hope it will help someone.