Sylius: Test environment broken with php 7.2.20

Sylius version affected: >=1.4.2

Description

We encountered this error when we tried to clear the cache:

[Symfony\Component\Debug\Exception\FatalErrorException]
Error: During class fetch: Uncaught ReflectionException: Class HWI\Bundle\OAuthBundle\Connect\AccountConnectorInterface not found
  in /var/www/html/sylius/vendor/sylius/sylius/src/Sylius/Bundle/CoreBundle/OAuth/UserProvider.php:36
Stack trace:
#0 /var/www/html/sylius/vendor/symfony/debug/DebugClassLoader.php(160): require('/var/www/html/s...')
#1 [internal function]: Symfony\Component\Debug\DebugClassLoader->loadClass('Sylius\\Bundle\\C...')
#2 [internal function]: spl_autoload_call('Sylius\\Bundle\\C...')
#3 /var/www/html/sylius/vendor/symfony/config/Resource/ClassExistenceResource.php(78): class_exists('Sylius\\Bundle\\C...')
#4 /var/www/html/sylius/vendor/symfony/dependency-injection/ContainerBuilder.php(353): Symfony\Component\Config\Resource\ClassExistenceResource->isFresh(0)
#5 /var/www/html/sylius/vendor/symfony/dependency-injection/Compiler/AutowirePass.php(336): Symfony\Component\DependencyInjection\ContainerBuilder->getReflectionClass('Sylius\\Bundle\\C...', false)
#6 /var/www/html/sylius/vendor/symfony/dep      

I found out that it’s due to Symfony trying to load the Sylius\Bundle\CoreBundle\OAuth\UserProvider, when in the test environment, which implements the HWI\Bundle\OAuthBundle\Connect\AccountConnectorInterface. This interface comes with the hwi/oauth-bundle which doesn’t get installed by Sylius-standard.

The only reason it worked before was a bug in Php that made it possible to define a class that implements a non existent interface.

This bug got fixed in Php 7.2.20. https://www.php.net/ChangeLog-7.php#7.2.20 https://bugs.php.net/bug.php?id=76980

Steps to reproduce

Install Php 7.2.20 and try to clear the cache in the test environment.

Possible Solution

Two solutions come to mind:

1. Require hwi\oauth-bundle in Sylius-standard
2. Only load the service definition if the hwi\oauth-bundle is installed like here: https://github.com/Sylius/Sylius/blob/defb389775303ec9fffa1dab7bdbd594b3de841e/src/Sylius/Bundle/CoreBundle/DependencyInjection/SyliusCoreExtension.php#L85-L92