The DI component is acting weird in 4.0.0. I'm currently working on facile-it/paraunit#112 to allow Symfony 4, and the build is broken only with Symfony 4, no deprecations on 3.4.

I've already fixed the private/public service deprecations, and I'm currently debugging the issue. I found the root cause in the RemoveUnusedDefinitionsPass: many private services are considered unused and so removed from the DI. Maybe it's because they are inlined? If that's true, I'm not able to make them public in tests afterward to retrieve and inspect them.