Symfony version(s) affected: 4.2.2

Hello!

Description
I'm in pain to make the well known implicit ParamConverter trick work with Symfony 4.2.2:

/**
 * @Route("/{id}", name="admin_post_show")
 */
public function show(Post $post)

It tells that the Post argument cannot be autowired.

How to reproduce
Just follow the exemple given in the best practices guide.

Possible Solution
No idea, sorry!

Additional context
Could relate to #29819 (not sure as the ticket is closed).

Thank you for your feedback!