PathProcessorEntityResourceBC Class not found error after updating to Drupal 8.4 After updating to Drupal 8.4 I now have an error in my browser when trying to load my Drupal site:

Error: Class '\Drupal\rest\PathProcessor\PathProcessorEntityResourceBC' not found in Drupal\Component\DependencyInjection\Container->createService() (line 262 of core/lib/Drupal/Component/DependencyInjection/Container.php)

If I execute a drush or drupal console command the error is the same but with more of a stacktrace. 

I upgraded to Drush 9 which simplifies the error a bit more:

Service 'rest.path_processor_entity_resource_bc' for consumer 'path_processor_manager' does not implement Drupal\Core\PathProcessor\InboundPathProcessorInterface.

It looks like I have some dependency missing somewhere. I'm not sure how to track it down.

I have tried "composer install", "composer update", and removing the vendor directory and re-installing the composer packages but nothing has worked yet.

Has anyone had this error or know how to get the missing PathProcessorEntityResourceBC class?
I have fixed the error by replacing the 'core' directory with the latest 8.4.0 version. I then checked the git log and differences which were found in these files:

modified:   core/scripts/drupal.sh
modified:   core/scripts/password-hash.sh
modified:   core/scripts/rebuild_token_calculator.sh
modified:   core/scripts/run-tests.sh
modified:   core/scripts/update-countries.sh


The differences were similar for each file, for example:

--git a/core/scripts/update-countries.sh b/core/scripts/update-countries.sh


I'm not sure exactly how and why this fixed the missing class, but it seems like I have some differences between the 2 versions of the site. 

I then added the changes into git on a new branch and after switching between the 2 branches I don't get the fatal error any more (with or without the updated files). 

It's possible that something was cached and as my site and drush was broken I was unable to clear the cache, refreshing the core files must have fixed it.