Problem/Motivation
Symfony 3 is here, we decided to adopt it for Drupal 8 in #2395443: [policy, no patch] Follow symfony 2.7 or 3.0., this is the question.
Proposed resolution
While the current patch passes tests, Drush must be updated to use Symfony 3.2 as well - see this PR: https://github.com/drush-ops/drush/pull/2477
Updating to Symfony 3.2 means that Drush will need to update its PHP requirement from PHP 5.4 to PHP 5.5.9 to match Symfony's. This likely requires a new major release of drush, since people on older PHP installs should not update to that new version. There may also be further contributed module updates for Symfony 3 compatibility.
Due to this, we're planning to commit this patch to 8.4.x shortly, and monitor any additional breakage over time. The hope is that Drupal 8.4.0 will ship with an up-to-date release of Symfony 3, but Drupal 8.3.x will stay on Symfony 2.8.* to avoid disruption to contrib and sites.
Remaining tasks
Fix #2712633: Update symfony-cmf/routing to 1.4.0.
Fix #2712637: Update stack/builder for Symfony 3 compatibility.
Fix #2712643: Update behat/* to 1.7.1 and fabpot/goutte to 3.1.2.
Fix #2720869: Remove the use of deprecated CssSelector and use CssSelectorConverter instead.
Fix #2720891: Replace ContainerAware with ContainerAwareTrait
Fix #2721139: Replace deprecated files ParameterBag usage.
Fix #2721179: Replace deprecated Symfony ExecutionContextInterface.
Fix #2726645: Do not @deprecate EntityManager until all usages are removed (helps the Symfony 3 update).
Fix #2728815: Batch API uses request attributes instead of query in Symfony 3.
Fix #2730129: DrupalKernel must never persist service_container for Symfony 3 update.
Fix #2682373: Implement ContainerAwareEventDispatcher::getListenerPriority().
Fix #2776105: Update asm89/stack-cors for Symfony 3 compatibility and revert changes introduced by #1869548.
Fix #2823687: Use isMethodCacheable() instead of isMethodSafe() when checking for GET/HEAD.
Fix fails.
Review.
Commit.
User interface changes
None
API changes
Hopefully none
Data model changes
Not yet