I ran into the usual situation of Composer hair-pulling when updating to Drupal 8.5.
I received errors related to Symfony 3.2.14 not being the release needed. And so it turns out, that Symfony 3.4.5 is the release being used.
The problem was a Catch-22. Composer did not want to update Symfony because someone was requiring 3.2.14.
I tried updating individually, requiring, even updating both at the same time, but in the end, what worked for me was the following:
- Edit composer.lock in the Drupal root and search/replace 3.2.14 with 3.4.5
- composer require drupal/core:~8.5 --with-dependencies
The result is Composer and its components being updated to 3.4.5 and Drupal happily updating to 8.5.