Drupal 8 core update via Composer

Troubleshooting Composer can be frustrating. Composer is doing many complex things for you, and there's a degree of understanding required to effectively troubleshoot issues that arise.

This page is intended to provide troubleshooting techniques and to enumerate common problems. It is not intended to be exhaustive. If you're having a problem not described here, performing an internet search for your specific error message may be more helpful.