While trying to install civicrm-core by command:
composer require civicrm-core'
i get the following error:
civicrm/civicrm-core 5.12.x-dev requires zetacomponents/mail dev-1.7-civi -> no matching package found
You need to run:
composer config repositories.zetacomponents-mail vcs https://github.com/civicrm/zetacomponents-mail.git
though you have to provide git token.
You just have to do it once per server you're on.