8.5.0-stable was released just a few days ago, and I imagine other folks may run into the same installation problem as I did when attempting to upgrade. If you think this might be you, then read on...
Problem: When trying to upgrade from any previous version of Drupal core to
~8.5, Composer delivers
8.5.0-beta1 instead of the latest stable version.
Answer: Drupal core
8.5.0-beta2 introduced dependencies on some PHP extensions, which can't simply be downloaded from Packagist like other requirements. If you use a local development tool like Lando, DDEV, Docksal, or the like instead of a local stack, your local version of PHP may not have the necessary extensions. In that case, when you run composer locally instead of inside your development environment, the very latest version of Drupal it can grab is
8.5.0-beta1, the version before those PHP dependencies were introduced.
The quick fix: Just run the composer update command inside your local development environment. E.g.:
lando composer update drupal/core webflo/drupal-core-require-dev symfony/* --with-dependencies