When performing an emerge w/ emptytree option, if I understand the process correctly, portage will attempt to install all software at the exact cat/pkg atom as exists on a system at the time. However, emerge will fail if it is unable to download that exact cat/pkg atom. The problem only comes apparent after emerge fails deep into a rebuild of a multitude of packages and the user is not aware of admins upgrading to a new version and removed the outdated as part of distribution maintenance, even after user action to update.
Feature request: during "gathering" phase of emerge with emptytree option, determine that atom on system cannot first be upgraded. If `–ask` is used, offer upgrade option rather than restricting user to rebuild-only at existing cat/pkg atom.