Gem::Resolver#all_versions_for

# File tmp/rubies/ruby-4.1.0-preview1/lib/rubygems/resolver.rb, line 198
def all_versions_for(package)
  versions = @sorted_versions[package].reverse # highest first
  name = package.to_s

  if (skip_dep_gems = skip_gems[name]) && !skip_dep_gems.empty?
    # Conservative mode: float the already-installed (skip) versions to the
    # front so the solver prefers them. This sets *preference* only (it feeds
    # the strategy's version-index map); it does not restrict availability, so
    # every version stays selectable via versions_for. When an installed
    # version is made impossible by a downstream conflict, the solver
    # backtracks to a newer version instead of failing. Molinillo instead
    # hard-restricted the candidate set to skip versions and raised.
    #
    # This reaches the same outcome as Bundler (upgrade-over-raise) for the
    # common single-blocked-gem case, though the mechanism differs: Bundler
    # hard-pins locked gems and selectively unlocks + re-solves on conflict,
    # whereas we float as a preference and let PubGrub backtrack in one solve.
    # The float can therefore over-upgrade when several installed gems are
    # jointly involved in a conflict; that outcome-level divergence is
    # accepted (see test_conservative_upgrades_when_installed_blocked).
    skip_versions = skip_dep_gems.map(&:version)
    preferred, rest = versions.partition {|v| skip_versions.include?(v) }
    preferred + rest
  else
    # Prefer already-installed versions to avoid unnecessary upgrades
    installed_versions = @all_specs[name].
      select {|s| s.is_a?(Gem::Resolver::InstalledSpecification) }.
      map(&:version)
    if installed_versions.any?
      preferred, rest = versions.partition {|v| installed_versions.include?(v) }
      preferred + rest
    else
      versions
    end
  end
end