class Gem::Resolver::InstallerSet

def add_always_install(dependency)
  request = Gem::Resolver::DependencyRequest.new dependency, nil
  found = find_all request
  found.delete_if do |s|
    s.version.prerelease? && !s.local?
  end unless dependency.prerelease?
  found = found.select do |s|
    Gem::Source::SpecificFile === s.source ||
      Gem::Platform.match_spec?(s)
  end
  found = found.sort_by do |s|
    [s.version, Gem::Platform.sort_priority(s.platform)]
  end
  newest = found.last
  unless newest
    exc = Gem::UnsatisfiableDependencyError.new request
    exc.errors = errors
    raise exc
  end
  unless @force
    found_matching_metadata = found.reverse.find do |spec|
      metadata_satisfied?(spec)
    end
    if found_matching_metadata.nil?
      ensure_required_ruby_version_met(newest.spec)
      ensure_required_rubygems_version_met(newest.spec)
    else
      newest = found_matching_metadata
    end
  end
  @always_install << newest.spec
end