Gem::DependencyList#spec_predecessors

# File tmp/rubies/ruby-3.0.5/lib/rubygems/dependency_list.rb, line 193
def spec_predecessors
  result = Hash.new {|h,k| h[k] = [] }

  specs = @specs.sort.reverse

  specs.each do |spec|
    specs.each do |other|
      next if spec == other

      other.dependencies.each do |dep|
        if spec.satisfies_requirement? dep
          result[spec] << other
        end
      end
    end
  end

  result
end