class Bundler::Source::RubygemsAggregate

def double_check_for_index(idx, dependency_names)

each spec we found, we add all possible versions from all sources to the index.
of Foo specifically depends on a version of Bar that is only found in source B. This ensures that for
but will not have found any versions of Bar from source B, which is a problem if the requested version
sources A and B. At this point, the API request will have found all the versions of Bar in source A,
Suppose the gem Foo depends on the gem Bar. Foo exists in Source A. Bar has some versions that exist in both 
def double_check_for_index(idx, dependency_names)
  pinned_names = source_map.pinned_spec_names
  names = :names # do this so we only have to traverse to get dependency_names from the index once
  unmet_dependency_names = lambda do
    return names unless names == :names
    new_names = sources.all_sources.map(&:dependency_names_to_double_check)
    return names = nil if new_names.compact!
    names = new_names.flatten(1).concat(dependency_names)
    names.uniq!
    names -= pinned_names
    names
  end
  sources.all_sources.each do |source|
    source.double_check_for(unmet_dependency_names)
  end
end