OptionParser::Completion.candidate

# File tmp/rubies/ruby-3.1.3/lib/optparse.rb, line 445
def self.candidate(key, icase = false, pat = nil, &block)
  pat ||= Completion.regexp(key, icase)
  candidates = []
  block.call do |k, *v|
    (if Regexp === k
       kn = ""
       k === key
     else
       kn = defined?(k.id2name) ? k.id2name : k
       pat === kn
     end) or next
    v << k if v.empty?
    candidates << [k, v, kn]
  end
  candidates
end