Support for the Ruby 2.4 series has ended. See here for reference.
Hash with completion search feature. See OptionParser::Completion.
Completion for hash key.
 
               # File optparse.rb, line 899
def match(key)
  *values = fetch(key) {
    raise AmbiguousArgument, catch(:ambiguous) {return complete(key)}
  }
  return key, *values
end