IPAddr#ipv4_compat

# File tmp/rubies/ruby-4.0.0/lib/ipaddr.rb, line 380
def ipv4_compat
  warn "IPAddr\##{__callee__} is obsolete", uplevel: 1 if $VERBOSE
  if !ipv4?
    raise InvalidAddressError, "not an IPv4 address: #{@addr}"
  end
  clone = self.clone.set(@addr, Socket::AF_INET6)
  clone.instance_variable_set(:@mask_addr, @mask_addr | 0xffffffffffffffffffffffff00000000)
  clone
end