test.pp

class dhcp ($foo) {
  notice {"foo is ${foo}":}
}

class dhcp::pool ($bar=$dhcp::foo) {
  notice {"bar is ${bar}":}
  notice {inline_template('has_variable is <%= has_variable?("dhcp::foo") %>'):}
}

class { 'dhcp':
  foo => 'klassfoo'
}

class { 'dhcp::pool': }