test4.pp

class mod::config($param) {
}

class mod::uses_config {
  # Why hasn't $mod::config been evaluated yet?
  $var = $mod::config::param
  notify { $var: }
}

Class['mod::config'] -> Class['mod::uses_config']

class mod::main($config) {
  $hash = {
    'mod::config' => $config
  }
  # add mod::config to the catalog with parameters
  create_resources('class', $hash)

  class { 'mod::uses_config': }
}

class {
  'mod::main':
    config => {
      param => 'doot'
    }
}

test5.pp

class config {
  $x = 'abc'
}

class uses_config {
  Class['config'] -> Class['uses_config']
  $x = $config::x
  notify { $x: }
}

include uses_config
include config