putting alias_method above module_function allows the alias to be called

wtf.rb

~$ ruby -v                                                                                                                                       *[master]
ruby 2.1.2p95 (2014-05-08 revision 45877) [x86_64-darwin13.0]
~$ irb                                                                                                                                           *[master]
irb(main):001:0> module I18nHelper
irb(main):002:1>   def custom_translate
irb(main):003:2>     "boo"
irb(main):004:2>   end
irb(main):005:1>
irb(main):006:1*   module_function :custom_translate
irb(main):007:1>   alias_method :ct, :custom_translate
irb(main):008:1> end
=> I18nHelper
irb(main):009:0>
irb(main):010:0* class TestI18nHelper
irb(main):011:1>   include I18nHelper
irb(main):012:1> end
=> TestI18nHelper
irb(main):013:0>
irb(main):014:0* TestI18nHelper.new.custom_translate
NoMethodError: private method 'custom_translate' called for #<TestI18nHelper:0x007f9f528f7cc8>
  from (irb):14
  from /Users/tim/.rubies/ruby-2.1.2/bin/irb:11:in '<main>'
irb(main):015:0> TestI18nHelper.new.ct
NoMethodError: private method 'ct' called for #<TestI18nHelper:0x007f9f528e4ad8>
  from (irb):15
  from /Users/tim/.rubies/ruby-2.1.2/bin/irb:11:in '<main>'
irb(main):016:0> module I18nHelper
irb(main):017:1>   def custom_translate
irb(main):018:2>     "boo"
irb(main):019:2>   end
irb(main):020:1>
irb(main):021:1*   # now with alias_method first...
irb(main):022:1*   alias_method :ct, :custom_translate
irb(main):023:1>   module_function :custom_translate
irb(main):024:1> end
=> I18nHelper
irb(main):025:0>
irb(main):026:0* class TestI18nHelper
irb(main):027:1>   include I18nHelper
irb(main):028:1> end
=> TestI18nHelper
irb(main):029:0>
irb(main):030:0* TestI18nHelper.new.custom_translate
NoMethodError: private method 'custom_translate' called for #<TestI18nHelper:0x007f9f52836fa0>
  from (irb):30
  from /Users/tim/.rubies/ruby-2.1.2/bin/irb:11:in '<main>'
irb(main):031:0> TestI18nHelper.new.ct
=> "boo"
irb(main):032:0>

as @vowel_boy notes, alias_method copies the method as it stands. so if it comes after module_function (which makes the method available to the module, but makes it private in any class it's included into), the private status gets copied also.