A Ruby quirk with include. Quite annoying.

include_constants.rb

module A
  module B
  end
  module Wrapper
    module C
    end
  end

  #Cite:
  #When this module is included in another, Ruby calls append_features in this
  #module, passing it the receiving module in mod. Ruby’s default
  #implementation is to add the constants, methods, and module variables of
  #this module to mod if this module has not already been added to mod or one
  #of its ancestors. See also Module#include.

  #But it's not that easy...

  include Wrapper
end

p A.constants   # => [:B, :Wrapper, :C]
p A::B          # => A::B

p A::C          # => A::Wrapper::C

module A
  p B           # => A::B
end

module A
  p C           # => A::Wrapper::C
end

module A
  module Bracket
    p B         # => A::B
    #p C (see bottom)
  end
end

# Manual constant copying, from A::Wrapper
module X
  A::Wrapper.constants.each do |sym|
    const_set sym, A::Wrapper.const_get(sym)
  end

  p C           # => A::Wrapper::C
  # Quirk 1:
  p defined?(B) # => nil
end

# Manual constant copying, from A. Works as well.
module Y
  A.constants.each do |sym|
    const_set sym, A.const_get(sym)
  end

  p B           # => A::B
  p C           # => A::Wrapper::C
end

# This was a surprise to me:
module A
  module Bracket
    # Quirk 2:
    p C #raises uninitialized constant A::Bracket::C (NameError)
  end
end