Gemfile

source 'https://rubygems.org'

gem 'rspec', group: :test

ruby "2.3.0"

my_flatten.rb

module MyFlatten

  def self.included(base)
    if base.ancestors.include?(Array)

      base.class_eval {
        def my_flatten
          result = []

          self.each do |object|
            object_has_my_flatten = object.class.ancestors.include?(MyFlatten) && object.respond_to?(:my_flatten)
            object_has_my_flatten ? result += object.my_flatten : result << object
          end

          result
        end
      }

    else
      raise Exception.new('MyFlatten included in non-Array class.')
    end
  end

end

my_flatten_spec.rb

ENV['BUNDLE_GEMFILE'] ||= File.expand_path('../Gemfile', __FILE__)
Bundler.require(:test)

$LOAD_PATH.unshift '.'
require 'my_flatten'

describe MyFlatten do

  it "defines #my_flatten without exception when included into Array or an Array subclass" do
    class MyArray < Array; end

    expect(Array.include(MyFlatten)).to be_truthy
    expect(MyArray.include(MyFlatten)).to be_truthy

    expect(Array.new.respond_to?(:my_flatten)).to be_truthy
    expect(MyArray.new.respond_to?(:my_flatten)).to be_truthy
  end

  it "raises an excetion & doesn't define #my_flatten when included into a non Array class" do
    expect { String.include(MyFlatten) }.to raise_error('MyFlatten included in non-Array class.')
    expect(String.new.respond_to?(:my_flatten)).to be_falsy
  end

  it "defines #my_flatten to behave like array#flatten" do
    arrays = [ [], [1,2,3], [[1,2,[3]],4], [nil, {}, '', [[1]]] ]
    arrays.each { |array| expect(array.my_flatten).to eq(array.flatten) }
  end

end