`extend self` vs `extend A::B`

modules_extending.rb

module A
  # evil module:
  module A
    module B
      def foo
        :evil_foo
      end
    end
  end
  # good module:
  module B
    def foo
      :good_foo
    end
    extend A::B
  end
end

A::B.foo
# => :evil_foo
# it is not :good_foo!