refinement_enumerator_fail.rb

module Foo
  refine Hash do
    def each_smtn(&block)
      return to_enum(__method__) unless block
      block.call(1)
      block.call(2)
    end
  end
end

using Foo

Hash.new.each_smtn.each { |x| p x }
#=> undefined method `each_smtn' for {}:Hash (NoMethodError)

You can do this instead:

return Enumerator.new { |y| each_smtn { |x| y << x } } unless block