# Minimal enumerable class
class List
  include ::Enumerable

  def initialize(*items)
    @items = items
  end

  def each(&block)
    @items.each(&block)
  end
end

# Works with array
a = [2]
puts a.find_index(2.0).inspect
#0 # mri
#0 # jruby

# Doesn't work with enumerable
b = List.new(2)
puts b.find_index(2.0).inspect
#0   # mri
# nil # jruby

# Works in another direction
c = List.new(2.0)
puts c.find_index(2).inspect
#0 # mri
#0 # jruby

# Works with comparison in block
d = List.new(2)
puts c.find_index { |i| i == 2.0 }.inspect
#0 # mri
#0 # jruby