Ruby Set#include? strangeness

set_test.rb

require 'minitest/autorun'
require 'set'

class Foo
  def initialize(s)
    @s = s
  end

  def ==(other)
    @s == other
  end

  def eql?(other)
    @s.eql?(other)
  end

  def hash
    @s.hash
  end
end

module FooEquality
  def ==(other)
    if other.is_a?(Foo)
      other == self
    else
      super(other)
    end
  end

  def eql?(other)
    if other.is_a?(Foo)
      other.eql?(self)
    else
      super(other)
    end
  end
end

class String
  prepend FooEquality
end

class TestSet < MiniTest::Unit::TestCase
  def setup
    @foo = Foo.new("foo")
    @set = Set.new([@foo])
    @hsh = { @foo => true }
  end

  def test_set_include_obj
    assert @set.include?(@foo)
  end

  def test_set_include_str
    assert @set.include?("foo")
  end

  def test_hsh_include_obj
    assert @hsh.include?(@foo)
  end

  def test_hsh_include_str
    assert @hsh.include?("foo")
  end

  def test_eql
    assert @foo.eql?("foo")
  end

  def test_hash
    assert @foo.hash == "foo".hash
  end
end

test.output

Run options: --seed 36646

# Running:

......

Finished in 0.001399s, 4288.7777 runs/s, 4288.7777 assertions/s.

6 runs, 6 assertions, 0 failures, 0 errors, 0 skips