Demonstrating unexpected behavior in cancancan

demo.rb

require 'cancan'

Hat = Struct.new(:color)

class MyAbility
  include CanCan::Ability

  def initialize
    can :see, Hat, color: "red"
  end
end

# ---

ability = MyAbility.new

if ability.can?(:see, Hat.new("red"))
  $stderr.puts "✅ You can see a particular red hat, as expected"
else
  $stderr.puts "❌ You can't see a particular red hat. Weird."
end

if ability.can?(:see, Hat.new("blue"))
  $stderr.puts "❌ You can see a particular blue hat. That shouldn't happen."
else
  $stderr.puts "✅ You can't see a particular blue hat. Good."
end

if ability.can?(:see, Hat, color: "red")
  $stderr.puts "✅ You can see any red hat, as expected"
else
  $stderr.puts "❌ You can't see any red hat. Weird."
end

if ability.can?(:see, Hat, color: "blue")
  $stderr.puts "❌ You can see any blue hat. That shouldn't happen."
else
  $stderr.puts "✅ You can't see any blue hat. Good."
end

if ability.can?(:see, Hat)
  $stderr.puts "❌ You can see any hat. That shouldn't happen."
else
  $stderr.puts "✅ You can't see just any hat. Good."
end

Gemfile

source 'https://rubygems.org'

gem 'cancancan'

Issue is here: cancancan/issues#277. Summary: working as intended.