Demo on why `case x = @x` is necessary

arcane_ivar.cr

class Foo
  @stuff : String | Int32
  
  @waiter = Channel(Nil).new
  
  def initialize(@stuff)
    spawn do
      @waiter.receive # Simulate waiting for some IO
      @stuff = "Oh noes!" # And store the result of it
    end
  end
  
  def do_stuff
    case @stuff
    when String
      puts "Don't mind me."
    when Int32
      puts "@stuff is a #{@stuff.class}"  # <- @stuff is a Int32, everything is great
      @waiter.send(nil)                   # <- Someone calls another fiber
      puts "@stuff is a #{@stuff.class}"  # <- @stuff is still a Int32 .. or is it?
    end
  end
end

Foo.new(123).do_stuff