windows.cr

class App
  @@windows : Array(Window) = [] of Window

  def self.windows=(windows)
    @@windows = windows
  end
end

class Window; end

class ShapedWindow < Window; end

class CircularWindow < ShapedWindow; end
class SquareWindow < ShapedWindow; end

App.windows = [CircularWindow.new, SquareWindow.new]

# class variable '@@windows' of App must be (Array(Window) | Nil), not Array(ShapedWindow)