This is what I’m trying to do:

class Foo; end

Foo.class_exec do
  class_variable_set(:@@x, 42)
  def get
    @@x
  end
end

puts Foo.new.get

I’m getting (Ruby 3.2.1):

$ ruby a.rb
a.rb:6:in `get': class variable access from toplevel (RuntimeError)
    from a.rb:10:in `<main>'

How can I read this @@x, which I just set?