gistfile1.rb

class Thing
  if ENV['DEFINE_NESTED_CLASS']
    class SpecificThing
      def identify
        'nested'
      end
    end
  end
end

class SpecificThing < Thing
  def identify
    'top-level'
  end
end

t = Thing::SpecificThing.new
puts t.identify

# $ ruby /tmp/things.rb
# /tmp/things.rb:17: warning: toplevel constant SpecificThing referenced by Thing::SpecificThing
# top-level
#
# $ DEFINE_NESTED_CLASS=true ruby /tmp/things.rb
# nested