bar.rb

class Foo
  puts "defining Foo::Bar"
  class Bar
    sleep 5
  end
  puts "done defining Foo::Bar"
end

test.rb

class Foo
  autoload :Bar, 'bar.rb'
end

a = Thread.new do
  puts "A accessing const"
  Foo::Bar # should sleep for 5 seconds
  puts "A finished with const"
end

b = Thread.new do
  sleep 2
  if defined?(Foo::Bar)
    puts "B accessing const"
    Foo::Bar # should return immediately, before A has finished
    puts "B finished with const"
  else
    puts "B sees Foo::Bar not defined yet"
  end
end

[a,b].each(&:join)