On JRuby 1.7.4 defined? reports that BasicObject is undefined in 1.9 mode.

jruby-1.7.4 :018 > defined? ::BasicObject
 => nil
jruby-1.7.4 :019 > BasicObject
 => BasicObject

Yet it is defined and usable... Curiously when execing the same code from -e it works...

~$ ruby -v
jruby 1.7.4 (1.9.3p392) 2013-05-16
~$ ruby -e 'p defined?(::BasicObject)'
"constant"
~$ irb
jruby-1.7.4 > defined?(::BasicObject)
 => nil