Issue #1392 has been updated by Jason Earl.
My trunk build is: ruby 1.9.2dev (2010-01-27 trunk 26434) [x86_64-linux]
Your clone code above doesn't leak either for me.
However both of these simplified scenarios leak:
./ruby -e 'loop { Class.new.__send__ :include, Module.new }'
./ruby -e 'loop { Module.new.include Module.new }'
While I'm no C guru, I'm guessing the issue will be relating to include_class_new in class.c or at least it doesn't look like a garage collection issue given that your clone seems OK.
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http://redmine.ruby-lang.org/issues/show/1392
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