As per http://sbcl10.sbcl.org/materials/jsnell/0001-adjustable-array-transforms.patch and http://rvw.doc.gold.ac.uk/sullivan/cgi-bin/gitweb.cgi?p=sbcl.git;a=shortlog;h=refs/heads/array-type-cleverness
Bug watches keep track of this bug in other bug trackers.