Issue #1732 has been updated by Run Paint Run Run.
Thanks, Ryan. :-)
The remaining difference between Array#join and Enumerable#join w.r.t taint/trust involves empty data structures.
$ ruby -e 'p Array.new.untrust.join(":").untrusted?'
false
$ ruby -e 'p Hash.new.untrust.join(":").untrusted?'
true
$ ruby -e 'p Array.new.taint.join(":").tainted?'
false
$ ruby -e 'p Hash.new.taint.join(":").tainted?'
true
$ ruby -v
ruby 1.9.2dev (2009-07-08 trunk 23987) [i686-linux]
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http://redmine.ruby-lang.org/issues/show/1732
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http://redmine.ruby-lang.org