Bugs item #8539, was opened at 2007-02-09 17:27
You can respond by visiting: 
http://rubyforge.org/tracker/?func=detail&atid=1698&aid=8539&group_id=426

Category: Core
Group: 1.8.2
Status: Open
Resolution: None
Priority: 3
Submitted By: Gregor Cramer (gcramer)
Assigned to: Nobody (None)
Summary: String:upto loops forever if argument is modified inside block

Initial Comment:
$ ruby -e '"a".upto("b") {|s| s[0] = "c" }'

loops forever!

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You can respond by visiting: 
http://rubyforge.org/tracker/?func=detail&atid=1698&aid=8539&group_id=426