In tcl8.5.6 it succeeds. This is a problem.
In tcl8.4.19 it produced an error:
can't read "i": no such variable
(reading value of variable to increment)
invoked from within
(file "incr.tcl" line 2)
Donal K. Fellows
This was a deliberate change in 8.5. See http://tip.tcl.tk/215.html for reasoning.