From zefram@fysh.org

The reduce metaoperator iterates through its list of operands in a
way that will be prematurely terminated if the IterationEnd sentinel
appears in the list. This is trivially demonstrated by reducing over
list construction, which ought to amount to list copying​:

$ ./perl6 -e 'say ([,] 2, 3, 4, 5, 6).perl'
(2, 3, 4, 5, 6)
$ ./perl6 -e 'say ([,] 2, 3, IterationEnd, 5, 6).perl'
(2, 3)

Obviously, being a public object that's part of the Iterator API, this can
in general appear in a user's list. The sentinel wouldn't normally be
used as an operand for the kind of operations one would stereotypically
reduce over, but it does have well-defined behaviour for such operators,
which is lost by the premature list termination​:

$ ./perl6 -e 'say 2 * IterationEnd'
X​::Multi​::NoMatch exception produced no message
  in block <unit> at -e​:1
$ ./perl6 -e 'say ([*] 2, IterationEnd, 3)'
2

-zefram

From @jnthn

On Wed Sep 23 13​:19​:16 2015, zefram@​fysh.org wrote​:

The reduce metaoperator iterates through its list of operands in a
way that will be prematurely terminated if the IterationEnd sentinel
appears in the list. This is trivially demonstrated by reducing over
list construction, which ought to amount to list copying​:

$ ./perl6 -e 'say ([,] 2, 3, 4, 5, 6).perl'
(2, 3, 4, 5, 6)
$ ./perl6 -e 'say ([,] 2, 3, IterationEnd, 5, 6).perl'
(2, 3)

Obviously, being a public object that's part of the Iterator API, this can
in general appear in a user's list. The sentinel wouldn't normally be
used as an operand for the kind of operations one would stereotypically
reduce over, but it does have well-defined behaviour for such operators,
which is lost by the premature list termination​:

$ ./perl6 -e 'say 2 * IterationEnd'
X​::Multi​::NoMatch exception produced no message
in block <unit> at -e​:1
$ ./perl6 -e 'say ([*] 2, IterationEnd, 3)'
2

This is rejected for same reason as RT #​126147. Anyone using it for anything other than its intended purpose deserves what they get.

The RT System itself - Status changed from 'new' to 'open'

@jnthn - Status changed from 'open' to 'resolved'

@jnthn - Status changed from 'resolved' to 'rejected'