From @masak

<felher> nom​: my @​array; say @​array[4];
<p6eval> rakudo 1a468d​: OUTPUT�Any()␤�
<felher> nom​: my @​array; say @​array[*-1];
<p6eval> rakudo 1a468d​: OUTPUT�Failure.new(exception =>
X​::AdHoc.new(payload => "Cannot use negative index -1 on Array"))␤�
* felher hoped for 'Any', too
<masak> probably it's * getting expanded as 0, and then the failure
triggers (wrongly)
<masak> it could be argued that it should never trigger for whatever-y
expressions.
* masak submits rakudobug
<felher> \o/

From @usev6

The current error message is kind of funny and supports masak's assumption that * getting expanded as 0, and then the failure triggers (wrongly)​:

$ perl6-m -e 'my @​array; say @​array[*-1]'
Unsupported use of [-1] subscript to access from end of Array; in Perl 6 please use [*-1]

(An exception of type X​::Subscript​::FromEnd is thrown.)

@usev6 - Status changed from 'new' to 'open'

From @pmichaud

The fact that an Exception is thrown is correct -- negative subscripts are not allowed unless the subscript has been declared modular (from S09).

The Exception message needs to be changed. STD.pm6 has "in Perl 6 please use [*-1]" when it can statically determine that a negative subscript has been passed. For dynamically computed negative subscript, the message can hint that something like *-1 might be useful but definitely shouldn't recommend using the very thing that just failed.

Better would be to produce a different error message when a negative subscript is generated from a Code (WhateverCode) object.

Pm

From @usev6

TimToady++ recently worked on this (rakudo/rakudo@10aa8bf41c) and it should be in good shape now​:

$ perl6 -e 'my @​array; say @​array[*-1]'
Calculated index (-1) is negative, but Array allows only 0-based indexing
  in method gist at src/gen/m-CORE.setting​:13776
  in sub say at src/gen/m-CORE.setting​:16510
  in block <unit> at -e​:1

The exception is now of type X​::Subscript​::Negative and the error message no longer recommends to use [*-1]. It does so when using [-1]​:

$ perl6-m -e 'my @​array; say @​array[-1]'
===SORRY!=== Error while compiling -e
Unsupported use of a negative -1 subscript to index from the end; in Perl 6 please use a function such as *-1
at -e​:1
------> my @​array; say @​array[-1]⏏<EOL>

There are tests in S02-types/array.t. I'm closing this ticket now.

@usev6 - Status changed from 'open' to 'resolved'

From @lizmat

On 25 Nov 2014, at 22​:37, Christian Bartolomaeus via RT <bugs-comment@​bugs6.perl.org> wrote​:

TimToady++ recently worked on this (rakudo/rakudo@10aa8bf41c) and it should be in good shape now​:

$ perl6 -e 'my @​array; say @​array[*-1]'
Calculated index (-1) is negative, but Array allows only 0-based indexing
in method gist at src/gen/m-CORE.setting​:13776
in sub say at src/gen/m-CORE.setting​:16510
in block <unit> at -e​:1

The exception is now of type X​::Subscript​::Negative and the error message no longer recommends to use [*-1]. It does so when using [-1]​:

$ perl6-m -e 'my @​array; say @​array[-1]'
===SORRY!=== Error while compiling -e
Unsupported use of a negative -1 subscript to index from the end; in Perl 6 please use a function such as *-1
at -e​:1
------> my @​array; say @​array[-1]⏏<EOL>

There are tests in S02-types/array.t. I'm closing this ticket now.

That’s because the first one is a run-time exception, at which time we don’t know anymore whether the calculated index involved * or not (a patch to that effect was reverted by TimToady++). So the error message cannot mention *, because it could have been used already.

The second one is a compile time warning because you’re using a literal “-1”. That *clearly* doesn’t use a *, so it can be mentioned then.

Liz