Different implementation of nqp::index for Moar and [JVM] (negative startposition)

[coke]

Originally opened as https://rt.perl.org/Ticket/Display.html?id=127913 by Christian Bartolomaeus

> The following discussion from november 2015 was linked in ticket
> https://rt.perl.org/Ticket/Display.html?id=125784 which I'm going
> to close.
> 
> I'm opening this separate ticket for the differing behaviour 
> of rakudo-moar and rakudo-jvm with regard to using a negative
> startposition for index (and rindex).
> 
> <bartolin> lizmat: earlier today I found this differing behaviour 
> for moar and jvm:
> <bartolin> r: say index("xxy", "y", -1)
> <camelia> rakudo-moar: OUTPUT«Nil␤»
> <camelia> ..rakudo-jvm 0b6328: OUTPUT«2␤»
> <bartolin> do you think moar is right here?
> * lizmat looks into it
> <bartolin> lizmat++
> <lizmat> r: use nqp; say nqp::index("xxy","y",-1)
> <camelia> rakudo-jvm 0b6328: OUTPUT«2␤»
> <camelia> ..rakudo-moar: OUTPUT«-1␤»
> <lizmat> looks like a difference in implementation of nqp::index
> <lizmat> r: use nqp; say nqp::index("xxy","y",0)
> <camelia> rakudo-moar, rakudo-jvm 0b6328: OUTPUT«2␤»
> <lizmat> bartolin: seems worthy of an NQP ticket
> <lizmat> or perhaps just a rakudobug  :-)
> <bartolin> lizmat: and what do you think is the "better" answer? 
> Perl 5 allows negative start positions, if I'm not mistaken.
> <lizmat> yes, but P5 also allows to use -1 to indicate from end
> <lizmat> so perhaps it should just fail on a negative position
> <bartolin> I see. I'll open a RT for this (probably a bit later).
> <bartolin> thanks for looking!

With a followup narrowing this down to pure nqp behavior:

> Sorry, I got a bit confused:
> 
> The different behavior for routines index and rindex with negative startposition between rakudo-moar and rakudo-jvm was fixed (see https://rt.perl.org/Ticket/Display.html?id=126700).
> 
> What's still relevant is the different implementation of nqp::index:
> 
> $ perl6-m -e 'use nqp; say nqp::index("xxy","y",-1)'
> -1
> 
> $ perl6-j -e 'use nqp; say nqp::index("xxy","y",-1)'
> 2
> 
> Output generated with Rakudo version 2016.03-110-g5bfc8fb.

Which is still happening today:

15:46 < unmatched}> r: use nqp; say nqp::index("xxy","y",-1)
15:46 < camelia> rakudo-moar 3bea9d: OUTPUT«-1␤»
15:46 < camelia> ..rakudo-jvm cd19db: OUTPUT«2␤»