@a[-1;0]:exists returns True

[HarmtH]

The Problem

> my @a; my $neg=-1; say @a[$neg;0]:exists
True

Expected Behavior

It returns False

Environment

• Operating system: Ubuntu 18.04
• Compiler version (perl6 -v): This is Rakudo version 2018.11 built on MoarVM version 2018.11 implementing Perl 6.d.

[lizmat]

Fixed with ca7408e9fa , tests added with Raku/roast@4e732c620b

[HarmtH]

I was thinking a bit about this issue and about fix ca7408e9fa

Wouldn't the array-of-array notation still be broken?

my @a; my $neg=-1; say @a[$neg][0]:exists
True

I think the real issue is Failures 'exist'. Maybe Failures should behave more like Nil. As a Failure is a Nil, that would also make sense. Although I don't know what the language specs say about this.

Failure is a Nil is a Cool is an Any is a Mu

> Failure.new[0]:exists
True
> Nil.new[0]:exists
False
> Cool.new[0]:exists
True
> Any.new[0]:exists
True
> Mu.new[0]:exists
Type check failed in binding to parameter '<anon>'; expected Any but got Mu (Mu.new)
  in block <unit> at <unknown file> line 1