From @AlexDaniel

Code for 32-bit system​:
my @​a[-9999999999999999]; say +@​a

Result​:
-9999999999999999

Woohoo! Array with negative size!

On 64-bit it is a bit more tricky​:
my @​a[-9223372036854775808,-2]; say +@​a

Result​:
-9223372036854775808

Just disallow negative numbers and that's it.

From @AlexDaniel

Oh, and don't forget about zeros. Zero has to be disallowed as well​:

Code​:
my @​a[0]; say @​a[0]

Result​:
Index 0 for dimension 1 out of range (must be 0..-1)
  in block <unit> at -e line 1

From @AlexDaniel

Fixed in rakudo/rakudo#737 , tests needed.

From @usev6

I added the above examples as tests to S02-types/array-shapes.t with commit Raku/roast@a2617480f4

I'm closing this ticket as 'resolved'.

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