%0 and &0 should probably be syntax errors

[lizmat]

This started off with:

$ perl6 -e 'my $width = 4; printf("%0{$width}d", 42)'
Use of uninitialized value element of type Any in string context.
Methods .^name, .perl, .gist, or .say can be used to stringify it to something meaningful.
  in block <unit> at -e line 1
Your printf-style directives specify 0 arguments, but 1 argument was supplied
  in block <unit> at -e line 1

which, in my view, is a completely legit way of specifying a variable width printf format.

Turns out, this does not do what you mean. Apparently, this is short for %($/[0]).

$ perl6 -e '"abcd" ~~ m/(\w)+/; say %0'
{a => ｢b｣, c => ｢d｣}

which looks nice, but with an odd number of captures, becomes:

$ perl6 -e '"abc" ~~ m/(\w)+/; say %0'
Odd number of elements found where hash initializer expected:
Found 3 (implicit) elements:
Last element seen: Match.new(list => (), from => 2, hash => Map.new(()), made => Any, pos => 3, orig => "abc")
  in block <unit> at -e line 1

It even gets weirder with &0:

$ perl6 -e '"abc" ~~ m/(\w)+/; say &0'
[｢a｣ ｢b｣ ｢c｣]

which, btw is also what @0 and $0 give.

I propose that %N and &N (where N is a 0..Inf) do not interpolate, and become syntax errors outside of interpolation.

[AlexDaniel]

FWIW, I didn't even know that @0 %0 &0 are allowed.

[lizmat]

I didn't either until someone pointed it out to me in a mail. @0 kinda makes sense, for the given example. %0 and &0 definitely do not, in my opinion.