From @LLFourn

Delegating array behaviour to an attribute is a common pattern.

class Foo {
  has @​!interal handles <AT-POS EXISTS-POS elems end iterator>
}

But the above is easy to get wrong. And is wrong. It doesn't work for [*-1]
like access because internally rakudo is using .cache.elems.

'class Foo does Positional { has $.val handles <AT-POS EXISTS-POS elems end
iterator>; }; my $f = Foo.new(val => [5,6]); say $f[1], $f[*-1]' # 5,6
(should be 6,6)

This RT is to suggest that there should be some way to say delegate "all
the things that would make &postcircumfix​:sym<[ ]> work" (and likewise for
{ }) and perhaps other things. These "delegation groups" would be
implementation independent, ie different Perl 6 implementations could
choose to delegate different sets of methods to get [] to work properly.

Of course the right way to do this most of the time is to do class Foo is
Array { ... } which works fine. But sometimes has-a relationships are
preferable to is-a.

So the questions to be answered are​:

1. is this a good idea?
2. how do we do this using the pre-existing "handles" trait syntax? (roles
with lists of methods maybe?)
3. how can we divide the groups of methods so you can opt-in only to
particular behaviour? ie you may want to delegate [] stuff like AT-POS but
not have <push pop shift unshift append prepend> (or vis versa or both).