From @zoffixznet

If you convert an Array with holes into a List, it makes sense for holes to be turned in Nils,
and if you individually AT-POS them, Nil is indeed what you get. However, if you .perl or .gist that List,
or assign it to another Array with some `is default`, the Nils turn up to be Mus​:

  m​: my @​a = <a b c>; @​a[1]​:delete; @​a[5] = 42; @​a.List.gist.put; @​a.List.perl.put; my @​b is default(42) = @​a.List; say @​b.perl
  rakudo-moar b8eda6​: OUTPUT​: «(a (Mu) c (Mu) (Mu) 42)␤("a", Mu, "c", Mu, Mu, 42)␤["a", Mu, "c", Mu, Mu, 42]␤»

Which is especially weird, if you assign some Mus into the Array​:

  m​: (my @​a)[1] = Mu; @​a.List.gist.say
  rakudo-moar b8eda6​: OUTPUT​: «((Mu) (Mu))␤»
  m​: (my @​a)[1] = Mu; @​a.List[0,1].say
  rakudo-moar b8eda6​: OUTPUT​: «(Nil (Mu))␤»

Looks like the logic that turns holes into Nils on AT-POS also needs to be added in the iterator.

From @zoffixznet

On Tue, 10 Oct 2017 05​:56​:34 -0700, cpan@​zoffix.com wrote​:

If you convert an Array with holes into a List, it makes sense for
holes to be turned in Nils,
and if you individually AT-POS them, Nil is indeed what you get.
However, if you .perl or .gist that List,
or assign it to another Array with some `is default`, the Nils turn up
to be Mus​:

m​: my @​a = <a b c>; @​a[1]​:delete; @​a[5] = 42; @​a.List.gist.put;
@​a.List.perl.put; my @​b is default(42) = @​a.List; say @​b.perl
rakudo-moar b8eda6​: OUTPUT​: «(a (Mu) c (Mu) (Mu) 42)␤("a", Mu, "c",
Mu, Mu, 42)␤["a", Mu, "c", Mu, Mu, 42]␤»

Which is especially weird, if you assign some Mus into the Array​:

m​: (my @​a)[1] = Mu; @​a.List.gist.say
rakudo-moar b8eda6​: OUTPUT​: «((Mu) (Mu))␤»
m​: (my @​a)[1] = Mu; @​a.List[0,1].say
rakudo-moar b8eda6​: OUTPUT​: «(Nil (Mu))␤»

Looks like the logic that turns holes into Nils on AT-POS also needs
to be added in the iterator.

Made Array.List shove Nils into holes.

Fix​: rakudo/rakudo@e135121914
Test​: Raku/roast@2257ab353f

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