Inconsistent resultant type of `+@` slurpy when given a Seq

[p6rt]

Migrated from rt.perl.org#131483 (status was 'rejected')

Searchable as RT131483$

[p6rt]

From @zoffixznet

Some discussion​: https://irclog.perlgeek.de/perl6-dev/2017-06-02#i_14675550

The crux of it is the type of `+@​` slurpy changes, depending on the input, though on further examinations it seems only Seq is affected​:

m​: sub grab(+@​a) { dd @​a }; grab %(​:42a, :72b); grab (1, (2, 3)); grab [1, (2, 3)]; grab (1, (2, 3)).Seq
rakudo-moar 348891​: OUTPUT​: «[​:a(42), :b(72)]␤[1, (2, 3)]␤[1, (2, 3)]␤(1, (2, 3))␤»

In all of the examples above, the `@​a` ends up being an Array, except for the last case (with Seq input), where it's a List.

This leads to two issues​:
1) The inner list of the Seq did not get containerized, so we have inconsistencies in how the `+@​` slurpy flattens
2) The List is immutable, so doing something like `@​a[42] = 70` is currently never safe with a `+@​` slurpy, because
  for some inputs it might end up being a List.

Seems like the Seq case needs fixing so it ends up being an Array as well.

[p6rt]

From @zoffixznet

On Fri, 02 Jun 2017 05​:26​:14 -0700, cpan@​zoffix.com wrote​:

Some discussion​: https://irclog.perlgeek.de/perl6-dev/2017-06-
02#i_14675550

The crux of it is the type of `+@​` slurpy changes, depending on the
input, though on further examinations it seems only Seq is affected​:

m​: sub grab(+@​a) { dd @​a }; grab %(​:42a, :72b); grab (1, (2, 3)); grab
[1, (2, 3)]; grab (1, (2, 3)).Seq
rakudo-moar 348891​: OUTPUT​: «[​:a(42), :b(72)]␤[1, (2, 3)]␤[1, (2,
3)]␤(1, (2, 3))␤»

In all of the examples above, the `@​a` ends up being an Array, except
for the last case (with Seq input), where it's a List.

This leads to two issues​:
1) The inner list of the Seq did not get containerized, so we have
inconsistencies in how the `+@​` slurpy flattens
2) The List is immutable, so doing something like `@​a[42] = 70` is
currently never safe with a `+@​` slurpy, because
for some inputs it might end up being a List.

Seems like the Seq case needs fixing so it ends up being an Array as
well.

Closing this in favour of rakudo/rakudo#1355 , which points out additional inconsistencies.

[p6rt]

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