[perl #131699] [LTA] Errors indexing past the end of a List

[brian d foy]

# New Ticket Created by  "brian d foy" 
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Accessing a List element beyond the end of the List returns Nil,
although accessing an element before the beginning returns an out of
bounds failure. I think there's two things that can be better here
since we know the size of the List.

    my $list = < a b c >;
    put "I have a {$list.^name}";

First, in the "before" case, we have more information than the error
message lets on. The index should be from 0 to 2:

    {
    my $i = -1;
    $list[$i];  # Index out of range. Is: -1, should be in 0..^Inf
    }

But this requires the change I think is more helpful. Since the List
size won't change, we can have the same out-of-bounds error on
accesses past the end. At the moment it's no warning:

    {
    my $i = $list.end + 1;
    $list[$i]; # No warning
    }

This would then be the error for assigning into a position beyond the
end. The existing error doesn't say what went wrong even though Perl 6
has enough information to figure that out:

    {
    my $i = $list.end + 1;
    $list[$i] = 5;  # Cannot modify an immutable Nil
    }