/language/traps should warn against using [Z] to get the transpose of a list-of-lists

[smls]

Every now and then, someone gets they idea that they can use [Z] to create the transpose of a list-of-lists:

my @matrix = <X Y>, <a b>, <1 2>;
my @transpose = [Z] @matrix;

say @transpose;  # [(X a 1) (Y b 2)] -- so far so good

And everything works fine... until you get an input @matrix with exactly one row (child list):

my @matrix = <X Y>,;
my @transpose = [Z] @matrix;

say @transpose;  # [(X Y)] -- not the expected transpose [(X) (Y)]

I've explained the cause of this surprising edge-case behavior, which basically ruins [Z] for variable-size inputs, in this SO answer (which should be easy to understand without also having to read the associated SO question).

I think this has all the markings of a trap:

• The syntax is simple (so people will continue to "discover" it), and elegant (so they'll be happy to use it).
• It's quite sensible to expect, on first sight, that it will do the right thing.
• It does in fact do the right thing for most inputs, meaning that it might slip through unit tests etc.

Thus, I think the Perl 6 documentation should mention it in its "Traps to avoid" document.

[AlexDaniel]

FWIW, we now have this ticket: https://rt.perl.org/Ticket/Display.html?id=131686

[AlexDaniel]

Another interesting ticket (although maybe unrelated): rakudo/rakudo#1210

[AlexDaniel]

I think this is now resolved in 6519ef5. I copied some parts of your OP here, let me know if that's a problem.