I put the full code up here: https://github.com/frezik/Games-Tetrachromat/tree/perl6_bug. It's demonstrated by doing:

$ perl6 -I lib t/010_packet_ack.t 
1..7
Type check failed for return value; expected Game::Tetrachromat::Packe
+t but got Any (Any)
  in method read_packet at /home/tmurray/proj/Game-Tetrachromat/lib/Ga
+me/Tetrachromat/PacketFactory.pm6 (Game::Tetrachromat::PacketFactory)
+ line 79
  in block <unit> at t/010_packet_ack.t line 33

# Looks like you planned 7 tests, but ran 0
[download]

The factory is in lib/Game/Tetrachromat/PacketFactory.pm6, and the class it's supposed to be using for this test is lib/Game/Tetrachromat/Packet/Ack.pm6. All packet types do the role lib/Game/Tetrachromat/Packet.pm6.

I was able to work around things by converting Game::Tetrachromat::Packet to a class instead of a role, and using BUILDALL for Packet::Ack. Inheritance is arguably the right way to go for this particular case, so I'll probably just leave it that way. Still, if there's a bug in perl6, I figured it was worth putting up the code that breaks it.

Thanks for the code. Fwiw I found it very readable.

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Mu's BUILDALL returns the constructed object (per the self line at the end). I imagine a custom BUILDALL needs to do the same thing.

If you have the patience I'd appreciate you confirming that adding a trailing self line to Ack's BUILDALL fixes the problem you first posted about. (If so I'll update my original response to make it clear my guess about public attributes was a red herring.)

It's a mystery to me why converting a role to a class helped.

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To wrap up I'll try to explain the error you just quoted. I'll go in to far more detail than I imagine you need in the hope it's helpful to someone some day.

I would appreciate confirmation that this error now makes sense to you as follows:

1. Ack's BUILDALL returns Nil

   A block of code in Perl 6 is interpreted as a semilist (semicolon separated list) of statements. When a block is evaluated it returns the value of its last statement unless the code explicitly specifies otherwise. In this case the last statement is a for loop:

   unit class Game::Tetrachromat::Packet::Ack; # FTFY :)
   ...
   method BUILDALL(|)
   {
       ...
       for ... {
           ...
       }
   }
   [download]

   A loop construct as the last statement is evaluated in sink context. Sink context means any value returned by an expression or statement gets thrown away -- down the sink as it were.

   Perl 6 uses Nil to communicate nothingness. So Ack's BUILDALL attempts to return Nil.

   Because Ack's BUILDALL's signature doesn't constrain what it returns, returning a Nil is fine and doesn't trigger a type error.

   The code continues to run...

2. The Nil becomes an Any

   When a Scalar container is empty it pretends it contains a value, by default, an undefined Any. So if Nil gets assigned to any Scalar container with the default default the effect is indistinguishable from assigning it an undefined Any.

   Somewhere between the return of Ack's BUILDALL and the completion of the PacketFactory read_buffer method code my $packet = $packet_class.new... the Nil becomes an Any. The my $packet declaration doesn't specify a type constraint. So the assignment of an Any (or a Nil which becomes an Any) proceeds without a type error.

   The code continues to run...

3. return $packet triggers a type error

   Rakudo finally gets around to reporting a type error at line 79 of PacketFactory because the enclosing method, read_buffer, defines a return type in its signature: returns Game::Tetrachromat::Packet but $packet contains an Any.

»ö« ^{. o O ( "the celebrity tell-all of the Perl-6 cult?" )}

Thanks--returning self at the end does fix that branch of the code. As for why converting away from a role helped, doing that was also meant using BUILD rather than BUILDALL; apparently, you don't have to return the object at the end of BUILD.

It doesn't seem clear from the documentation that returning from BUILDALL would matter. Of these spots:

https://doc.perl6.org/language/objects#Object_Construction

https://doc.perl6.org/language/traps#BUILD_prevents_automatic_attribute_initialization_from_constructor_arguments

None of them talk about the return value of BUILDALL, nor is it shown in the example code.

Frankly, the perl6 docs seem to spend a lot of effort on showing how clever the language can be, and not enough on explaining what's going on.

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"There is no shame in being self-taught, only in not trying to learn in the first place." -- Atrus, Myst: The Book of D'ni.