From vittore.scolari@gmail.com

The following script

  role DataNo {
  method Blob() {
  my \T = uint8;
  say T;
  Blob[T].new;
  }
  }

  my DataNo $a = DataNo.new;
  say $a.Blob;

  role Data[​::T] {
  method Blob() {
  say T;
  Blob[T].new;
  }
  }

my Data[uint8] $b = Data[uint8].new;
say $b.Blob;

dies with error​:
concatenate requires a concrete string, but got null
  in any protect at gen/moar/stage2/NQPCORE.setting line 1033

From @smls

Although that particular error message seems to be specific to `Blob`, it also breaks with other parametric roles (such as `Array`), because the generic parameter T of the outer role seems to be passed through without being specialized first.

Shorter examples​:

  ➜ role R[​::T] { method a { Array[T].new } }; say R[Int].a.perl;
  No such method 'perl' for invocant of type 'T'
  in block <unit> at <unknown file> line 1

  ➜ role R[​::T] { method a { Blob[T].new } }; say R[Int].a.perl;
  concatenate requires a concrete string, but got null
  in any protect at gen/moar/stage2/NQPCORE.setting line 1033
  in method a at <unknown file> line 1
  in block <unit> at <unknown file> line 1

Looks related to https://rt-archive.perl.org/perl6/Ticket/Display.html?id=131947 .

The RT System itself - Status changed from 'new' to 'open'

This seems to work fine nowadays:

The original report:

$ ./rakudo-m -e 'role DataNo { method Blob() { my \T = uint8; say T; Blob[T].new } }; my DataNo $a = DataNo.new; say $a.Blob; role Data[::T] { method Blob() { say T; Blob[T].new } }; my Data[uint8] $b = Data[uint8].new; say $b.Blob'
(uint8)
Blob[uint8]:0x<>
(uint8)
Blob[uint8]:0x<>

The two shortened evaluations:

$ ./rakudo-m -e 'role R[::T] { method a { Array[T].new } }; say R[Int].a.raku'
Array[Int].new()
$ ./rakudo-m -e 'role R[::T] { method a { Blob[T].new } }; say R[Int].a.raku'
Could not instantiate role 'Blob':
Can only parameterize with native int types, not 'Int'.

The second one errors out for a valid reason -- it works with e.g. int:

$ ./rakudo-m -e 'role R[::T] { method a { Blob[T].new } }; say R[int].a.raku'
Blob[int].new()

Tagging "testneeded".