[perl #132051] [BUG] Parametric type object passed as literal die with error

[via RT]

# New Ticket Created by  Vittore Scolari 
# Please include the string:  [perl #132051]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=132051 >


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