[perl #127832] Parameterizing a role via a variable results in an error

[via RT]

# New Ticket Created by  Rob Hoelz 
# Please include the string:  [perl #127832]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=127832 >


See the attached file.  If you store a parameterized role in a variable and 
attempt to parameterize it via postcircumfix:sym<[ ]>, you get a cryptic error 
message:

> No appropriate parametric role variant available for 'R'
>   in any specialize at gen/moar/m-Metamodel.nqp line 2609
>   in any compose at gen/moar/m-Metamodel.nqp line 2988
>   in any make_pun at gen/moar/m-Metamodel.nqp line 1692
>   in any find_method at gen/moar/m-Metamodel.nqp line 1720
>   in block <unit> at test.pl line 11

use v6;

role R[$value] {}

class C {}

my $obj = C.new;
my $r = R;

$obj but $r[1];

=output
No appropriate parametric role variant available for 'R'
  in any specialize at gen/moar/m-Metamodel.nqp line 2609
  in any compose at gen/moar/m-Metamodel.nqp line 2988
  in any make_pun at gen/moar/m-Metamodel.nqp line 1692
  in any find_method at gen/moar/m-Metamodel.nqp line 1720
  in block <unit> at test.pl line 11