[perl #63170] Null PMC access when calling method on proto-object on imported role, when the method has a parameter with another role as a type

[Carl Mäsak]

# New Ticket Created by  "Carl Mäsak" 
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In Rakudo b2e7ac, Parrot r36619:

$ cat A.pm
use v6;
role A { method x(B $b) {} }
$ perl6 -e 'role B {}; use A; A.x(B)'
Null PMC access in isa()
[...]

Observations:

* The role A must be imported for this to happen.
* The parameter in method x must refer to B.
* The argument to A.x doesn't actually have to be B, or of type B. The
error occurs anyway.