[perl #119643] [BUG] Null PMC access when composing a stubbed role into a class in Rakudo

[Christian Bartolomaeus via RT]

This dies now with a typed exception X::Role::Parametric::NoSuchCandidate and a 
descriptive error message (IMHO):

$ perl6 -e 'my role Foo { ... }; class A does Foo {}'
===SORRY!===
No appropriate parametric role variant available for 'Foo'

I added a test to S14-roles/stubs.t with commit 
https://github.com/perl6/roast/commit/62633d4987

I'm closing this ticket now. (And I'll merge ticket 117169 since it's a 
duplicate.)