From @zoffixznet

<ZoffixW> m​: role A {...}; class B does A {}
<camelia> rakudo-moar a822bc​: OUTPUT«Use of uninitialized value of
type Any in string context.␤Methods .^name, .perl, .gist, or .say can
be used to stringify it to something meaningful.␤ in any try_select
at gen/moar/m-Metamodel.nqp line 2653␤===SORRY!=== Error while
compiling <t…»

The line references try_select at
https://github.com/rakudo/rakudo/blob/a822bcf9cf1570cbf6f1c0285f6e8a66523988c7/src/Perl6/Metamodel/ParametricRoleGroupHOW.nqp#L104

The current error message looks better to me. There is no gut-referencing anymore and it states that role A was not found:

$ ./rakudo-m -e 'role A {...}; class B does A {}'
===SORRY!=== Error while compiling -e
No appropriate parametric role variant available for 'A':
    Cannot resolve caller (B); Routine does not have any candidates. Is only the proto defined?
at -e:1

It talks about Routine, which might be a bit surprising. So, still LTA or closeable with test?

Yeah, LTA:

Comment in the code: # Use multi-dispatcher to pick the body block of the best role.

That fails, and the error is quoted verbatim in src/Perl6/Metamodel/ParametricRoleGroupHOW.nqp (line 100).

So it feels like the error that is being quoted, needs a bit of mangling.