From @masak

<masak> rn​: class A {}; class B is A { has $!x = 5; our method foo(A​:)
{ say $!x } }; &B​::foo(A.new)
<p6eval> rakudo d70363​: OUTPUT«Can not get non-existent attribute
'$!x' on class 'B'␤ in method foo [...]
<p6eval> ..niecza v21-1-ga8aa70b​: OUTPUT«Unhandled exception​:
Attribute $!x not defined in A or any superclass [...]
* masak submits LTA error message rakudobug

The attribute *does* exist on class 'B', but Rakudo seems to think
that's where it didn't find it, rather than on class 'A'.

From @usev6

This now gives a segfault on Moar and Parrot and an internal error on JVM​:

debian7$ perl6-m --version # same rakudo version for perl6-p and perl6-j
This is perl6 version 2015.01-6-g25da7a9 built on MoarVM version 2015.01-5-ga29eaa9

debian7$ perl6-m -e 'class A {}; class B is A { has $!x = 5; our method foo(A​:) { say $!x } }; &B​::foo(A.new)'
Segmentation fault

debian7$ perl6-p -e 'class A {}; class B is A { has $!x = 5; our method foo(A​:) { say $!x } }; &B​::foo(A.new)'
Segmentation fault

debian7$ perl6-j -e 'class A {}; class B is A { has $!x = 5; our method foo(A​:) { say $!x } }; &B​::foo(A.new)'
P6opaque representation does not support attributes
  in method foo at -e​:1
  in block <unit> at -e​:1

The segfault seems to happen while saying $!x -- if I remove that the code runs fine​:

debian7$ perl6-m -e 'class A {}; class B is A { has $!x = 5; our method foo(A​:) { say "alive" } }; &B​::foo(A.new)'
alive

The above happens on Debian 7 (64 bit).

Interestingly on FreeBSD 10.1 (64 bit) there is no segfault with Parrot (Moar and JVM behave as on Debian)​:

freebsd10.1$ perl6-p -e 'class A {}; class B is A { has $!x = 5; our method foo(A​:) { say $!x } }; &B​::foo(A.new)'
No such method 'gist' for invocant of type 'NQPMu'
  in sub say at gen/parrot/CORE.setting​:16823
  in sub say at gen/parrot/CORE.setting​:16815
  in method foo at -e​:1
  in block <unit> at -e​:1

freebsd10.1$ perl6-p --version
This is perl6 version 2015.01-7-gf120204 built on parrot 7.0.1 revision RELEASE_7_0_1

The Rakudo version is a few commits newer than in the first examples, but I don't think that is relevant, since on Debian even the latest Rakudo on Parrot segfaults​:

debian7$ perl6-p -e 'class A {}; class B is A { has $!x = 5; our method foo(A​:) { say $!x } }; &B​::foo(A.new)'
Segmentation fault

debian7$ perl6-p --version
This is perl6 version 2015.01-10-g5dd603b built on parrot 7.0.1 revision RELEASE_7_0_1

@usev6 - Status changed from 'new' to 'open'

From @diakopter

Moar still segfaults​:

13​:35 <diakopter> m​: class A {}; class B is A { has $!x = 5; our method foo(A​:) { say $!x } }; &B​::foo(A.new)
13​:36 <camelia> rakudo-moar 61d231​: OUTPUT«(signal SEGV)»

From @zoffixznet

Fixed by timotimo in MoarVM as per http://irclog.perlgeek.de/perl6-dev/2016-09-02#i_13139713

Tests needed

From @zoffixznet

Tests added in Raku/roast@3815be036a

@zoffixznet - Status changed from 'open' to 'resolved'