[perl #126302] Use of uninitialized value in Perl6/World.nqp

[via RT]

# New Ticket Created by  Timothy Smith 
# Please include the string:  [perl #126302]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=126302 >


#! /bin/sh

set -x
if [ ! -r lib/A/B.pm6 ]; then
    mkdir -p lib/A
    echo 'class A { use A::B; }' > lib/A.pm6
    echo 'class A::B { has $!foo is rw; }' > lib/A/B.pm6
fi

perl6 --version
perl6 -Ilib -e 'use A; say "OK";'


+ '[' '!' -r lib/A/B.pm6 ']'
+ mkdir -p lib/A
+ echo 'class A { use A::B; }'
+ echo 'class A::B { has $!foo is rw; }'
+ perl6 --version
This is perl6 version 2015.09-273-g53969ab built on MoarVM version
2015.09-55-gf09c782
+ perl6 -Ilib -e 'use A; say "OK";'
Use of uninitialized value % of type Any in string context
Any of .^name, .perl, .gist, or .say can stringify undefined things,
if needed.  in any mop_up_and_check at src/Perl6/World.nqp:401
Potential difficulties:
    useless use of 'is rw' on $!foo
    at /home/tsmith/p6/tmp/lib/A/B.pm6:1
    ------> class A::B { has $!foo is rw⏏; }
      from :1
      from -e:1
OK