LTA error with is rw defaults in an unpacked slurpy

[zoffixznet]

https://irclog.perlgeek.de/perl6-dev/2017-10-25#i_15352946

2017.09.474 zoffix@VirtualBox~$ perl6 -e 'm: sub foo(+$x [$ is rw = False]) { $x }; say foo().perl;'
Unhandled exception: concatenate requires a concrete string, but got null
   at SETTING::src/core/Exception.pm:395  (/home/zoffix/.rakudobrew/moar-nom/install/share/perl6/runtime/CORE.setting.moarvm:print_exception)
 from SETTING::src/core/Exception.pm:452  (/home/zoffix/.rakudobrew/moar-nom/install/share/perl6/runtime/CORE.setting.moarvm:<anon>)
 from gen/moar/stage2/NQPHLL.nqp:1708  (/home/zoffix/.rakudobrew/moar-nom/install/share/nqp/lib/NQPHLL.moarvm:command_eval)
 from src/Perl6/Compiler.nqp:42  (/home/zoffix/.rakudobrew/moar-nom/install/share/nqp/lib/Perl6/Compiler.moarvm:command_eval)
 from gen/moar/stage2/NQPHLL.nqp:1630  (/home/zoffix/.rakudobrew/moar-nom/install/share/nqp/lib/NQPHLL.moarvm:command_line)
 from gen/moar/main.nqp:47  (/home/zoffix/.rakudobrew/moar-nom/install/share/perl6/runtime/perl6.moarvm:MAIN)
 from gen/moar/main.nqp:38  (/home/zoffix/.rakudobrew/moar-nom/install/share/perl6/runtime/perl6.moarvm:<mainline>)
 from <unknown>:1  (/home/zoffix/.rakudobrew/moar-nom/install/share/perl6/runtime/perl6.moarvm:<main>)
 from <unknown>:1  (/home/zoffix/.rakudobrew/moar-nom/install/share/perl6/runtime/perl6.moarvm:<entry>)


2017.09.474 zoffix@VirtualBox~$ perl6 -v
This is Rakudo version 2017.09-474-g765dd69 built on MoarVM version 2017.09.1-609-g1a9be0a
implementing Perl 6.c.

[jsimonet]

The error is generated by the usage of an empty string (the attribute name) in (https://github.com/rakudo/rakudo/blob/master/src/Perl6/Metamodel/BOOTSTRAP.nqp#L1899).

It could be fixed by adding a test on the $varname definiteness.

I don't know why the variable name is not set, but a previous test in the same routine tests for its definiteness, so I guess its a normal possibility.