class/role problem

[mt1957]

Ls,

Following piece of code gets an error when calling $b.get-t0.
---
#!/usr/bin/env perl6
#
use v6;
role A {
  has $!t0;

  method get-t0 ( ) {
    return self!get-t0-helper;
  }

  method !get-t0-helper ( ) {
    my $t0 = $!t0 // 'T0';
  }
}

class B does A {  }

my $a = A.new;
say "A: ", $a.get-t0;

my $b = B.new;
say "B: ", $b.get-t0;
---

The output and error is;

Attr: Int $!t0
A: T0
Attr: Int $!t0
P6opaque: no such attribute '$!t0'
  in method get-t0-helper at class3.pl6:14
  in method get-t0 at class3.pl6:9
  in block <unit> at class3.pl6:25

The strange thing is when you call self.^attributes in the helper method 
the name is printed as shown in the output. The error disappears after 
removing the hide(!) operator.

Perl version;
This is perl6 version 2015.04-5-g59f57a8 built on MoarVM version 
2015.04-3-gbb50941

Greetings,
Marcel