[perl #126176] subroutine with trait rw "return" a scalar cann't be lvalue

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``` perl6 code
my $lastval = 4;
sub lastval () is rw { return $lastval }
dd $lastval;
lastval() = 5;
dd $lastval; 
```

Int $lastval = 4
 Cannot assign to a readonly variable or a value   
in block  at t3.p6:7

This is perl6 version 2015.09-115-g4653d98 built on MoarVM version 
2015.09-36-g51dcbe4