oop – 在mixins中使用角色签名

#!/usr/bin/env perl6

role by-n[$n=1] {
    method multiply(Str $str) { return $str x $n; }
}

role by-string[$n=1] does by-n[$n] {
    method whatever(Str $str) { return $n ~ "→" ~ $.multiply( $str ); }
}

class mighty-tor does by-string[2] {};

say mighty-tor.whatever("*");

这会返回一个错误,这似乎表明$n有一个Mu类型：

Cannot resolve caller infix:<x>(Str,Mu); none of these signatures match:
    ()
    ($x)
    ($s,Num:D $n)
    ($s,Any:D $n)
    ($s,Any:U $n)
    (Str:D $s,Int:D $repetition --> Str:D)
    (str $s,int $repetition --> str)
  in method multiply at composable-roles-fail.p6 line 4
  in method whatever at composable-roles-fail.p6 line 8
  in block <unit> at composable-roles-fail.p6 line 13

但是,字符串[$n = 1]的作用是由-n [1]或任何其他数字运行良好.尝试修复签名中的类型：

role by-n[Int $n=1] {
    method multiply(Str $str) { return $str x $n; }
}

role by-string[Int $n=1] does by-n[$n] {
    method whatever(Str $str) { return $n ~ "→" ~ $.multiply( $str ); }
}

产生不同的错误：

===SORRY!=== Error while compiling /home/jmerelo/Code/perl6/dev.to-code/perl6/composable-roles-fail.p6
No appropriate parametric role variant available for 'by-n'
at /home/jmerelo/Code/perl6/dev.to-code/perl6/composable-roles-fail.p6:11

然而,再一次,将它更改为角色by-string [Int $n = 1]确实可以通过-n [1]起作用.知道这里发生了什么或者可以做些什么来解决这个问题？