title: raku typed enums not working with custom types
body: <p>Should Raku typed enums work with custom types?  I get compile errors when trying the following:</p>
  <pre><code>role A { }
  class B does A { }
  class C does A { }
  my A enum E ( b =&gt; B, c =&gt; C );
  </code></pre>
  <p>results in</p>
  <blockquote>
  <p>Cannot find method 'mro' on object of type Perl6::Metamodel::ParametricRoleGroupHOW</p>
  </blockquote>
  <p>and</p>
  <pre><code>class D { }
  my D enum F ( b =&gt; D.new, c =&gt; D.new );
  </code></pre>
  <p>results in</p>
  <blockquote>
  <p>Cannot auto-generate a proto method for 'Int' in the setting</p>
  </blockquote>
  <p>or does this fall under</p>
  <blockquote>
  <p>Complex expressions for generating key-value pairs are not supported.</p>
  </blockquote>
  <p><strong>EDIT</strong></p>
  <p>Regarding the first example - it looks like Raku doesn't like roles as the type constraint.  The same error is given when trying, e.g.</p>
  <pre><code>my Rational enum G ( one =&gt; 1/1, two =&gt; 1/2 );
  </code></pre>
  <p>and as for what I was really hoping for:</p>
  <blockquote>
  <p>Using a type object as a value for an enum not yet implemented. Sorry.</p>
  </blockquote>