sub echo(Int:U $x) {
    say $x.^name;
}

echo(3);

--output:--
Parameter '$x' of routine 'echo' must be a type object of type 'Int',
not an object instance of type 'Int'.  Did you forget a 'multi'?
  in sub echo at b.raku line 14
  in block <unit> at b.raku line 22