# Pastebin SBm0s6KH
class Content {}
class Page {}

subset NavItem of Pair where *.value ~~ Content | Page;

class Nav {
    has NavItem() @.items;
}

my $n = Nav.new: items => [Page1 => Page.new, Page2 => Page.new];

#Impossible coercion from 'Pair' into 'NavItem': method Pair returned an instance of Pair
  in block <unit> at xxx.raku line 10