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   use Types::Common::Numeric qw(PositiveInt);
   
   has user_id => (
      is   => 'ro',
      isa  => PositiveInt,
   );

   package MyApp::Types {
      use Type::Library
         -base,
...
      
      ...;
   }

   declare User, as InstanceOf['MyApp::User'];
   
   coerce User,
...
   
   coerce UserId,
      from User, via { $_->user_id };

   package MyApp::Types {
      use Type::Library -base;
      use Type::Utils (); # don't import any keywords
...
      
      __PACKAGE__->make_immutable;
   }

   # Imagine this is some code in a class...
   #
   
...
      isa    => UserId->plus_coercions(Str, sub { ... }),
      coerce => 1,
   );