$ cat lib/C1.pm
package C1;

use Moose;

extends 'C';
use A;
use D;

has a => ( is => 'ro', default => sub { A->new } );
has d => ( is => 'ro', default => sub { D->new } );

sub foo {
    my ($self) = @_;

    print $self->a, "\n";
    print $self->a->bar, "\n";
    print $self->d, "\n";
    print $self->d->bar, "\n";
    $self->whos_who;

    return;
}

__PACKAGE__->meta->make_immutable;