package Bit::Vector {
	use Moose;
}

package Bit::Thing {
	use Moose;
	has thing => (
		isa     => "Bit::Vector",
		is      =>"rw",
		default => sub { return Bit::Vector->new },
	);
}

Bit::Thing->new;  # fine, "thing" gets the default value
Bit::Thing->new(thing => undef);  # croaks