I think maybe does the job. For the standard Moose types it works well like isa => 'Maybe[Str]'. The problem is I created some subtypes for my needs.

package Foo::Types::Library;

use Data::Dumper;
use MooseX::Types -declare => [
   qw/
      GooglePrimaryEmail
      WWWGoogleContactsTypeEmail
   /
];

use MooseX::Types::Email qw/EmailAddress/;


# WWWGoogleContactsTypeEmail
class_type WWWGoogleContactsTypeEmail, {
    class   => 'WWW::Google::Contacts::Type::Email',
};



# GooglePrimaryEmail
subtype GooglePrimaryEmail,
    as      EmailAddress,
    message {
        'validation failed for: '. Dumper $_;
    };
   
    
coerce GooglePrimaryEmail,
    from    ArrayRef[WWWGoogleContactsTypeEmail],
    via     {
        my $p   = (grep { $_->{primary} } @$_)[0];
        $p      = $p ? $p->{value} : @$_->[0]{value};
        return $p;
    };
[download]

This works for something like:

use MooseX::Declare;

class Foo {

    use Foo::Types::Library qw /GooglePrimaryEmail/;

    has 'email' => (
        is     => 'rw',
        isa    => GooglePrimaryEmail,
        coerce => 1
    );

}
[download]

Works fine unless email is initialized undef. In this case the attribute check fails.
Now maybe solves this but I don't know how I can create my own Maybe. I use MooseX::Types for creating my own types. As I can't see any syntax to create my own Maybe type I used the one in Moose::Util::TypeConstraints:

maybe_type (GooglePrimaryEmail);

I added this line to my type library, exported it with declare as Maybe[GooglePrimaryEmail and tested it with:

use MooseX::Declare;

class Foo {

    use Foo::Types::Library qw /Maybe[GooglePrimaryEmail]/;

    has 'email' => (
        is     => 'rw',
        isa    => 'Maybe[GooglePrimaryEmail]',
        coerce => 1
    );

}
[download]

Now every attribute type check fails. :(

Any help?