a.p6

use Foo::Node;

unit class Main is Foo::Node;

use Foo::Stmt;
use Foo::Use;

has Array[Foo::Stmt] %.foo{Str};

method bar {
    my Foo::Stmt @a;
    my @cur-sub-docs = [@a];
    %!foo<abc> = @a;  # this line throws out a type constraint failure exception
}

Foo::Node.pm6

unit class Foo::Node;

Foo::Stmt.pm6

use Foo::Node;

unit class Foo::Stmt is Foo::Node;

Foo::Use.pm6

use Foo::Stmt;

unit class Foo::Use is Foo::Stmt;