Make sure you also look at the design document section titled "Multiple constraints":

http://design.perl6.org/S12.html#Multiple_constraints

It concludes with:

> As a first approximation for 6.0.0, subsets of enums are static, and other subsets are dynamic. We may refine this in subsequent versions of Perl.

It allows arbitrary code to define the subset. So it's very flexible, but it also means that compiler usually can't be clever about subsets. We can't solve the Halting problem for you.