Policy-based design in D

[TheGag96 via Digitalmars-d-learn]

Tonight I stumbled upon Andrei's concept of policy-based design 
(https://en.wikipedia.org/wiki/Policy-based_design) and tried to 
implement their example in D with the lack of multiple 
inheritance in mind.

https://dpaste.dzfl.pl/adc05892344f (btw, any reason why 
certificate validation on dpaste fails right now?)

The implementation isn't perfect, as I'm not sure how to check 
members of mixin templates so that you  could verify whether 
print() and message() are actually where they should be. How 
would you do that? Is there any use for this kind of thing in D, 
and if so, what would it be? I've hardly dabbled in OOP patterns, 
but the abstraction seems kinda interesting.