Is this actually supposed to be legal?

[Chris NS]

It is indeed supposed to work, and was actually touted as a 
common and lauded example way back in the day.  However, with the 
advent of this-params for templates it seems less useful now 
(once they've been through the ringer a little more at least).

I did use this-params to great effect in Zeal, to auto-inject 
behavior into subclasses with the proper scoping and other 
concerns.  The base class didn't even have to be a template 
itself (just the magical internals).
https://github.com/csauls/zeal.d/blob/master/source/zeal/base/controller.d

So, to repeat, yes it is supposed to work... but I'm not so sure 
it is such a good idea anymore -- assuming this-params will work 
on the class declaration.

-- Chris NS