"SFINAE is Evil"

[Bill Baxter]

The description of traits in Boost is quite succinct and hits the good 
points -- particularly about the non-intrusive part:
http://www.boost.org/more/generic_programming.html#traits

I think most traits-like things can be done in D if you require that 
people modify their classes.  But the non-intrusiveness is one of key 
parts of what makes traits templates a good building block.

--bb