While you can do lots of horrible things with generics (cough typenum [1] cough), it has a rather strict rule that prevents it from ever having the same degree of freedom as C++ template has. In particular, this entire thing is correctly type-checked and you won't see hacks like SFINAE. Probably things like this should be encapsulated in a separate crate, reviewed and maintained by the community.

[1] http://paholg.com/typenum/typenum/index.html