There should be a primitive int / float trait in core / stdlib

Yeah, I agree coherence-aware sealing would be interesting -- though complex since the local impls might themselves be dependent on external traits -- for lots of things.

But even if we had that, I still like impl<N> MyTrait for u<N> better. If nothing else because it doesn't also need mutually-exclusive traits to be able to also impl<N> MyTrait for i<N>.