(D::Larger)::Smaller != D ?

[goertzenator]

I am writing an ndarray function that is generic in dimension (D: Dimension) and am ending up with the following error when I try to Zip some things together:

    = note: expected associated type `<<<D as ndarray::Dimension>::Larger as ndarray::Dimension>::Smaller as ndarray::Dimension>::Smaller`
               found associated type `<D as ndarray::Dimension>::Smaller`

Intuitively these are the same dimension types, but I see how the Dimension trait doesn't enforce (D::Larger)::Smaller to be the same as D. Any tips on establishing this constraint?

In the meantime I'm going to see if I can avoid using Larger to begin with.

[akern40]

Any chance you can provide a minimal example that produces this error? I'd be happy to look into it, but am honestly not sure how to trigger it!

[goertzenator]

Here is an example: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=9729023ceaf478356d661899d80ab5b9

@jturner314 explains why it can't work, and I have found a work around for my application. However I am still mentioning this because I saw something about a redesign of the Dimension system. Perhaps a different design could make this work?

[jturner314]

Unfortunately, they aren't always the same type. In the latest version of ndarray, constant-ndim types only go up to 6 dimensions, so Ix6::Larger = IxDyn. Since IxDyn::Smaller = IxDyn, <Ix6::Larger as Dimension>::Smaller = IxDyn::Smaller = IxDyn.