I'm wondering if anyone has considered sub-byte user types as part of the API design. The assumption that types have element sizes that are integer numbers of bytes seems to be baked into current user dtypes design.

For example, consider a packedint4 type, whose elements are nibbles (half-bytes). It doesn't seem possible to express such a type in the user dtype API, e.g., the type descriptor requires a byte size for each element, and ufuncs require byte strides. I was toying with the idea of trying to write such a type, but it doesn't seem to be possible without API changes.

I see this was mentioned briefly in NEP 41 (int2) but there is no further discussion I can see. Has there been any more thinking along those lines?