Fix collect of non-reshaped reinterpret of an IndexSCartesian2 array

[greekera1000]

Fixes #63305

A non-reshaped reinterpret inherited its parent's IndexSCartesian2{K} index style. But eachindex for that style is forwarded to the parent, so when the element size changes (and with it the array's axes), iteration and therefore collect visited the wrong elements. This makes it fall back to IndexCartesian() in that case. Same-size reinterprets keep the fast path.

The same pattern may also affect reinterpret(reshape, T, ...) when it drops the leading dimension of an IndexSCartesian2 parent. Happy to handle that here or in a follow-up.

Disclosure: I used claude to help find the root cause and draft this patch. I have reviewed the change and understand it.

[nhz2]

I wonder if it would be safer to have a white list of IndexStyle to pass through, instead of an explicit set of IndexStyles that need to be blocked from passing through.

[greekera1000]

Good point, thanks @nhz2 . I'll switch to a whitelist so that only IndexLinear passes through and everything else falls back to IndexCartesian.

[nhz2]

I'm having a hard time understanding IndexSCartesian2, the docs at https://docs.julialang.org/en/v1/manual/interfaces/#man-interface-array seem to imply only IndexLinear and IndexCartesian are valid IndexStyle.

[greekera1000]

Yeah, that confused me at first too. IndexSCartesian2 isn't a documented style. It's an internal one defined in base/reinterpretarray.jl, and Base only uses it for reinterpret(reshape, T, A) when T is smaller than S and the parent is IndexLinear (like the reinterpret(reshape, UInt16, a) step in the issue). It speeds up iteration by assuming the new first axis is always 1:K.

The problem is that the non-reshaped reinterpret just forwarded its parent's style, and eachindex for IndexSCartesian2 gets forwarded to the parent too. So reinterpret(UInt8, ...) ended up iterating the 8×2 array as if it were still 4×2, which is why collect returned the wrong values.

With the whitelist change, this method now only ever returns IndexLinear or IndexCartesian, which matches the docs you linked.