AbstractArray vector subtypes get wrongly handled

[lkdvos]

It seems like whenever the custom vector-type is <:AbstractArray, and has IndexLinear indexing style, we eagerly assume that it can be treated as if it is an Array, taking this path in the code for basistransform!. I think it would probably be more reasonable to have <:DenseArray or even just <:Array there, as we are no longer adhering to the VectorInterface set of allowed operations there.

For example, similar is being used instead of zerovector (

KrylovKit.jl/src/orthonormal.jl

Line 336 in 1b175b7

let b2 = [similar(b[1]) for j in 1:n], K = K, m = m, n = n

), and then we index into these vectors (

KrylovKit.jl/src/orthonormal.jl

Line 342 in 1b175b7

b2j[i] = zero(b2j[i])

), which fails for non-isbits types.

Additionally, there really is no guarantee that AbstractArrays are efficiently indexable (or the indexing even means the same thing).
For example in MPSKit (where I ran into this issue), we have that QuasiParticle states subtype AbstractVector, because you can index into them by site, while we also have that they are elements of a vectorspace, for which we overload VectorInterface. While this design might be questionable, I would still expect KrylovKit to use the VectorInterface methods, instead of the AbstractVector methods.

[Edit] Actually, the reason it was going wrong is that in the Multiline tests, we used a RecursiveVec for the different lines, which I now changed for a regular Vector, thus changing this to <:DenseArray or <:Array also does not suffice.

[Jutho]

I am certainly happy to further restrict this to DenseArray, or maybe even just Array, as we want to exclude GPU arrays, and this condition is probably one of the only reasons why we depend on GPUArraysCore.jl. Furthermore, I am also happy to see similar replaced with zerovector.

[lkdvos]

I will try and have a look at it later this week. <:DenseArray actually does not really solve the problem, I think it would even have to be <:DenseArray{<:Number}, as the case I am having is Vector{Quasiparticle}, which is dense.