Should `broadcast` on `Adjoint` should use the parent array's `BroadcastStyle`?

[andyferris]

It seems that BroadcastStyle(::Type{<:Adjoint}) uses the default style instead of that of its parent.

E.g. at an earlier stage today I was upgrading StaticArrays to work on v0.7 and came across this:

julia> using StaticArrays

julia> Broadcast.BroadcastStyle(SVector{3, Float64}))
Base.Broadcast.DefaultArrayStyle{2}()

This gives different results in vector + vector' than I originally expected, even though we've defined a StaticArrayStyle <: AbstractArrayStyle.

Would using the parent's array style work, or might there be some issues with that?

[nalimilan]

Cc: @timholy

[ajkeller34]

I noticed this when I was playing around with getting broadcasting to work with AxisArrays as well. Gory details at JuliaArrays/AxisArrays.jl#131.

[mbauman]

My gut reaction here is that — unfortunately — we probably need to make you do this yourself. A fallback definition won't always be right since we don't know how you'll use the styles.

I've been working with more BroadcastStyles in the context of #25377 (but for these purposes I don't think it matters). One potential new broadcast style is for LinearAlgebra's structured matrices. I'm trying out a StructuredMatrixStyle{LowerTriangular} to replace some of the currently-existing broadcast methods. That would be one obvious case where the adjoint cannot share its parent's style.