`VectorOfArrayStyle` potentially relies on bug in `Base.AbstractArrayStyle` promotion rule

[adienes]

revealed during a PkgEval run on JuliaLang/julia#35948, which contained errors like this

  MethodError: no method matching RecursiveArrayTools.VectorOfArrayStyle{2}(::Val{2})
  The type `RecursiveArrayTools.VectorOfArrayStyle{2}` exists, but no method is defined for this combination of argument types when trying to construct it.
  Closest candidates are:
    (::Type{RecursiveArrayTools.VectorOfArrayStyle{N}} where N)()
     @ RecursiveArrayTools ~/.julia/packages/RecursiveArrayTools/X30HP/src/vector_of_array.jl:462
  Stacktrace:
    [1] Base.Broadcast.BroadcastStyle(::Base.Broadcast.DefaultArrayStyle{0}, ::RecursiveArrayTools.VectorOfArrayStyle{2})
      @ Base.Broadcast ./broadcast.jl:141
    [2] result_style(s1::RecursiveArrayTools.VectorOfArrayStyle{2}, s2::Base.Broadcast.DefaultArrayStyle{0})
      @ Base.Broadcast ./broadcast.jl:465
    [3] combine_styles(c1::RecursiveArrayTools.VectorOfArray{ComplexF64, 2, Vector{Vector{ComplexF64}}}, c2::Float64)
      @ Base.Broadcast ./broadcast.jl:439
    [4] broadcasted
      @ ./broadcast.jl:1356 [inlined]
    [5] initial_step(f0::RecursiveArrayTools.VectorOfArray{ComplexF64, 2, Vector{Vector{ComplexF64}}}, u0::RecursiveArrayTools.VectorOfArray{ComplexF64, 2, Vector{Vector{ComplexF64}}}, atol::Float64, rtol::Float64)

potential errors in the definition of VectorOfArrayStyle here

RecursiveArrayTools.jl/src/vector_of_array.jl

Line 916 in 016807f

struct VectorOfArrayStyle{N} <: Broadcast.AbstractArrayStyle{N} end # N is only used when voa sees other abstract arrays

could have been exposed by the bugfix

[ChrisRackauckas]

What is the "bug"? What is the fix?

[adienes]

IIUC, this fallback in Base

BroadcastStyle(a::AbstractArrayStyle{M}, ::DefaultArrayStyle{N}) where {M,N} =
    typeof(a)(Val(max(M, N)))

is trying to call VectorOfArrayStyle{2}(Val(2)) constructor on the (concrete) type VectorOfArrayStyle{2}, but this package defines

VectorOfArrayStyle(::Val{N}) where {N} = VectorOfArrayStyle{N}()

constructor on the (parametric) type VectorOfArrayStyle. so a remedy I believe would be to simply add {N} there

The reason this doesn't fail before JuliaLang/julia#35948 is that BroadcastStyle is checking typename(a) === typename(b) and then exiting with Unknown() . There is some compelling argument in that PR that this fallback is questionable, and the proposed new behavior would require calling the Val constructor (on a concrete type) here.

the label "bug" here is maybe not 100% accurate --- I really just meant to flag what I saw in PkgEval