Views of `VectorOfArray` with differently sized vectors gives wrong result

[JoshuaLampert]

Describe the bug 🐞

Creating a view of a VectorOfArray with inner vectors, which have different lengths does not return the expected slice.

Expected behavior

It should return the same slice as the corresponding slice without using a view.

Minimal Reproducible Example 👇

julia> f = VectorOfArray([[1.0], [2.0, 3.0]])
VectorOfArray{Float64,2}:
2-element Vector{Vector{Float64}}:
 [1.0]
 [2.0, 3.0]

julia> f[:, 1]
1-element Vector{Float64}:
 1.0

julia> view(f, :, 1)
1-element view(::VectorOfArray{Float64, 2, Vector{Vector{Float64}}}, :, 1) with eltype Float64:
 1.0

julia> f[:, 2]
2-element Vector{Float64}:
 2.0
 3.0

julia> view(f, :, 2) # Here, I would expect the view on [2.0, 3.0]
1-element view(::VectorOfArray{Float64, 2, Vector{Vector{Float64}}}, :, 2) with eltype Float64:
 2.0

Similarly:

julia> f2 = VectorOfArray([[1.0, 2.0], [3.0]])
VectorOfArray{Float64,2}:
2-element Vector{Vector{Float64}}:
 [1.0, 2.0]
 [3.0]

julia> f2[:, 1]
2-element Vector{Float64}:
 1.0
 2.0

julia> f2[:, 2]
1-element Vector{Float64}:
 3.0

julia> view(f2, :, 1)
2-element view(::VectorOfArray{Float64, 2, Vector{Vector{Float64}}}, :, 1) with eltype Float64:
 1.0
 2.0

julia> view(f2, :, 2) # There should be no `#undef` here
2-element view(::VectorOfArray{Float64, 2, Vector{Vector{Float64}}}, :, 2) with eltype Float64:
   3.0
 #undef

Error & Stacktrace ⚠️

No error, but wrong result. See above.

Environment (please complete the following information):

• Output of using Pkg; Pkg.status()

Status `/tmp/jl_U8APBX/Project.toml`
  [731186ca] RecursiveArrayTools v3.33.0

• Output of using Pkg; Pkg.status(; mode = PKGMODE_MANIFEST)

Status `/tmp/jl_U8APBX/Manifest.toml`
  [7d9f7c33] Accessors v0.1.42
  [79e6a3ab] Adapt v4.3.0
  [4fba245c] ArrayInterface v7.19.0
  [a33af91c] CompositionsBase v0.1.2
  [187b0558] ConstructionBase v1.5.8
  [a8cc5b0e] Crayons v4.1.1
  [9a962f9c] DataAPI v1.16.0
  [e2d170a0] DataValueInterfaces v1.0.0
  [ffbed154] DocStringExtensions v0.9.4
  [e2ba6199] ExprTools v0.1.10
  [46192b85] GPUArraysCore v0.2.0
  [3587e190] InverseFunctions v0.1.17
  [82899510] IteratorInterfaceExtensions v1.0.0
  [b964fa9f] LaTeXStrings v1.4.0
  [1914dd2f] MacroTools v0.5.16
  [bac558e1] OrderedCollections v1.8.1
⌅ [aea7be01] PrecompileTools v1.2.1
  [21216c6a] Preferences v1.4.3
  [08abe8d2] PrettyTables v2.4.0
  [3cdcf5f2] RecipesBase v1.3.4
  [731186ca] RecursiveArrayTools v3.33.0
  [189a3867] Reexport v1.2.2
  [ae029012] Requires v1.3.1
  [7e49a35a] RuntimeGeneratedFunctions v0.5.15
  [1e83bf80] StaticArraysCore v1.4.3
  [10745b16] Statistics v1.11.1
  [892a3eda] StringManipulation v0.4.1
  [2efcf032] SymbolicIndexingInterface v0.3.40
  [3783bdb8] TableTraits v1.0.1
  [bd369af6] Tables v1.12.0
  [56f22d72] Artifacts v1.11.0
  [2a0f44e3] Base64 v1.11.0
  [ade2ca70] Dates v1.11.0
  [8f399da3] Libdl v1.11.0
  [37e2e46d] LinearAlgebra v1.11.0
  [d6f4376e] Markdown v1.11.0
  [de0858da] Printf v1.11.0
  [9a3f8284] Random v1.11.0
  [ea8e919c] SHA v0.7.0
  [9e88b42a] Serialization v1.11.0
  [fa267f1f] TOML v1.0.3
  [cf7118a7] UUIDs v1.11.0
  [4ec0a83e] Unicode v1.11.0
  [e66e0078] CompilerSupportLibraries_jll v1.1.1+0
  [4536629a] OpenBLAS_jll v0.3.27+1
  [8e850b90] libblastrampoline_jll v5.11.0+0

• Output of versioninfo()

Julia Version 1.11.5
Commit 760b2e5b739 (2025-04-14 06:53 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 12 × 13th Gen Intel(R) Core(TM) i5-1345U
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, goldmont)
Threads: 1 default, 0 interactive, 1 GC (on 12 virtual cores)

Additional context

Add any other context about the problem here.

[JoshuaLampert]

I ran into this issue again. It looks like the problem reduces to to_indices returning something wrong, which in turn is probably due axes (due to https://github.com/JuliaLang/julia/blob/46c2a5c7e1f970e83b408b6ddcba49aaa31d8329/base/indices.jl#L359):

julia> f = VectorOfArray([[1.0], [2.0, 3.0]])
VectorOfArray{Float64,2}:
2-element Vector{Vector{Float64}}:
 [1.0]
 [2.0, 3.0]

julia> to_indices(f, (:, 2))
(Base.Slice(Base.OneTo(1)), 2)

julia> axes(f)
(Base.OneTo(1), Base.OneTo(2))

The problem is that axes is now dependent on the outer index. So I think, we should not use to_indices, but use some workarounds instead (maybe similar to how f[:, 2] is implemented?). Are there any plans to fix this soon or can you give me some advice on how to fix it if it doesn't require too much work?