`setindex` on `SizedArray` fails when target array has uninitialised data

[penelopeysm]

julia> using StaticArrays

julia> v = @SVector zeros(3)
3-element SVector{3, Float64} with indices SOneTo(3):
 0.0
 0.0
 0.0

julia> v2 = similar(v, Real)
3-element SizedVector{3, Real, Vector{Real}} with indices SOneTo(3):
 #undef
 #undef
 #undef

julia> setindex(v2, 1.0, 1)
ERROR: UndefRefError: access to undefined reference
Stacktrace:
 [1] getindex
   @ ./essentials.jl:917 [inlined]
 [2] getindex
   @ ~/.julia/packages/StaticArrays/IZgIP/src/SizedArray.jl:92 [inlined]
 [3] macro expansion
   @ ~/.julia/packages/StaticArrays/IZgIP/src/deque.jl:193 [inlined]
 [4] _setindex
   @ ~/.julia/packages/StaticArrays/IZgIP/src/deque.jl:186 [inlined]
 [5] setindex(a::SizedVector{3, Real, Vector{Real}}, x::Float64, index::Int64)
   @ StaticArrays ~/.julia/packages/StaticArrays/IZgIP/src/deque.jl:185
 [6] top-level scope
   @ REPL[4]:1

[mateuszbaran]

For reference: Julia doesn't support setindex for Array at all. See JuliaLang/julia#55129 for a discussion. Accessors.jl has an implementation though: https://github.com/JuliaObjects/Accessors.jl/blob/4603471383ab0641f8e28c1e90771974374585c6/src/setindex.jl#L7-L16 .

[penelopeysm]

Thanks for the link! That's really useful.

I actually got here via BangBang:

https://github.com/JuliaFolds2/BangBang.jl/blob/9a255bdd9d046b781796c1ef119965a44cc45600/ext/BangBangStaticArraysExt.jl#L27

The semantics of NoBang._setindex are essentially the same as what Base.setindex tries to be, and calling setindex works fine for e.g. SArray, but not for SizedArray. I guess the problem is that the existing method at

StaticArrays.jl/src/deque.jl

Line 185 in 58bf1e1

@propagate_inbounds setindex(a::StaticArray, x, index::Int) = _setindex(Length(a), a, convert(eltype(typeof(a)), x), index)

is more specific so the Accessors method is never called.

Looking at Lilith's last comment here, if you aren't willing to change the behaviour of setindex inside StaticArrays.jl, maybe a solution on the BangBang side could be to overload NoBang._setindex for SizedArray to use Accessors' implementation?

[mateuszbaran]

I'm a bit hesitant to have a fix in StaticArrays.jl. Doing it without a performance regression might be a bit involved. I think BangBangStaticArraysExt.jl would be a good place to resolve this issue.