BangBang._setindex! is broken on Julia 1.7 for undef entries

[torfjelde]

MWE on Julia 1.7.3

julia> using BangBang

julia> xs = Vector{Vector{Float64}}(undef, 2)
2-element Vector{Vector{Float64}}:
 #undef
 #undef

julia> BangBang.@set!! xs[1] = [1.0]
ERROR: UndefRefError: access to undefined reference
Stacktrace:
 [1] getindex
   @ ./array.jl:861 [inlined]
 [2] copy!(dst::Vector{Vector{Float64}}, src::Vector{Vector{Float64}})
   @ Base ./abstractarray.jl:874
 [3] _setindex
   @ ~/.julia/packages/BangBang/g5v4f/src/NoBang/base.jl:133 [inlined]
 [4] may
   @ ~/.julia/packages/BangBang/g5v4f/src/core.jl:11 [inlined]
 [5] setindex!!
   @ ~/.julia/packages/BangBang/g5v4f/src/base.jl:478 [inlined]
 [6] set(obj::Vector{Vector{Float64}}, lens::BangBang.AccessorsImpl.Lens!!{Accessors.IndexLens{Tuple{Int64}}}, value::Vector{Float64})
   @ BangBang.AccessorsImpl ~/.julia/packages/BangBang/g5v4f/src/accessors.jl:35 [7] top-level scope
   @ REPL[42]:1

while on Julia 1.10 it works without issues.

It comes down to the usage of copy!(ys, xs) here

BangBang.jl/src/NoBang/base.jl

Lines 129 to 137 in cc65483

	function _setindex(xs::AbstractArray, v, I...)
	T = promote_type(eltype(xs), typeof(v))
	ys = similar(xs, T)
	if eltype(xs) !== Union{}
	copy!(ys, xs)
	end
	ys[I...] = v
	return ys
	end

which, on Julia 1.7 requires getindex:

function copy!(dst::AbstractVector, src::AbstractVector)
    if length(dst) != length(src)
        resize!(dst, length(src))
    end
    for i in eachindex(dst, src)
        @inbounds dst[i] = src[i]
    end
    dst
end

while on Julia 1.10 we hit

function copy!(dst::AbstractVector, src::AbstractVector)
    firstindex(dst) == firstindex(src) || throw(ArgumentError(
        "vectors must have the same offset for copy! (consider using `copyto!`)"))
    if length(dst) != length(src)
        resize!(dst, length(src))
    end
    copyto!(dst, src)
end

and eventually

function _copyto_impl!(dest::Array, doffs::Integer, src::Array, soffs::Integer, n::Integer)
    n == 0 && return dest
    n > 0 || _throw_argerror("Number of elements to copy must be nonnegative.")
    @boundscheck checkbounds(dest, doffs:doffs+n-1)
    @boundscheck checkbounds(src, soffs:soffs+n-1)
    unsafe_copyto!(dest, doffs, src, soffs, n)
    return dest
end

where the unsafe_copyto! saves us.

I'm not really certain if this is a "bug" on the end of BangBang.jl or if it's a bug in copy! on Julia 1.7 😕

Manifest.toml

  [7d9f7c33] Accessors v0.1.36
  [198e06fe] BangBang v0.4.1
  [34da2185] Compat v4.14.0
  [a33af91c] CompositionsBase v0.1.2
  [187b0558] ConstructionBase v1.5.5
  [22cec73e] InitialValues v0.3.1
  [3587e190] InverseFunctions v0.1.13
  [1914dd2f] MacroTools v0.5.13
  [ae029012] Requires v1.3.0
  [56f22d72] Artifacts
  [2a0f44e3] Base64
  [ade2ca70] Dates
  [b77e0a4c] InteractiveUtils
  [8f399da3] Libdl
  [37e2e46d] LinearAlgebra
  [56ddb016] Logging
  [d6f4376e] Markdown
  [de0858da] Printf
  [9a3f8284] Random
  [ea8e919c] SHA
  [9e88b42a] Serialization
  [fa267f1f] TOML
  [8dfed614] Test
  [cf7118a7] UUIDs
  [4ec0a83e] Unicode
  [e66e0078] CompilerSupportLibraries_jll
  [4536629a] OpenBLAS_jll
  [8e850b90] libblastrampoline_jll

[torfjelde]

Note that this example shouldn't really hit the NoBang implementation in the first place 😬

But it does because some knobhead (me) introduced this check:

BangBang.jl/src/base.jl

Lines 520 to 525 in cc65483

	# This will still return `false` for scenarios such as
	#
	# setindex!!([1, 2, 3], [4, 5, 6], :, 1)
	#
	# which are in fact valid. However, this cases are rare.
	(_index_dimension(indices) == M || _index_dimension(indices) == 1)

which returns false in this scenario because M = 1 but _index_dimension(indices) = 0.

EDIT: Though it seems this was a concern in the original PR (though that was just based on a hunch) #16 (comment)

[torfjelde]

Fixed by #22