`copyto!` with `CartesianIndices` assumes identical step in source and destination indices

[jishnub]

julia/base/multidimensional.jl

Lines 1105 to 1129 in 9a0e797

	function copyto!(dest::AbstractArray{T1,N}, Rdest::CartesianIndices{N},
	src::AbstractArray{T2,N}, Rsrc::CartesianIndices{N}) where {T1,T2,N}
	isempty(Rdest) && return dest
	if size(Rdest) != size(Rsrc)
	throw(ArgumentError("source and destination must have same size (got $(size(Rsrc)) and $(size(Rdest)))"))
	end
	checkbounds(dest, first(Rdest))
	checkbounds(dest, last(Rdest))
	checkbounds(src, first(Rsrc))
	checkbounds(src, last(Rsrc))
	src′ = unalias(dest, src)
	ΔI = first(Rdest) - first(Rsrc)
	if @generated
	quote
	@nloops $N i (n->Rsrc.indices[n]) begin
	@inbounds @nref($N,dest,n->i_n+ΔI[n]) = @nref($N,src′,i)
	end
	end
	else
	for I in Rsrc
	@inbounds dest[I + ΔI] = src′[I]
	end
	end
	dest
	end

Now that CartesianIndices may contain StepRanges, the implementation doesn't seem correct.

julia> a = rand(4)
4-element Vector{Float64}:
 0.7766672217743518
 0.5138339000883105
 0.7793932733439368
 0.6377375487030642

julia> b = zeros(2);

julia> copyto!(b, CartesianIndices(b), a, CartesianIndices((1:2:3,)))
2-element Vector{Float64}:
 0.7766672217743518
 0.0

julia> a[CartesianIndices((1:2:3,))]
2-element Vector{Float64}:
 0.7766672217743518
 0.7793932733439368