Forbid offsets

[mcabbott]

This follows LinearAlgebra in explicitly forbidding offset arrays. They don't work anyway, but this replaces a BoundsError with something deliberate:

julia> ForwardDiff.gradient(sum, OffsetArray([1,2,3], 3))  # before
ERROR: BoundsError: attempt to access 3-element OffsetArray(::Vector{Dual{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3}}, 4:6) with eltype Dual{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3} with indices 4:6 at index [1:3]
Stacktrace:
  [1] throw_boundserror(A::OffsetVector{Dual{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3}, Vector{Dual{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3}}}, I::Tuple{UnitRange{Int64}})
    @ Base ./abstractarray.jl:703
  [2] checkbounds
    @ ./abstractarray.jl:668 [inlined]
  [3] view
    @ ./subarray.jl:177 [inlined]
  [4] maybeview
    @ ./views.jl:148 [inlined]
  [5] dotview
    @ ./broadcast.jl:1201 [inlined]
  [6] seed!(duals::OffsetVector{Dual{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3}, Vector{Dual{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3}}}, x::OffsetVector{Int64, Vector{Int64}}, seeds::Tuple{ForwardDiff.Partials{3, Int64}, ForwardDiff.Partials{3, Int64}, ForwardDiff.Partials{3, Int64}})
    @ ForwardDiff ~/.julia/dev/ForwardDiff/src/apiutils.jl:65
  [7] vector_mode_dual_eval!
    @ ~/.julia/dev/ForwardDiff/src/apiutils.jl:36 [inlined]
  [8] vector_mode_gradient(f::typeof(sum), x::OffsetVector{Int64, Vector{Int64}}, cfg::ForwardDiff.GradientConfig{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3, OffsetVector{Dual{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3}, Vector{Dual{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3}}}})
    @ ForwardDiff ~/.julia/dev/ForwardDiff/src/gradient.jl:106
  [9] gradient(f::Function, x::OffsetVector{Int64, Vector{Int64}}, cfg::ForwardDiff.GradientConfig{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3, OffsetVector{Dual{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3}, Vector{Dual{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3}}}}, ::Val{true})
    @ ForwardDiff ~/.julia/dev/ForwardDiff/src/gradient.jl:0
 [10] gradient(f::Function, x::OffsetVector{Int64, Vector{Int64}}, cfg::ForwardDiff.GradientConfig{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3, OffsetVector{Dual{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3}, Vector{Dual{ForwardDiff.Tag{typeof(sum), Int64}, Int64, 3}}}}) (repeats 2 times)
    @ ForwardDiff ~/.julia/dev/ForwardDiff/src/gradient.jl:17
 [11] top-level scope
    @ REPL[162]:1

julia> ForwardDiff.gradient(sum, OffsetArray([1,2,3], 3))  # after
ERROR: ArgumentError: offset arrays are not supported but got an array with index other than 1

[andreasnoack]

I think it would be better to put the checks in the functions that assume one-based indexing

ForwardDiff.jl/src/apiutils.jl

Lines 62 to 84 in 78c73af

	function seed!(duals::AbstractArray{Dual{T,V,N}}, x,
	seeds::NTuple{N,Partials{N,V}}) where {T,V,N}
	dual_inds = 1:N
	duals[dual_inds] .= Dual{T,V,N}.(view(x,dual_inds), seeds)
	return duals
	end
	function seed!(duals::AbstractArray{Dual{T,V,N}}, x, index,
	seed::Partials{N,V} = zero(Partials{N,V})) where {T,V,N}
	offset = index - 1
	dual_inds = (1:N) .+ offset
	duals[dual_inds] .= Dual{T,V,N}.(view(x, dual_inds), Ref(seed))
	return duals
	end
	function seed!(duals::AbstractArray{Dual{T,V,N}}, x, index,
	seeds::NTuple{N,Partials{N,V}}, chunksize = N) where {T,V,N}
	offset = index - 1
	seed_inds = 1:chunksize
	dual_inds = seed_inds .+ offset
	duals[dual_inds] .= Dual{T,V,N}.(view(x, dual_inds), getindex.(Ref(seeds), seed_inds))
	return duals
	end

where the error you show is thrown from. Or maybe just change the indexing to be generic?

[mcabbott]

My hope is that eventually general indexing could be supported, and these checks can be removed as soon as there are tests that it works.

I didn't bother to figure out precisely where the failures were, but did check that all the functions to which this error was added do currently fail.