Error trying to differentiate eachindex

[gdalle]

This came up in a test suite:

julia> import AbstractDifferentiation as AD

julia> import Diffractor: DiffractorForwardBackend

julia> h(x::AbstractVector) = eachindex(x) .* x
h (generic function with 1 method)

julia> AD.jacobian(DiffractorForwardBackend(), h, [1, 2])
ERROR: MethodError: no method matching size(::ChainRulesCore.Tangent{Base.OneTo{Int64}, @NamedTuple{stop::Int64}})

Closest candidates are:
  size(::Base.MethodList)
   @ Base reflection.jl:1042
  size(::BitVector)
   @ Base bitarray.jl:104
  size(::BitVector, ::Integer)
   @ Base bitarray.jl:107
  ...

Stacktrace:
  [1] axes
    @ ./abstractarray.jl:98 [inlined]
  [2] call_composed(fs::Tuple{typeof(axes)}, x::Tuple{ChainRulesCore.Tangent{Base.OneTo{…}, @NamedTuple{…}}}, kw::@Kwargs{})
    @ Base ./operators.jl:1045
  [3] call_composed
    @ ./operators.jl:1044 [inlined]
  [4] (::ComposedFunction{…})(x::ChainRulesCore.Tangent{…}; kw::@Kwargs{})
    @ Base ./operators.jl:1041
  [5] map
    @ ./tuple.jl:292 [inlined]
  [6] findconsistentvalue(f::typeof(axes), cols::Tuple{Base.OneTo{…}, ChainRulesCore.Tangent{…}})
    @ StructArrays ~/.julia/packages/StructArrays/CjQ4L/src/utils.jl:208
  [7] (StructArrays.StructVector{…})(c::Tuple{…})
    @ StructArrays ~/.julia/packages/StructArrays/CjQ4L/src/structarray.jl:18
  [8] (StructArrays.StructArray{Diffractor.TangentBundle{…} where P})(c::Tuple{Base.OneTo{…}, ChainRulesCore.Tangent{…}})
    @ StructArrays ~/.julia/packages/StructArrays/CjQ4L/src/structarray.jl:94
  [9] unbundle(atb::Diffractor.TangentBundle{1, Base.OneTo{Int64}, Diffractor.TaylorTangent{Tuple{…}}})
    @ Diffractor ~/.julia/packages/Diffractor/v1WuP/src/tangent.jl:365
 [10] (::Diffractor.var"#205#206"{1, Diffractor.∂☆{…}, Diffractor.TangentBundle{…}})(i::Int64)
    @ Diffractor ~/.julia/packages/Diffractor/v1WuP/src/stage1/broadcast.jl:23
 [11] ntuple(f::Diffractor.var"#205#206"{1, Diffractor.∂☆{…}, Diffractor.TangentBundle{…}}, n::Int64)
    @ Base ./ntuple.jl:19
 [12] (::Diffractor.∂☆{…})(zc::Diffractor.TangentBundle{…}, bc::Diffractor.TangentBundle{…})
    @ Diffractor ~/.julia/packages/Diffractor/v1WuP/src/stage1/broadcast.jl:16
 [13] materialize
    @ ./broadcast.jl:903 [inlined]
 [14] (::Diffractor.∂☆recurse{…})(::Diffractor.TangentBundle{…}, ::Diffractor.TangentBundle{…})
    @ Diffractor ~/.julia/packages/Diffractor/v1WuP/src/stage1/recurse_fwd.jl:0
 [15] (::Diffractor.∂☆internal{…})(::Diffractor.TangentBundle{…}, ::Vararg{…})
    @ Diffractor ~/.julia/packages/Diffractor/v1WuP/src/stage1/forward.jl:114
 [16] ∂☆
    @ ~/.julia/packages/Diffractor/v1WuP/src/stage1/forward.jl:161 [inlined]
 [17] h
    @ ~/Work/GitHub/Julia/DifferentiationInterface.jl/test/diffractor.jl:13 [inlined]
 [18] ∂☆internal
    @ ~/.julia/packages/Diffractor/v1WuP/src/stage1/forward.jl:114 [inlined]
 [19] ∂☆
    @ ~/.julia/packages/Diffractor/v1WuP/src/stage1/forward.jl:161 [inlined]
 [20] (::Diffractor.var"#pushforward#370"{typeof(h), Tuple{Vector{Int64}}})(vs::Tuple{Vector{Int64}})
    @ Diffractor ~/.julia/packages/Diffractor/v1WuP/src/AbstractDifferentiation.jl:17
 [21] #369
    @ ~/.julia/packages/AbstractDifferentiation/RGmzD/src/AbstractDifferentiation.jl:644 [inlined]
 [22] mapslices(f::Diffractor.var"#369#373"{Diffractor.var"#pushforward#370"{…}}, A::Matrix{Int64}; dims::Int64)
    @ Base ./abstractarray.jl:3191
 [23] jacobian(b::DiffractorForwardBackend, f::Function, args::Vector{Int64})
    @ Diffractor ~/.julia/packages/AbstractDifferentiation/RGmzD/src/AbstractDifferentiation.jl:642
 [24] top-level scope
    @ ~/Work/GitHub/Julia/DifferentiationInterface.jl/test/diffractor.jl:15
Some type information was truncated. Use `show(err)` to see complete types.

[oxinabox]

I suspect this would be resolved by #276
though really we need to make it so that we don't try to AD things like this, off the active path.