ForwardDiff derivative on complex interpolation

[jtravs]

If I make a complex interpolation:

A_x = [1.0, 1.7, 8.0, 9.7, 10.3, 12.5, 32]
A = [4.0, 2.7, 1.0, 0.7, 4.3, 17.4, 43]
intp = LinearInterpolation(A_x, A .+ im.*A./3)

I can interpolate and also take the gradient just fine:

julia> intp(3.0)
2.3492063492063493 + 0.783068783068783im

julia> Interpolations.gradient(intp, 3.0)
1-element StaticArrays.SArray{Tuple{1},Complex{Float64},1,1}:
 -0.2698412698412699 - 0.08994708994708994im

but I cannot use ForwardDiff:

julia> ForwardDiff.derivative(intp, 3.0)
ERROR: MethodError: no method matching extract_derivative(::Type{ForwardDiff.Tag{Interpolations.Extrapolation{Complex{Float64},1,Interpolations.GriddedInterpolation{Complex{Float64},1,Complex{Float64},Gridded{Linear},Tuple{Array{Float64,1}}},Gridded{Linear},Throw{Nothing}},Float64}}, ::Complex{ForwardDiff.Dual{ForwardDiff.Tag{Interpolations.Extrapolation{Complex{Float64},1,Interpolations.GriddedInterpolation{Complex{Float64},1,Complex{Float64},Gridded{Linear},Tuple{Array{Float64,1}}},Gridded{Linear},Throw{Nothing}},Float64},Float64,1}})
Closest candidates are:
  extract_derivative(::Type{T}, ::ForwardDiff.Dual) where T at C:\Users\John\.julia\packages\ForwardDiff\N0wMF\src\derivative.jl:79
  extract_derivative(::Type{T}, ::Real) where T at C:\Users\John\.julia\packages\ForwardDiff\N0wMF\src\derivative.jl:80
  extract_derivative(::Type{T}, ::AbstractArray) where T at C:\Users\John\.julia\packages\ForwardDiff\N0wMF\src\derivative.jl:81
Stacktrace:
 [1] derivative(::Interpolations.Extrapolation{Complex{Float64},1,Interpolations.GriddedInterpolation{Complex{Float64},1,Complex{Float64},Gridded{Linear},Tuple{Array{Float64,1}}},Gridded{Linear},Throw{Nothing}}, ::Float64) at C:\Users\John\.julia\packages\ForwardDiff\N0wMF\src\derivative.jl:14
 [2] top-level scope at none:0

If I use a real valued interpolation object all works fine:

julia> intp = LinearInterpolation(A_x, A);
julia> ForwardDiff.derivative(intp, 3.0)
-0.2698412698412699

Do you know why? Is this a problem with ForwardDiff?

[jtravs]

I'm going to close this because I think it is actually an issue with ForwardDiff, see JuliaDiff/ForwardDiff.jl#364