Incorrect gradient

[willtebbutt]

This is example is a simplified version of something I'm doing with FillArrays and Zygote, and definitely computes the wrong result. Any thoughts?

using Zygote, Flux

# A vector of length 10
struct MyFill{T} <: AbstractVector{T}
    value::T
end
Base.getindex(m::MyFill, n::Int) = m.value
Base.size(m::MyFill) = (10,)

foo(x) = sum(MyFill(x))

# This yields the right answer :)
Flux.gradient(foo, 5.0)

# This yields the wrong answer :(
Zygote.gradient(foo, 5.0)

[MikeInnes]

This now throws an error, rather than giving wrong gradients. You can fix the error by doing:

Zygote.@adjoint MyFill(x) = MyFill(x), x -> (sum(x),)