Array Indexing Error

[lhnguyen-vn]

I ran into a Zygote bug related to array slicing. Here's a MWE:

array = [[1,2], [3,4], [5,6]]
_, back = Zygote.pullback(x -> x[1:2], array)
back([[1,1], [1,1]]) # MethodError: no method matching +(::Nothing, ::Array{Int64,1})

This is a relatively well-known issue, Zygote uses nothing as a stronger zero. Internally, this is the source of the error:

Zygote.jl/src/lib/array.jl

Lines 45 to 47 in 17a017f

	dx = _zero(x, eltype(dy))
	dxv = view(dx, inds...)
	dxv .+= _droplike(dy, dxv)

Shouldn't the last line just be dxv .= _droplike(dy, dxv)? This gives the correct behavior and I can't think of why one would want to add to an array filled with nothing instead?

[lhnguyen-vn]

On second thoughts, .+= is correct. Perhaps _zero should be extended for arrays of arrays, something along the line of

_zero(xs::AbstractArray{<:AbstractArray}, T=Any) = [_zero(x) for x in xs]

This gives the correct gradient for my case, and I notice also solves issue #454.

[AzamatB]

This seems to be fixed on master.