Rule for `fill!` is wrong

[mcabbott]

I believe these should all be (Zero, 3):

julia> Zygote.gradient((x,y) -> sum(fill!(x,y)), [1,2,3], 4)
(nothing, nothing)

julia> Zygote.gradient((x,y) -> sum(map(_ -> y, x)), [1,2,3], 4)
([nothing, nothing, nothing], 3)

julia> Diffractor.gradient((x,y) -> sum(fill!(x,y)), [1,2,3], 4)
(ChainRulesCore.NoTangent(), ChainRulesCore.NoTangent())

julia> Diffractor.gradient((x,y) -> sum(map(_ -> y, x)), [1,2,3], 4)
ERROR: AssertionError: Base.issingletontype(typeof(f))
Stacktrace:
 [1] (::∂⃖{1})(#unused#::typeof(map), f::Function, a::Vector{Int64})
   @ Diffractor ~/.julia/packages/Diffractor/K2lJa/src/jet.jl:185

More generally, should in-place functions which completely overwrite their input be allowed? Like circshift! or sum! or mul!.

[oxinabox]

In forward mode they should be allowed.
In reverse they shouldn't (yet)

[mcabbott]

Oh right. Because some pullback may be relying on this array still having the same values, when it eventually gets called.