Zygote and ChainRules are broken

[torfjelde]

Currently

using ComponentArrays, Zygote

f(a, sym) = getproperty(a, sym)
ca = ComponentArray(x = [1, 2], θ = [1.0, 100.0], deg = 2)
y, dy = Zygote.pullback(f, ca, Val{:x}())
dy(similar(y) .= 1)

fails with

BoundsError: attempt to access Tuple{Nothing, ComponentVector{Float64, Vector{Float64}, Tuple{Axis{(s = 1, m = 2:101)}}}} at index [3]

Stacktrace:
 [1] getindex
   @ ./tuple.jl:29 [inlined]
 [2] gradindex(x::Tuple{Nothing, ComponentVector{Float64, Vector{Float64}, Tuple{Axis{(s = 1, m = 2:101)}}}}, i::Int64)
   @ Zygote ~/.julia/packages/Zygote/6HN9x/src/compiler/reverse.jl:12
 [3] Pullback
   @ ./In[200]:1 [inlined]
 [4] (::typeof(∂(f)))(Δ::Vector{Float64})
   @ Zygote ./compiler/interface2.jl:0
 [5] (::Zygote.var"#41#42"{typeof(∂(_f))})(Δ::Vector{Float64})
   @ Zygote ~/.julia/packages/Zygote/6HN9x/src/compiler/interface.jl:41
 [6] top-level scope
   @ In[200]:3
 [7] eval
   @ ./boot.jl:360 [inlined]
 [8] include_string(mapexpr::typeof(REPL.softscope), mod::Module, code::String, filename::String)
   @ Base ./loading.jl:1094

AFAIK the following occurs:

1. The rules in src/compat/zygote.jl is never hit, but instead we reach the rules in src/compat/chainrulescore.jl.
   • Interestingly Zygote.pullback(getproperty, ca, Val{:x}()) actually works. I'm guessing this is because this hits the adjoint defined for Zygote.literal_property while in every other case you hit ChainRules's impl.
2. The rules defined in src/compat/chainrulescore.jl seem incorrect, as they're missing the NoTangent for the key used in getproperty, hence the getindex error above.

AFAIK this also means that src/compat/zygote.jl can just be removed.

Also, is there a reason why ChainRulesCore.jl shouldn't be a dependency of ComponentArrays.jl? This would allow specifying compat-bounds and thus avoiding impls "silently" going out of date, as seems to have happened above.