Prevent this rule from being misapplied to Symbol types. (#151)

* Prevent this rule from being misapplied to `Symbol` types. See SciML/ModelingToolkit.jl#972

* Constrain all the indices in the general case to be integers.

Author: lamorton

src/zygote.jl

@@ -1,18 +1,18 @@
-function ChainRulesCore.rrule(::typeof(getindex),VA::AbstractVectorOfArray, i)
+function ChainRulesCore.rrule(::typeof(getindex),VA::AbstractVectorOfArray, i::Int)
   function AbstractVectorOfArray_getindex_adjoint(Δ)
     Δ′ = [ (i == j ? Δ : zero(x)) for (x,j) in zip(VA.u, 1:length(VA))]
     (NoTangent(),Δ′,NoTangent())
   end
   VA[i],AbstractVectorOfArray_getindex_adjoint
 end
 
-function ChainRulesCore.rrule(::typeof(getindex),VA::AbstractVectorOfArray, i, j...)
+function ChainRulesCore.rrule(::typeof(getindex),VA::AbstractVectorOfArray, indices::Vararg{Int,N}) where {N}
   function AbstractVectorOfArray_getindex_adjoint(Δ)
     Δ′ = zero(VA)
-    Δ′[i,j...] = Δ
-    (NoTangent(), Δ′, i,map(_ -> NoTangent(), j)...)
+    Δ′[indices...] = Δ
+    (NoTangent(), Δ′, indices[1],map(_ -> NoTangent(), indices[2:end])...)
   end
-  VA[i,j...],AbstractVectorOfArray_getindex_adjoint
+  VA[indices...],AbstractVectorOfArray_getindex_adjoint
 end
 
 function ChainRulesCore.rrule(::Type{<:ArrayPartition}, x::S, ::Type{Val{copy_x}} = Val{false}) where {S<:Tuple,copy_x}