correct and test getindex rrule

Author: oxinabox

src/rulesets/Base/array.jl

@@ -105,16 +105,16 @@ end
 ##### getindex
 #####
 
-function rrule(::typeof(getindex), x::Array{<:Number}, inds::Union{Int, Vararg{Int}})
+function rrule(::typeof(getindex), x::Array{<:Number}, inds::Vararg{Int})
     y = getindex(x, inds...)
     function getindex_pullback(ȳ)
         function getindex_add!(Δ)
-            Δ[inds...] .+= ȳ;
+            Δ[inds...] = Δ[inds...] .+ ȳ
             return Δ
         end
 
         x̄ = InplaceableThunk(
-            @thunk(getindex_add!(zeros(x))),
+            @thunk(getindex_add!(zero(x))),
             getindex_add!
         )
         return (NO_FIELDS, x̄, (DoesNotExist() for _ in inds)...)

test/rulesets/Base/array.jl

@@ -78,7 +78,7 @@ end
     (ds, dv, dd) = pullback(ones(4))
     @test ds === NO_FIELDS
     @test dd isa DoesNotExist
-    @test extern(dv) == 4
+    @test extern(dv) == 4   
 
     y, pullback = rrule(fill, 2.0, (3, 3, 3))
     @test y == fill(2.0, (3, 3, 3))
@@ -87,3 +87,17 @@ end
     @test dd isa DoesNotExist
     @test dv ≈ 27.0
 end
+
+@testset "getindex" begin
+    x = [1.0 2.0 3.0; 10.0 20.0 30.0]
+    ind = [2,3]
+    ȳ = 7.2
+    x̄_fd, = j′vp(ChainRulesTestUtils._fdm, a->getindex(a, ind...), ȳ, x)
+    y, pullback = rrule(getindex, x, ind...)
+    _, x̄_ad, = pullback(ȳ)
+
+    @test unthunk(x̄_ad) ≈ x̄_fd
+
+    x_like = x .+ 1.0
+    @test x̄_ad.add!(copy(x_like)) ≈ x_like + x̄_fd
+end