fix view & its zero-arrays

src/rulesets/Base/indexing.jl

@@ -52,7 +52,6 @@ function rrule(::typeof(getindex), x::Tuple, ::Colon)
     return x, getindex_back_4
 end
 
-
 #####
 ##### getindex(::AbstractArray)
 #####
@@ -174,6 +173,15 @@ function rrule(::typeof(view), x::AbstractArray, inds...)
     return view(x, inds...), view_pullback
 end
 
+function rrule(::typeof(view), x::AbstractArray, i::Integer, jkl::Integer...)
+    # This case returns a zero-dim array, unlike getindex. So we fool ∇getindex:
+    function view_pullback_0(dy)
+        nots = map(Returns(NoTangent()), (i, jkl...))
+        return (NoTangent(), thunked∇getindex(x, dy, i:i, jkl...), nots...)
+    end
+    return view(x, i, jkl...), view_pullback_0
+end
+
 #####
 ##### setindex!
 #####

test/rulesets/Base/array.jl

@@ -366,6 +366,7 @@ end
     @test [5 0; 6 0] == @inferred unthunk(rrule(findmin, [1 2; 3 4], dims=2)[2]((hcat([5,6]), nothing))[2])
     test_rrule(findmin, rand(3,4), fkwargs=(dims=1,), output_tangent = (rand(1,4), NoTangent()))
     test_rrule(findmin, rand(3,4), fkwargs=(dims=2,))
+    test_rrule(findmin, rand(3,4), fkwargs=(dims=(1,2),))
 end
 
 @testset "$imum" for imum in [maximum, minimum]

test/rulesets/Base/indexing.jl

@@ -169,7 +169,7 @@ end
 
     test_rrule(view, rand(3, 4), :, 1)
     test_rrule(view, rand(3, 4), 2, [1, 1, 2])
-    @test_broken test_rrule(view, rand(3, 4), 3, 4)  # This is why ∇getindex needs one more argument, dammit
+    test_rrule(view, rand(3, 4), 3, 4)
 end
 
 @testset "setindex!" begin