Restrict the sum forward rule to AbstractArray

Project.toml

@@ -1,6 +1,6 @@
 name = "ChainRules"
 uuid = "082447d4-558c-5d27-93f4-14fc19e9eca2"
-version = "1.74.12"
+version = "1.74.13"
 
 [deps]
 Adapt = "79e6a3ab-5dfb-504d-930d-738a2a938a0e"

src/rulesets/Base/mapreduce.jl

@@ -6,7 +6,7 @@ function frule((_, ẋ), ::typeof(sum), x::Tuple)
     return sum(x), sum(ẋ)
 end
 
-function frule((_, ẋ), ::typeof(sum), x; dims=:)
+function frule((_, ẋ), ::typeof(sum), x::AbstractArray; dims=:)
     return sum(x; dims=dims), sum(ẋ; dims=dims)
 end
 

test/rulesets/Base/mapreduce.jl

@@ -5,6 +5,18 @@ Base.sum(xs::AbstractArray, weights::AbstractArray) = dot(xs, weights)
 struct SumRuleConfig <: RuleConfig{Union{HasReverseMode}} end
 
 @testset "Reductions" begin
+    @testset "sum falls back to AD for unsupported inputs" begin
+        xs = (x + 1.0 for x in 1:3)
+        dxs = Tangent{typeof(xs)}(f=NoTangent(), iter=NoTangent())
+        @test frule((NoTangent(), dxs), sum, xs) === nothing
+
+        @test frule((NoTangent(), 1.0), sum, 2.0) === nothing
+
+        nt = (a=1.0, b=2.0)
+        dnt = Tangent{typeof(nt)}(a=1.0, b=2.0)
+        @test frule((NoTangent(), dnt), sum, nt) === nothing
+    end
+
     @testset "sum(::Tuple)" begin
         test_frule(sum, Tuple(rand(5)))
         test_frule(sum, (rand(2), rand(2)))