add view and sum for abstract zeros

Author: Miha Zgubic

src/differentials/abstract_zero.jl

@@ -28,6 +28,9 @@ Base.convert(::Type{T}, x::AbstractZero) where T <: Number = zero(T)
 
 Base.getindex(z::AbstractZero, k) = z
 
+Base.view(z::AbstractZero, ind...) = z
+Base.sum(z::AbstractZero; dims=:) = z
+
 """
     ZeroTangent() <: AbstractZero
 

test/differentials/abstract_zero.jl

@@ -4,6 +4,14 @@
         @test iszero(NoTangent())
     end
 
+    @testset "Linear operators" begin
+        @test view(ZeroTangent(), 1) == ZeroTangent()
+        @test view(NoTangent(), 1, 2) == NoTangent()
+
+        @test sum(ZeroTangent()) == ZeroTangent()
+        @test sum(NoTangent(); dims=2) == NoTangent()
+    end
+
     @testset "ZeroTangent" begin
         z = ZeroTangent()
         @test z + z === z