Implement zero for NotImplemented

Author: sethaxen

src/tangent_types/notimplemented.jl

@@ -48,16 +48,8 @@ Base.:/(x::AbstractZero, ::NotImplemented) = x
 Base.:/(x::NotImplemented, ::AbstractThunk) = throw(NotImplementedException(x))
 Base.:/(::AbstractThunk, x::NotImplemented) = throw(NotImplementedException(x))
 
-Base.zero(x::NotImplemented) = throw(NotImplementedException(x))
-function Base.zero(::Type{<:NotImplemented})
-    return throw(
-        NotImplementedException(
-            @not_implemented(
-                "`zero` is not defined for missing tangents of type `NotImplemented`"
-            )
-        ),
-    )
-end
+Base.zero(::NotImplemented) = ZeroTangent()
+Base.zero(::Type{<:NotImplemented}) = ZeroTangent()
 
 Base.iterate(x::NotImplemented) = throw(NotImplementedException(x))
 Base.iterate(x::NotImplemented, ::Any) = throw(NotImplementedException(x))

test/tangent_types/notimplemented.jl

@@ -9,6 +9,11 @@
         x = rand()
         thunk = @thunk(x^2)
 
+        # zero
+        @test @inferred(zero(ni)) === ZeroTangent()
+        @test @inferred(zero(typeof(ni2))) === ZeroTangent()
+        @test !iszero(ni)
+
         # conjugate
         @test conj(ni) === ni
 
@@ -58,8 +63,6 @@
             @test_throws E a / ni
         end
         @test_throws E ni / ni2
-        @test_throws E zero(ni)
-        @test_throws E zero(typeof(ni))
         @test_throws E iterate(ni)
         @test_throws E iterate(ni, nothing)
         @test_throws E adjoint(ni)