Support types that have no tangent space in zero_tangent

Author: oxinabox

src/tangent_types/abstract_zero.jl

@@ -96,10 +96,11 @@ struct NoTangent <: AbstractZero end
     zero_tangent(primal)
 
 This returns an appropriate zero tangent suitable for accumulating tangents of the primal.
-For mutable composites types this is a structural []`MutableTangent`](@ref)
+For mutable composites types this is a structural [`MutableTangent`](@ref)
 For `Array`s, it is applied recursively for each element.
-For immutable types, this is simply [`ZeroTangent()`](@ref) as accumulation is default out-of-place for contexts where mutation does not apply.
-(Where mutation is not to be supported even for mutable types, then [`ZeroTangent()`](@ref) should be used for everything)
+For other types, in particular immutable types, we do not make promises beyond that it will be `iszero`
+and suitable for accumulating against.
+In general though, it is more likely to produce a structural tangent.
 
 !!! warning Exprimental
     `zero_tangent`is an experimental feature, and is part of the mutation support featureset.
@@ -110,7 +111,10 @@ function zero_tangent end
 
 zero_tangent(x::Number) = zero(x)
 
+zero_tangent(::Type) = NoTangent()
+
 @generated function zero_tangent(primal)
+    fieldcount(primal) == 0 && return NoTangent()  # no tangent space at all, no need for structural zero.
     zfield_exprs = map(fieldnames(primal)) do fname
         fval = :(
             if isdefined(primal, $(QuoteNode(fname)))

test/tangent_types/abstract_zero.jl

@@ -162,25 +162,38 @@
 end
 
 @testset "zero_tangent" begin
-    @test zero_tangent(1) === 0
-    @test zero_tangent(1.0) === 0.0
-    mutable struct MutDemo
-        x::Float64
-    end
-    struct Demo
-        x::Float64
-    end
-    @test zero_tangent(MutDemo(1.5)) isa MutableTangent{MutDemo}
-    @test iszero(zero_tangent(MutDemo(1.5)))
+    @testset "basics" begin
+        @test zero_tangent(1) === 0
+        @test zero_tangent(1.0) === 0.0
+        mutable struct MutDemo
+            x::Float64
+        end
+        struct Demo
+            x::Float64
+        end
+        @test zero_tangent(MutDemo(1.5)) isa MutableTangent{MutDemo}
+        @test iszero(zero_tangent(MutDemo(1.5)))
 
-    @test zero_tangent((; a=1)) isa Tangent{typeof((; a = 1))}
-    @test zero_tangent(Demo(1.2)) isa Tangent{Demo}
-    @test zero_tangent(Demo(1.2)).x === 0.0
+        @test zero_tangent((; a=1)) isa Tangent{typeof((; a = 1))}
+        @test zero_tangent(Demo(1.2)) isa Tangent{Demo}
+        @test zero_tangent(Demo(1.2)).x === 0.0
 
-    @test zero_tangent([1.0, 2.0]) == [0.0, 0.0]
-    @test zero_tangent([[1.0, 2.0], [3.0]]) == [[0.0, 0.0], [0.0]]
+        @test zero_tangent([1.0, 2.0]) == [0.0, 0.0]
+        @test zero_tangent([[1.0, 2.0], [3.0]]) == [[0.0, 0.0], [0.0]]
+
+        @test zero_tangent((1.0, 2.0)) == Tangent{Tuple{Float64,Float64}}(0.0, 0.0)
+    end
+
+    @testset "Weird types" begin
+        @test iszero(zero_tangent(typeof(Int)))  # primative type
+        @test iszero(zero_tangent(typeof(Base.RefValue)))  # struct
+        @test iszero(zero_tangent(Vector))  # UnionAll
+        @test iszero(zero_tangent(Union{Int, Float64}))  # Union
+        @test iszero(zero_tangent(:abc))
+        @test iszero(zero_tangent("abc"))
+        @test iszero(zero_tangent(sin))
+    end
 
-    @test zero_tangent((1.0, 2.0)) == Tangent{Tuple{Float64,Float64}}(0.0, 0.0)
     @testset "undef elements Vector" begin
         x = Vector{Vector{Float64}}(undef, 3)
         x[2] = [1.0, 2.0]