From 2f2c941712f9e2cd11f476666a63dc462ed6440a Mon Sep 17 00:00:00 2001
From: Nicholas Bauer <nicholasbauer@outlook.com>
Date: Tue, 17 Sep 2024 18:15:22 -0400
Subject: [PATCH] Add type promotion rules for `NoTangent` and `ZeroTangent`,
 and add `eltype` for `NoTangent` (#682)

* Add promotion rules for ZeroTangent and NoTangent

* Make NoTangent have an eltype of itself.

* bump version
---
 Project.toml                        |  2 +-
 src/tangent_types/abstract_zero.jl  |  3 +++
 test/tangent_types/abstract_zero.jl | 21 +++++++++++++++++++++
 3 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/Project.toml b/Project.toml
index bad5f567f..e60124086 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,6 +1,6 @@
 name = "ChainRulesCore"
 uuid = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
-version = "1.24.0"
+version = "1.25.0"
 
 [deps]
 Compat = "34da2185-b29b-5c13-b0c7-acf172513d20"
diff --git a/src/tangent_types/abstract_zero.jl b/src/tangent_types/abstract_zero.jl
index d526abffe..c5260489e 100644
--- a/src/tangent_types/abstract_zero.jl
+++ b/src/tangent_types/abstract_zero.jl
@@ -32,6 +32,7 @@ Base.:/(z::AbstractZero, ::Any) = z
 
 Base.convert(::Type{T}, x::AbstractZero) where {T<:Number} = zero(T)
 # (::Type{T})(::AbstractZero, ::AbstractZero...) where {T<:Number} = zero(T)
+Base.promote_rule(T::Type{<:Number}, S::Type{<:AbstractZero}) = T
 
 (::Type{Complex})(x::AbstractZero, y::Real) = Complex(false, y)
 (::Type{Complex})(x::Real, y::AbstractZero) = Complex(x, false)
@@ -92,6 +93,8 @@ end
 """
 struct NoTangent <: AbstractZero end
 
+Base.eltype(::Type{NoTangent}) = NoTangent
+
 """
     zero_tangent(primal)
 
diff --git a/test/tangent_types/abstract_zero.jl b/test/tangent_types/abstract_zero.jl
index 7da7cfadc..85a9e1d07 100644
--- a/test/tangent_types/abstract_zero.jl
+++ b/test/tangent_types/abstract_zero.jl
@@ -82,6 +82,15 @@
         @test convert(Float32, ZeroTangent()) === 0.0f0
         @test convert(ComplexF64, ZeroTangent()) === 0.0 + 0.0im
 
+        @test promote_type(ZeroTangent, Bool) == Bool
+        @test promote_type(Bool, ZeroTangent) == Bool
+        @test promote_type(ZeroTangent, Int64) == Int64
+        @test promote_type(Int64, ZeroTangent) == Int64
+        @test promote_type(ZeroTangent, Float32) == Float32
+        @test promote_type(Float32, ZeroTangent) == Float32
+        @test promote_type(ZeroTangent, ComplexF64) == ComplexF64
+        @test promote_type(ComplexF64, ZeroTangent) == ComplexF64
+
         @test z[1] === z
         @test z[1:3] === z
         @test z[1, 2] === z
@@ -110,6 +119,18 @@
         @test dot(dne, 17.2) == dne
         @test dot(11.9, dne) == dne
 
+        @test eltype(dne) === NoTangent
+        @test eltype(NoTangent) === NoTangent
+
+        @test promote_type(NoTangent, Bool) == Bool
+        @test promote_type(Bool, NoTangent) == Bool
+        @test promote_type(NoTangent, Int64) == Int64
+        @test promote_type(Int64, NoTangent) == Int64
+        @test promote_type(NoTangent, Float32) == Float32
+        @test promote_type(Float32, NoTangent) == Float32
+        @test promote_type(NoTangent, ComplexF64) == ComplexF64
+        @test promote_type(ComplexF64, NoTangent) == ComplexF64
+
         @test ZeroTangent() + dne == dne
         @test dne + ZeroTangent() == dne
         @test ZeroTangent() - dne == dne