From e63f62bc0be02d5db2ec81e072adb81f72a81d06 Mon Sep 17 00:00:00 2001
From: Lukas Weber <49278367+lukas-weber@users.noreply.github.com>
Date: Thu, 14 Dec 2023 17:48:20 -0500
Subject: [PATCH] tangent_arithmetic: add `norm` for `NoTangent` and
 `ZeroTangent`

---
 src/tangent_types/abstract_zero.jl  | 2 ++
 test/tangent_types/abstract_zero.jl | 3 +++
 test/tangent_types/tangent.jl       | 4 ++++
 3 files changed, 9 insertions(+)

diff --git a/src/tangent_types/abstract_zero.jl b/src/tangent_types/abstract_zero.jl
index bc1a4239b..77c455c04 100644
--- a/src/tangent_types/abstract_zero.jl
+++ b/src/tangent_types/abstract_zero.jl
@@ -23,6 +23,8 @@ Base.last(x::AbstractZero) = x
 Base.Broadcast.broadcastable(x::AbstractZero) = Ref(x)
 Base.Broadcast.broadcasted(::Type{T}) where {T<:AbstractZero} = T()
 
+LinearAlgebra.norm(::AbstractZero) = 0
+
 # Linear operators
 Base.adjoint(z::AbstractZero) = z
 Base.transpose(z::AbstractZero) = z
diff --git a/test/tangent_types/abstract_zero.jl b/test/tangent_types/abstract_zero.jl
index 390b4887b..028d942ea 100644
--- a/test/tangent_types/abstract_zero.jl
+++ b/test/tangent_types/abstract_zero.jl
@@ -120,6 +120,9 @@
         @test dot(ZeroTangent(), dne) == ZeroTangent()
         @test dot(dne, ZeroTangent()) == ZeroTangent()
 
+        @test norm(ZeroTangent()) == 0
+        @test norm(ZeroTangent(), 0.4) == 0
+
         for x in dne
             @test x === dne
         end
diff --git a/test/tangent_types/tangent.jl b/test/tangent_types/tangent.jl
index 8a5f1666a..b0cb5577e 100644
--- a/test/tangent_types/tangent.jl
+++ b/test/tangent_types/tangent.jl
@@ -329,11 +329,15 @@ end
         @test c * NoTangent() == NoTangent()
         @test dot(NoTangent(), c) == NoTangent()
         @test dot(c, NoTangent()) == NoTangent()
+        @test norm(Tangent{Foo}(; y=c.y, x=NoTangent())) == c.y
+        @test norm(NoTangent(), Inf) == 0
 
         @test ZeroTangent() * c == ZeroTangent()
         @test c * ZeroTangent() == ZeroTangent()
         @test dot(ZeroTangent(), c) == ZeroTangent()
         @test dot(c, ZeroTangent()) == ZeroTangent()
+        @test norm(ZeroTangent()) == 0
+        @test norm(ZeroTangent(), 0.4) == 0
 
         @test true * c === c
         @test c * true === c