From 063ee3dc334ed44a5c8e2235919a678fffb15c2f Mon Sep 17 00:00:00 2001
From: Tamme Claus <tamme.claus@rwth-aachen.de>
Date: Fri, 27 Jun 2025 11:51:40 +0200
Subject: [PATCH 1/4] specialize kron!(::Diagonal, ::Diagonal, ::Diagonal)

---
 src/diagonal.jl | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/src/diagonal.jl b/src/diagonal.jl
index b6e26249..7a304738 100644
--- a/src/diagonal.jl
+++ b/src/diagonal.jl
@@ -806,6 +806,11 @@ end
 
 kron(A::Diagonal, B::Diagonal) = Diagonal(kron(A.diag, B.diag))
 
+function kron!(C::Diagonal, A::Diagonal, B::Diagonal)
+    kron!(C.diag, A.diag, B.diag)
+    return C
+end
+
 function kron(A::Diagonal, B::SymTridiagonal)
     kdv = kron(A.diag, B.dv)
     # We don't need to drop the last element

From 5c2d962afbaef47128b417838e54abaebc87cea1 Mon Sep 17 00:00:00 2001
From: Tamme Claus <tamme.claus@rwth-aachen.de>
Date: Fri, 27 Jun 2025 17:03:24 +0200
Subject: [PATCH 2/4] extend "kron! for Diagonal" testset

---
 test/diagonal.jl | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/test/diagonal.jl b/test/diagonal.jl
index 55d493e5..70ae4444 100644
--- a/test/diagonal.jl
+++ b/test/diagonal.jl
@@ -1396,14 +1396,25 @@ end
 end
 
 @testset "kron! for Diagonal" begin
-    a = Diagonal([2,2])
-    b = Diagonal([1,1])
+    a = Diagonal([1,2])
+    b = Diagonal([3,4])
+    # Diagonal out
     c = Diagonal([0,0,0,0])
     kron!(c,b,a)
-    @test c == Diagonal([2,2,2,2])
+    @test c == Diagonal([3, 6, 4, 8])
+    @test c == kron!(fill(0, 4, 4), Matrix(b), Matrix(a)) # against dense kron!
     c=Diagonal(Vector{Float64}(undef, 4))
     kron!(c,a,b)
-    @test c == Diagonal([2,2,2,2])
+    @test c == Diagonal([3.0, 4.0, 6.0, 8.0])
+
+    # AbstractArray out
+    c = fill(0, 4, 4)
+    kron!(c, b, a) 
+    @test c == diagm([3, 6, 4, 8])
+    @test c == kron!(fill(0, 4, 4), Matrix(b), Matrix(a)) # against dense kron!
+    c = Matrix{Float64}(undef, 4, 4)
+    kron!(c, a, b)
+    @test c == diagm([3.0, 4.0, 6.0, 8.0])
 end
 
 @testset "uppertriangular/lowertriangular" begin

From 57ebfb4cd098c0c2e3a6a7618cb4a0d0835200ad Mon Sep 17 00:00:00 2001
From: Tamme Claus <tamme.claus@rwth-aachen.de>
Date: Mon, 30 Jun 2025 10:20:59 +0200
Subject: [PATCH 3/4] add whitespaces

---
 test/diagonal.jl | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/test/diagonal.jl b/test/diagonal.jl
index 70ae4444..0c4be6c0 100644
--- a/test/diagonal.jl
+++ b/test/diagonal.jl
@@ -1396,15 +1396,15 @@ end
 end
 
 @testset "kron! for Diagonal" begin
-    a = Diagonal([1,2])
-    b = Diagonal([3,4])
+    a = Diagonal([1, 2])
+    b = Diagonal([3, 4])
     # Diagonal out
-    c = Diagonal([0,0,0,0])
-    kron!(c,b,a)
+    c = Diagonal([0, 0, 0, 0])
+    kron!(c, b, a)
     @test c == Diagonal([3, 6, 4, 8])
     @test c == kron!(fill(0, 4, 4), Matrix(b), Matrix(a)) # against dense kron!
-    c=Diagonal(Vector{Float64}(undef, 4))
-    kron!(c,a,b)
+    c = Diagonal(Vector{Float64}(undef, 4))
+    kron!(c, a, b)
     @test c == Diagonal([3.0, 4.0, 6.0, 8.0])
 
     # AbstractArray out

From 63339e9d6301435ad06d9d5da8364135f609784e Mon Sep 17 00:00:00 2001
From: Daniel Karrasch <daniel.karrasch@posteo.de>
Date: Mon, 30 Jun 2025 16:20:00 +0200
Subject: [PATCH 4/4] Update test/diagonal.jl

---
 test/diagonal.jl | 1 +
 1 file changed, 1 insertion(+)

diff --git a/test/diagonal.jl b/test/diagonal.jl
index 0c4be6c0..712f426c 100644
--- a/test/diagonal.jl
+++ b/test/diagonal.jl
@@ -1415,6 +1415,7 @@ end
     c = Matrix{Float64}(undef, 4, 4)
     kron!(c, a, b)
     @test c == diagm([3.0, 4.0, 6.0, 8.0])
+    @test_throws DimensionMismatch kron!(Diagonal(zeros(5)), Diagonal(zeros(2)), Diagonal(zeros(2)))
 end
 
 @testset "uppertriangular/lowertriangular" begin