Fix kron with Diagonal (#40509)

Author: dkarrasch

stdlib/LinearAlgebra/src/diagonal.jl

@@ -472,14 +472,13 @@ rdiv!(A::AbstractMatrix{T}, transD::Transpose{<:Any,<:Diagonal{T}}) where {T} =
     invoke(\, Tuple{Union{QR,QRCompactWY,QRPivoted}, AbstractVecOrMat}, A, B)
 
 
-@inline function kron!(C::AbstractMatrix{T}, A::Diagonal, B::Diagonal) where T
-    fill!(C, zero(T))
+@inline function kron!(C::AbstractMatrix, A::Diagonal, B::Diagonal)
     valA = A.diag; nA = length(valA)
     valB = B.diag; nB = length(valB)
     nC = checksquare(C)
     @boundscheck nC == nA*nB ||
         throw(DimensionMismatch("expect C to be a $(nA*nB)x$(nA*nB) matrix, got size $(nC)x$(nC)"))
-
+    isempty(A) || isempty(B) || fill!(C, zero(A[1,1] * B[1,1]))
     @inbounds for i = 1:nA, j = 1:nB
         idx = (i-1)*nB+j
         C[idx, idx] = valA[i] * valB[j]
@@ -497,9 +496,12 @@ end
 
 @inline function kron!(C::AbstractMatrix, A::Diagonal, B::AbstractMatrix)
     Base.require_one_based_indexing(B)
-    (mA, nA) = size(A); (mB, nB) = size(B); (mC, nC) = size(C);
+    (mA, nA) = size(A)
+    (mB, nB) = size(B)
+    (mC, nC) = size(C)
     @boundscheck (mC, nC) == (mA * mB, nA * nB) ||
         throw(DimensionMismatch("expect C to be a $(mA * mB)x$(nA * nB) matrix, got size $(mC)x$(nC)"))
+    isempty(A) || isempty(B) || fill!(C, zero(A[1,1] * B[1,1]))
     m = 1
     @inbounds for j = 1:nA
         A_jj = A[j,j]
@@ -517,9 +519,12 @@ end
 
 @inline function kron!(C::AbstractMatrix, A::AbstractMatrix, B::Diagonal)
     require_one_based_indexing(A)
-    (mA, nA) = size(A); (mB, nB) = size(B); (mC, nC) = size(C);
+    (mA, nA) = size(A)
+    (mB, nB) = size(B)
+    (mC, nC) = size(C)
     @boundscheck (mC, nC) == (mA * mB, nA * nB) ||
         throw(DimensionMismatch("expect C to be a $(mA * mB)x$(nA * nB) matrix, got size $(mC)x$(nC)"))
+    isempty(A) || isempty(B) || fill!(C, zero(A[1,1] * B[1,1]))
     m = 1
     @inbounds for j = 1:nA
         for l = 1:mB
@@ -535,18 +540,6 @@ end
     return C
 end
 
-function kron(A::Diagonal{T}, B::AbstractMatrix{S}) where {T<:Number, S<:Number}
-    (mA, nA) = size(A); (mB, nB) = size(B)
-    R = zeros(Base.promote_op(*, T, S), mA * mB, nA * nB)
-    return @inbounds kron!(R, A, B)
-end
-
-function kron(A::AbstractMatrix{T}, B::Diagonal{S}) where {T<:Number, S<:Number}
-    (mA, nA) = size(A); (mB, nB) = size(B)
-    R = zeros(promote_op(*, T, S), mA * mB, nA * nB)
-    return @inbounds kron!(R, A, B)
-end
-
 conj(D::Diagonal) = Diagonal(conj(D.diag))
 transpose(D::Diagonal{<:Number}) = D
 transpose(D::Diagonal) = Diagonal(transpose.(D.diag))

stdlib/LinearAlgebra/test/diagonal.jl

@@ -299,6 +299,10 @@ Random.seed!(1)
         M4 = rand(elty, n÷2, n÷2)
         @test kron(D3, M4) ≈ kron(DM3, M4)
         @test kron(M4, D3) ≈ kron(M4, DM3)
+        X = [ones(1,1) for i in 1:2, j in 1:2]
+        @test kron(I(2), X)[1,3] == zeros(1,1)
+        X = [ones(2,2) for i in 1:2, j in 1:2]
+        @test kron(I(2), X)[1,3] == zeros(2,2)
     end
     @testset "iszero, isone, triu, tril" begin
         Dzero = Diagonal(zeros(elty, 10))