specialise kron for sparse mat/vec and Diagonal -> sparse matrix (#32793)

Author: ranocha

stdlib/SparseArrays/src/linalg.jl

@@ -1307,6 +1307,10 @@ kron(x::SparseVector, A::SparseMatrixCSC) = kron(SparseMatrixCSC(x), A)
 kron(A::Union{SparseVector,SparseMatrixCSC}, B::VecOrMat) = kron(A, sparse(B))
 kron(A::VecOrMat, B::Union{SparseVector,SparseMatrixCSC}) = kron(sparse(A), B)
 
+# sparse vec/mat ⊗ Diagonal and vice versa
+kron(A::Diagonal{T}, B::Union{SparseVector{S}, SparseMatrixCSC{S}}) where {T<:Number, S<:Number} = kron(sparse(A), B)
+kron(A::Union{SparseVector{T}, SparseMatrixCSC{T}}, B::Diagonal{S}) where {T<:Number, S<:Number} = kron(A, sparse(B))
+
 # sparse outer product
 kron(A::SparseVectorUnion, B::AdjOrTransSparseVectorUnion) = A .* B
 

stdlib/SparseArrays/test/sparse.jl

@@ -374,10 +374,20 @@ end
         v = view(a, :, 1); v_d = Vector(v)
         x = sprand(m, 0.4); x_d = Vector(x)
         y = sprand(n, 0.3); y_d = Vector(y)
+        c_di = Diagonal(rand(m)); c = sparse(c_di); c_d = Array(c_di)
+        d_di = Diagonal(rand(n)); d = sparse(d_di); d_d = Array(d_di)
         # mat ⊗ mat
         @test Array(kron(a, b)) == kron(a_d, b_d)
         @test Array(kron(a_d, b)) == kron(a_d, b_d)
         @test Array(kron(a, b_d)) == kron(a_d, b_d)
+        @test issparse(kron(c, d_di))
+        @test Array(kron(c, d_di)) == kron(c_d, d_d)
+        @test issparse(kron(c_di, d))
+        @test Array(kron(c_di, d)) == kron(c_d, d_d)
+        @test issparse(kron(c_di, y))
+        @test Array(kron(c_di, y)) == kron(c_di, y_d)
+        @test issparse(kron(x, d_di))
+        @test Array(kron(x, d_di)) == kron(x_d, d_di)
         # vec ⊗ vec
         @test Vector(kron(x, y)) == kron(x_d, y_d)
         @test Vector(kron(x_d, y)) == kron(x_d, y_d)