overload Base.kron, add tests

Author: vpuri3

src/tensor.jl

@@ -1,7 +1,5 @@
 #
-"""
-$SIGNATURES
-
+TENSOR_PROD_DOC = """
 Computes the lazy pairwise Kronecker product, or tensor product,
 operator of `AbstractMatrix`, and `AbstractSciMLOperator` subtypes.
 Calling `⊗(ops...)` is equivalent to `Base.kron(ops...)`. Fast
@@ -10,11 +8,18 @@ product operator.
 
 ```
 TensorProductOperator(A, B) = A ⊗ B
+TensorProductOperator(A, B, C) = A ⊗ B ⊗ C
 
 (A ⊗ B)(u) = vec(B * reshape(u, M, N) * transpose(A))
 ```
 where `M = size(B, 2)`, and `N = size(A, 2)`
 """
+
+"""
+$SIGNATURES
+
+$TENSOR_PROD_DOC
+"""
 struct TensorProductOperator{T,O,C} <: AbstractSciMLOperator{T}
     ops::O
     cache::C
@@ -54,18 +59,7 @@ TensorProductOperator(ii1::IdentityOperator, ii2::IdentityOperator) = IdentityOp
 """
 $SIGNATURES
 
-Computes the lazy pairwise Kronecker product, or tensor product,
-operator of `AbstractMatrix`, and `AbstractSciMLOperator` subtypes.
-Calling `⊗(ops...)` is equivalent to `Base.kron(ops...)`. Fast
-operator evaluation is performed without forming the full tensor
-product operator.
-
-```
-TensorProductOperator(A, B) = A ⊗ B
-
-(A ⊗ B)(u) = vec(B * reshape(u, M, N) * transpose(A))
-```
-where `M = size(B, 2)`, and `N = size(A, 2)`
+$TENSOR_PROD_DOC
 """
 ⊗(ops::Union{AbstractMatrix,AbstractSciMLOperator}...) = TensorProductOperator(ops...)
 
@@ -82,8 +76,7 @@ Base.kron(A::AbstractSciMLOperator, B::AbstractSciMLOperator) = TensorProductOpe
 Base.kron(A::AbstractMatrix, B::AbstractSciMLOperator) = TensorProductOperator(A, B)
 Base.kron(A::AbstractSciMLOperator, B::AbstractMatrix) = TensorProductOperator(A, B)
 
-# convert to matrix
-Base.kron(ops::AbstractSciMLOperator...) = kron(convert.(AbstractMatrix, ops)...)
+Base.kron(ops::AbstractSciMLOperator...) = TensorProductOperator(ops...)
 
 function Base.convert(::Type{AbstractMatrix}, L::TensorProductOperator)
     kron(convert.(AbstractMatrix, L.ops)...)

test/matrix.jl

@@ -1,7 +1,7 @@
 using SciMLOperators, LinearAlgebra
 using Random
 
-using SciMLOperators: InvertibleOperator, InvertedOperator, ⊗
+using SciMLOperators: InvertibleOperator, InvertedOperator, ⊗, AbstractSciMLOperator
 using FFTW
 
 Random.seed!(0)
@@ -263,6 +263,18 @@ for square in [false, true] #for K in [1, K]
     AB  = kron(A, B)
     ABC = kron(A, B, C)
 
+    # test Base.kron overload
+    # ensure kron(mat, mat) is not a TensorProductOperator
+    @test !isa(AB, AbstractSciMLOperator)
+    @test !isa(ABC, AbstractSciMLOperator)
+
+    # test Base.kron overload
+    _A = rand(N, N)
+    @test kron(_A, MatrixOperator(_A)) isa TensorProductOperator
+    @test kron(MatrixOperator(_A), _A) isa TensorProductOperator
+
+    @test kron(MatrixOperator(_A), MatrixOperator(_A)) isa TensorProductOperator
+
     # Inputs
     u2 = rand(n1*n2, K)
     u3 = rand(n1*n2*n3, K)