Fix neutral_element for AbstractArrays (#320)

Author: odow

src/reduce.jl

@@ -22,7 +22,8 @@ reduce_op(op::AddSubMul) = add_sub_op(op)
 
 reduce_op(::typeof(add_dot)) = +
 
-neutral_element(::typeof(+), T::Type) = zero(T)
+neutral_element(::typeof(+), ::Type{T}) where {T} = zero(T)
+neutral_element(::typeof(+), ::Type{T}) where {T<:AbstractArray} = Zero()
 
 map_op(::AddSubMul) = *
 

test/interface.jl

@@ -215,3 +215,21 @@ end
     @test ret == reshape([3.0], 1, 1)
     @test y == reshape([1.0], 1, 1, 1)
 end
+
+@testset "issue_318_neutral_element" begin
+    a = rand(3)
+    A = [rand(2, 2) for _ in 1:3]
+    @test_throws DimensionMismatch MA.operate(LinearAlgebra.dot, a, A)
+    y = a' * A
+    @test isapprox(MA.fused_map_reduce(MA.add_mul, a', A), y)
+    z = MA.operate(LinearAlgebra.dot, Int[], Int[])
+    @test iszero(z) && z isa Int
+    z = MA.operate(LinearAlgebra.dot, BigInt[], Int[])
+    @test iszero(z) && z isa BigInt
+    z = MA.operate(LinearAlgebra.dot, Int[], Float64[])
+    @test iszero(z) && z isa Float64
+    z = MA.operate(LinearAlgebra.dot, Matrix{Int}[], Matrix{Float64}[])
+    @test iszero(z) && z isa Float64
+    @test MA.fused_map_reduce(MA.add_mul, Matrix{Int}[], Float64[]) isa MA.Zero
+    @test MA.fused_map_reduce(MA.add_mul, Float64[], Matrix{Int}[]) isa MA.Zero
+end