Add similar_array_type for LinearAlgebra.Diagonal (#205)

Author: odow

src/implementations/LinearAlgebra.jl

@@ -155,6 +155,14 @@ function similar_array_type(
     return LinearAlgebra.Symmetric{S,similar_array_type(MT, S)}
 end
 
+function similar_array_type(
+    ::Type{LinearAlgebra.Diagonal{T,VT}},
+    ::Type{S},
+) where {S,T,VT<:AbstractVector{T}}
+    return LinearAlgebra.Diagonal{S,similar_array_type(VT, S)}
+end
+
+similar_array_type(::Type{<:AbstractVector}, ::Type{T}) where {T} = Vector{T}
 similar_array_type(::Type{Array{T,N}}, ::Type{S}) where {S,T,N} = Array{S,N}
 
 similar_array_type(::Type{BitArray{N}}, ::Type{S}) where {S,N} = Array{S,N}

test/interface.jl

@@ -6,6 +6,7 @@
 
 using Test
 import MutableArithmetics as MA
+import LinearAlgebra
 
 struct DummyMutable end
 
@@ -142,3 +143,15 @@ end
     @test MA.similar_array_type(BitArray{2}, Int) == Array{Int,2}
     @test MA.similar_array_type(BitArray{2}, Bool) == BitArray{2}
 end
+
+@testset "similar_array_type_Diagonal" begin
+    z = zeros(2, 2)
+    y = MA.operate!!(MA.add_mul, z, big(1), LinearAlgebra.I(2))
+    @test y == BigFloat[1 0; 0 1]
+    y = MA.operate!!(MA.add_mul, z, 2.4, LinearAlgebra.I(2))
+    @test y === z
+    @test y == Float64[2.4 0; 0 2.4]
+    z = zeros(2, 2)
+    y = MA.operate!!(MA.add_mul, z, 2.4, LinearAlgebra.Diagonal(1:2))
+    @test y == LinearAlgebra.Diagonal(2.4 * (1:2))
+end