Forward structure-preserving broadcasting to diag for Diagonal

src/structuredbroadcast.jl

@@ -208,6 +208,30 @@ end
 # All structured matrices are square, and therefore they only broadcast out if they are size (1, 1)
 Broadcast.newindex(D::StructuredMatrix, I::CartesianIndex{2}) = size(D) == (1,1) ? CartesianIndex(1,1) : I
 
+# Recursively replace wrapped matrices by their parents to improve broadcasting performance
+# We may do this because the indexing within `copyto!` is restricted to the stored indices
+preprocess_broadcasted(::Type{T}, A) where {T} = _preprocess_broadcasted(T, A)
+function preprocess_broadcasted(::Type{T}, bc::Broadcasted) where {T}
+    args = map(x -> preprocess_broadcasted(T, x), bc.args)
+    Broadcast.broadcasted(bc.f, args...)
+end
+# fallback case that doesn't unwrap at all
+_preprocess_broadcasted(::Type, x) = x
+
+_preprocess_broadcasted(::Type{Diagonal}, d::Diagonal) = d.diag
+# fallback for types that might opt into Diagonal-like structured broadcasting, e.g. wrappers
+_preprocess_broadcasted(::Type{Diagonal}, d::AbstractMatrix) = diag(d)
+
+function copy(bc::Broadcasted{StructuredMatrixStyle{Diagonal}})
+    if isstructurepreserving(bc) || fzeropreserving(bc)
+        # forward the broadcasting operation to the diagonal
+        bc2 = preprocess_broadcasted(Diagonal, bc)
+        return Diagonal(copy(bc2))
+    else
+        @invoke copy(bc::Broadcasted)
+    end
+end
+
 function copyto!(dest::Diagonal, bc::Broadcasted{<:StructuredMatrixStyle})
     isvalidstructbc(dest, bc) || return copyto!(dest, convert(Broadcasted{Nothing}, bc))
     axs = axes(dest)
@@ -269,13 +293,6 @@ function copyto!(dest::Tridiagonal, bc::Broadcasted{<:StructuredMatrixStyle})
     return dest
 end
 
-# Recursively replace wrapped matrices by their parents to improve broadcasting performance
-# We may do this because the indexing within `copyto!` is restricted to the stored indices
-preprocess_broadcasted(::Type{T}, A) where {T} = _preprocess_broadcasted(T, A)
-function preprocess_broadcasted(::Type{T}, bc::Broadcasted) where {T}
-    args = map(x -> preprocess_broadcasted(T, x), bc.args)
-    Broadcast.Broadcasted(bc.f, args, bc.axes)
-end
 _preprocess_broadcasted(::Type{LowerTriangular}, A) = lowertridata(A)
 _preprocess_broadcasted(::Type{UpperTriangular}, A) = uppertridata(A)
 

test/structuredbroadcast.jl

@@ -398,4 +398,10 @@ end
     end
 end
 
+@testset "forwarding for Diganoal" begin
+    D = Diagonal(1:4)
+    D2 = D .* 2
+    @test D2 isa Diagonal{Int, <:AbstractRange{Int}}
+end
+
 end