Use views for wrappers

Author: jishnub

src/structuredbroadcast.jl

@@ -242,7 +242,7 @@ _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)
+_preprocess_broadcasted(::Type{Diagonal}, d::AbstractMatrix) = diagview(d)
 
 function copy(bc::Broadcasted{StructuredMatrixStyle{Diagonal}})
     if isstructurepreserving(bc) || fzeropreserving(bc)

test/structuredbroadcast.jl

@@ -410,10 +410,19 @@ end
     @test UH2 isa UpperHessenberg
 end
 
-@testset "forwarding for Diganoal" begin
+@testset "forwarding broadcast to the diag for a Diagonal" begin
     D = Diagonal(1:4)
     D2 = D .* 2
     @test D2 isa Diagonal{Int, <:AbstractRange{Int}}
+
+    # test for wrappers that opt into Diagonal-like broadcasting
+    U = UpperTriangular(D)
+    bc = Broadcast.broadcasted(+, D, U)
+    bcD = Broadcast.broadcasted(+, D, D)
+    S = typeof(Broadcast.BroadcastStyle(typeof(bcD)))
+    bc2 = convert(Broadcast.Broadcasted{S}, bc)
+    @test copy(bc2) == copy(bc) == copy(bcD)
+    @test copy(bc2) isa Diagonal
 end
 
 end