fix #31674, error when storing nonzeros into structural zeros with .=

Previously, broadcasted assignment (`.=`) would happily ignore all nonstructured portions of the destination, regardless of whether the broadcasted expression would actually evaluate to zero or not. This changes these in-place methods to use the same infrastructure that out-of-place broadcast uses to determine the result type. If we are unsure of the structural properties of the output, we fall back to the generic implementation, which will attempt to store into every single location of the destination -- including those structural zeros. Thus we now error in cases where we generate nonzeros in those locations.

stdlib/LinearAlgebra/src/structuredbroadcast.jl

@@ -102,6 +102,7 @@ function Base.similar(bc::Broadcasted{StructuredMatrixStyle{T}}, ::Type{ElType})
 end
 
 function copyto!(dest::Diagonal, bc::Broadcasted{<:StructuredMatrixStyle})
+    !isstructurepreserving(bc) && !fzeropreserving(bc) && copyto!(dest, convert(Broadcasted{Nothing}, bc))
     axs = axes(dest)
     axes(bc) == axs || Broadcast.throwdm(axes(bc), axs)
     for i in axs[1]
@@ -111,6 +112,7 @@ function copyto!(dest::Diagonal, bc::Broadcasted{<:StructuredMatrixStyle})
 end
 
 function copyto!(dest::Bidiagonal, bc::Broadcasted{<:StructuredMatrixStyle})
+    !isstructurepreserving(bc) && !fzeropreserving(bc) && copyto!(dest, convert(Broadcasted{Nothing}, bc))
     axs = axes(dest)
     axes(bc) == axs || Broadcast.throwdm(axes(bc), axs)
     for i in axs[1]
@@ -129,18 +131,22 @@ function copyto!(dest::Bidiagonal, bc::Broadcasted{<:StructuredMatrixStyle})
 end
 
 function copyto!(dest::SymTridiagonal, bc::Broadcasted{<:StructuredMatrixStyle})
+    !isstructurepreserving(bc) && !fzeropreserving(bc) && copyto!(dest, convert(Broadcasted{Nothing}, bc))
     axs = axes(dest)
     axes(bc) == axs || Broadcast.throwdm(axes(bc), axs)
     for i in axs[1]
         dest.dv[i] = Broadcast._broadcast_getindex(bc, CartesianIndex(i, i))
     end
     for i = 1:size(dest, 1)-1
-        dest.ev[i] = Broadcast._broadcast_getindex(bc, CartesianIndex(i, i+1))
+        v = Broadcast._broadcast_getindex(bc, CartesianIndex(i, i+1))
+        v == Broadcast._broadcast_getindex(bc, CartesianIndex(i+1, i)) || throw(ArgumentError("broadcasted assignment breaks symmetry between locations ($i, $(i+1)) and ($(i+1), $i)"))
+        dest.ev[i] = v
     end
     return dest
 end
 
 function copyto!(dest::Tridiagonal, bc::Broadcasted{<:StructuredMatrixStyle})
+    !isstructurepreserving(bc) && !fzeropreserving(bc) && copyto!(dest, convert(Broadcasted{Nothing}, bc))
     axs = axes(dest)
     axes(bc) == axs || Broadcast.throwdm(axes(bc), axs)
     for i in axs[1]
@@ -154,6 +160,7 @@ function copyto!(dest::Tridiagonal, bc::Broadcasted{<:StructuredMatrixStyle})
 end
 
 function copyto!(dest::LowerTriangular, bc::Broadcasted{<:StructuredMatrixStyle})
+    !isstructurepreserving(bc) && !fzeropreserving(bc) && copyto!(dest, convert(Broadcasted{Nothing}, bc))
     axs = axes(dest)
     axes(bc) == axs || Broadcast.throwdm(axes(bc), axs)
     for j in axs[2]
@@ -165,6 +172,7 @@ function copyto!(dest::LowerTriangular, bc::Broadcasted{<:StructuredMatrixStyle}
 end
 
 function copyto!(dest::UpperTriangular, bc::Broadcasted{<:StructuredMatrixStyle})
+    !isstructurepreserving(bc) && !fzeropreserving(bc) && copyto!(dest, convert(Broadcasted{Nothing}, bc))
     axs = axes(dest)
     axes(bc) == axs || Broadcast.throwdm(axes(bc), axs)
     for j in axs[2]

stdlib/LinearAlgebra/test/structuredbroadcast.jl

@@ -51,14 +51,37 @@ end
     A = rand(N, N)
     sA = A + copy(A')
     D = Diagonal(rand(N))
-    B = Bidiagonal(rand(N), rand(N - 1), :U)
+    Bu = Bidiagonal(rand(N), rand(N - 1), :U)
+    Bl = Bidiagonal(rand(N), rand(N - 1), :L)
     T = Tridiagonal(rand(N - 1), rand(N), rand(N - 1))
+    ◣ = LowerTriangular(rand(N,N))
+    ◥ = UpperTriangular(rand(N,N))
+
     @test broadcast!(sin, copy(D), D) == Diagonal(sin.(D))
-    @test broadcast!(sin, copy(B), B) == Bidiagonal(sin.(B), :U)
+    @test broadcast!(sin, copy(Bu), Bu) == Bidiagonal(sin.(Bu), :U)
+    @test broadcast!(sin, copy(Bl), Bl) == Bidiagonal(sin.(Bl), :L)
     @test broadcast!(sin, copy(T), T) == Tridiagonal(sin.(T))
+    @test broadcast!(sin, copy(◣), ◣) == LowerTriangular(sin.(◣))
+    @test broadcast!(sin, copy(◥), ◥) == UpperTriangular(sin.(◥))
     @test broadcast!(*, copy(D), D, A) == Diagonal(broadcast(*, D, A))
-    @test broadcast!(*, copy(B), B, A) == Bidiagonal(broadcast(*, B, A), :U)
+    @test broadcast!(*, copy(Bu), Bu, A) == Bidiagonal(broadcast(*, Bu, A), :U)
+    @test broadcast!(*, copy(Bl), Bl, A) == Bidiagonal(broadcast(*, Bl, A), :L)
     @test broadcast!(*, copy(T), T, A) == Tridiagonal(broadcast(*, T, A))
+    @test broadcast!(*, copy(◣), ◣, A) == LowerTriangular(broadcast(*, ◣, A))
+    @test broadcast!(*, copy(◥), ◥, A) == UpperTriangular(broadcast(*, ◥, A))
+
+    @test_throws ArgumentError broadcast!(cos, copy(D), D) == Diagonal(sin.(D))
+    @test_throws ArgumentError broadcast!(cos, copy(Bu), Bu) == Bidiagonal(sin.(Bu), :U)
+    @test_throws ArgumentError broadcast!(cos, copy(Bl), Bl) == Bidiagonal(sin.(Bl), :L)
+    @test_throws ArgumentError broadcast!(cos, copy(T), T) == Tridiagonal(sin.(T))
+    @test_throws ArgumentError broadcast!(cos, copy(◣), ◣) == LowerTriangular(sin.(◣))
+    @test_throws ArgumentError broadcast!(cos, copy(◥), ◥) == UpperTriangular(sin.(◥))
+    @test_throws ArgumentError broadcast!(+, copy(D), D, A) == Diagonal(broadcast(*, D, A))
+    @test_throws ArgumentError broadcast!(+, copy(Bu), Bu, A) == Bidiagonal(broadcast(*, Bu, A), :U)
+    @test_throws ArgumentError broadcast!(+, copy(Bl), Bl, A) == Bidiagonal(broadcast(*, Bl, A), :L)
+    @test_throws ArgumentError broadcast!(+, copy(T), T, A) == Tridiagonal(broadcast(*, T, A))
+    @test_throws ArgumentError broadcast!(+, copy(◣), ◣, A) == LowerTriangular(broadcast(*, ◣, A))
+    @test_throws ArgumentError broadcast!(+, copy(◥), ◥, A) == UpperTriangular(broadcast(*, ◥, A))
 end
 
 @testset "map[!] over combinations of structured matrices" begin