Support broadcasting over structured block matrices (JuliaLang#53909)

Fix JuliaLang#48664

After this, broadcasting over structured block matrices with
matrix-valued elements works:
```julia
julia> D = Diagonal([[1 2; 3 4], [5 6; 7 8]])
2×2 Diagonal{Matrix{Int64}, Vector{Matrix{Int64}}}:
 [1 2; 3 4]      ⋅     
     ⋅       [5 6; 7 8]

julia> D .+ D
2×2 Diagonal{Matrix{Int64}, Vector{Matrix{Int64}}}:
 [2 4; 6 8]      ⋅     
     ⋅       [10 12; 14 16]

julia> cos.(D)
2×2 Matrix{Matrix{Float64}}:
 [0.855423 -0.110876; -0.166315 0.689109]  [1.0 0.0; 0.0 1.0]
 [1.0 0.0; 0.0 1.0]                        [0.928384 -0.069963; -0.0816235 0.893403]
```
Such operations show up when using `BlockArrays`.

The implementation is a bit hacky as it uses `0I` as the zero element in
`fzero`, which isn't really the correct zero if the blocks are
rectangular. Nonetheless, this works, as `fzero` is only used to
determine if the structure is preserved.

stdlib/LinearAlgebra/src/structuredbroadcast.jl

@@ -8,8 +8,8 @@ struct StructuredMatrixStyle{T} <: Broadcast.AbstractArrayStyle{2} end
 StructuredMatrixStyle{T}(::Val{2}) where {T} = StructuredMatrixStyle{T}()
 StructuredMatrixStyle{T}(::Val{N}) where {T,N} = Broadcast.DefaultArrayStyle{N}()
 
-const StructuredMatrix = Union{Diagonal,Bidiagonal,SymTridiagonal,Tridiagonal,LowerTriangular,UnitLowerTriangular,UpperTriangular,UnitUpperTriangular}
-for ST in Base.uniontypes(StructuredMatrix)
+const StructuredMatrix{T} = Union{Diagonal{T},Bidiagonal{T},SymTridiagonal{T},Tridiagonal{T},LowerTriangular{T},UnitLowerTriangular{T},UpperTriangular{T},UnitUpperTriangular{T}}
+for ST in (Diagonal,Bidiagonal,SymTridiagonal,Tridiagonal,LowerTriangular,UnitLowerTriangular,UpperTriangular,UnitUpperTriangular)
     @eval Broadcast.BroadcastStyle(::Type{<:$ST}) = $(StructuredMatrixStyle{ST}())
 end
 
@@ -133,6 +133,7 @@ fails as `zero(::Tuple{Int})` is not defined. However,
 iszerodefined(::Type) = false
 iszerodefined(::Type{<:Number}) = true
 iszerodefined(::Type{<:AbstractArray{T}}) where T = iszerodefined(T)
+iszerodefined(::Type{<:UniformScaling{T}}) where T = iszerodefined(T)
 
 fzeropreserving(bc) = (v = fzero(bc); !ismissing(v) && (iszerodefined(typeof(v)) ? iszero(v) : v == 0))
 # Like sparse matrices, we assume that the zero-preservation property of a broadcasted
@@ -144,6 +145,7 @@ fzero(::Type{T}) where T = T
 fzero(r::Ref) = r[]
 fzero(t::Tuple{Any}) = t[1]
 fzero(S::StructuredMatrix) = zero(eltype(S))
+fzero(::StructuredMatrix{<:AbstractMatrix{T}}) where {T<:Number} = haszero(T) ? zero(T)*I : missing
 fzero(x) = missing
 function fzero(bc::Broadcast.Broadcasted)
     args = map(fzero, bc.args)

stdlib/LinearAlgebra/test/special.jl

@@ -111,6 +111,7 @@ Random.seed!(1)
         struct TypeWithZero end
         Base.promote_rule(::Type{TypeWithoutZero}, ::Type{TypeWithZero}) = TypeWithZero
         Base.convert(::Type{TypeWithZero}, ::TypeWithoutZero) = TypeWithZero()
+        Base.zero(x::Union{TypeWithoutZero, TypeWithZero}) = zero(typeof(x))
         Base.zero(::Type{<:Union{TypeWithoutZero, TypeWithZero}}) = TypeWithZero()
         LinearAlgebra.symmetric(::TypeWithoutZero, ::Symbol) = TypeWithoutZero()
         LinearAlgebra.symmetric_type(::Type{TypeWithoutZero}) = TypeWithoutZero

stdlib/LinearAlgebra/test/structuredbroadcast.jl

@@ -280,4 +280,62 @@ end
 # structured broadcast with function returning non-number type
 @test tuple.(Diagonal([1, 2])) == [(1,) (0,); (0,) (2,)]
 
+@testset "broadcast over structured matrices with matrix elements" begin
+    function standardbroadcastingtests(D, T)
+        M = [x for x in D]
+        Dsum = D .+ D
+        @test Dsum isa T
+        @test Dsum == M .+ M
+        Dcopy = copy.(D)
+        @test Dcopy isa T
+        @test Dcopy == D
+        Df = float.(D)
+        @test Df isa T
+        @test Df == D
+        @test eltype(eltype(Df)) <: AbstractFloat
+        @test (x -> (x,)).(D) == (x -> (x,)).(M)
+        @test (x -> 1).(D) == ones(Int,size(D))
+        @test all(==(2), ndims.(D))
+        @test_throws MethodError size.(D)
+    end
+    @testset "Diagonal" begin
+        @testset "square" begin
+            A = [1 3; 2 4]
+            D = Diagonal([A, A])
+            standardbroadcastingtests(D, Diagonal)
+            @test sincos.(D) == sincos.(Matrix{eltype(D)}(D))
+            M = [x for x in D]
+            @test cos.(D) == cos.(M)
+        end
+
+        @testset "different-sized square blocks" begin
+            D = Diagonal([ones(3,3), fill(3.0,2,2)])
+            standardbroadcastingtests(D, Diagonal)
+        end
+
+        @testset "rectangular blocks" begin
+            D = Diagonal([ones(Bool,3,4), ones(Bool,2,3)])
+            standardbroadcastingtests(D, Diagonal)
+        end
+
+        @testset "incompatible sizes" begin
+            A = reshape(1:12, 4, 3)
+            B = reshape(1:12, 3, 4)
+            D1 = Diagonal(fill(A, 2))
+            D2 = Diagonal(fill(B, 2))
+            @test_throws DimensionMismatch D1 .+ D2
+        end
+    end
+    @testset "Bidiagonal" begin
+        A = [1 3; 2 4]
+        B = Bidiagonal(fill(A,3), fill(A,2), :U)
+        standardbroadcastingtests(B, Bidiagonal)
+    end
+    @testset "UpperTriangular" begin
+        A = [1 3; 2 4]
+        U = UpperTriangular([(i+j)*A for i in 1:3, j in 1:3])
+        standardbroadcastingtests(U, UpperTriangular)
+    end
+end
+
 end