Fix layout of cholesky(Symmetric(view(BandedMatrix))) (#451)

* Fix 450

* Change tests for LTS

* Change tests for LTS

* Should also test that U is actually correct

* Fix check of LTS

Project.toml

@@ -1,6 +1,6 @@
 name = "BandedMatrices"
 uuid = "aae01518-5342-5314-be14-df237901396f"
-version = "1.7.2"
+version = "1.7.3"
 
 [deps]
 ArrayLayouts = "4c555306-a7a7-4459-81d9-ec55ddd5c99a"

src/symbanded/BandedCholesky.jl

@@ -98,5 +98,7 @@ ArrayLayouts._cholesky(::Union{SymmetricLayout{<:AbstractBandedLayout},Hermitian
 
 cholcopy(A::RealHermSymComplexHerm{<:Any,<:BandedMatrix}) =
     copyto!(similar(A, LinearAlgebra.choltype(A)), A)
+cholcopy(A::RealHermSymComplexHerm{<:Any,<:SubArray{<:Number, 2, <:BandedMatrix}}) =
+    copyto!(similar(A, LinearAlgebra.choltype(A)), A)
 # cholesky(A::Hermitian{T,<:BandedMatrix{T}},
 #     ::Val{false}=Val(false); check::Bool = true) where T = cholesky!(cholcopy(A); check = check)

test/test_symbanded.jl

@@ -402,6 +402,23 @@ end
     @test isposdef(B) == isposdef(Matrix(B)) == false
     B = BandedMatrix(1=>Float64[1:4;], 0=>Float64[2:2:10;], -1=>Float64[1:4;])
     @test isposdef(B) == isposdef(Matrix(B)) == true
+
+    @testset "Issue #450: Cholesky of a BandedMatrix's view" begin
+        L = brand(10, 10, 2, 0) 
+        B = BandedMatrix(Symmetric(L * L'));
+        Bv = Symmetric(view(B, :, :))
+        chol = cholesky(Bv)
+        if VERSION >= v"1.7"
+            @test MemoryLayout(chol.U) isa TriangularLayout{'U', 'N', typeof(MemoryLayout(B))}
+            @test MemoryLayout(chol.L) isa TriangularLayout{'L', 'N', typeof(MemoryLayout(B'))}
+            @test isbanded(chol.L)
+        end
+        @test isbanded(chol.U)
+        cc = LinearAlgebra.cholcopy(Bv)
+        @test cc isa Symmetric{Float64, typeof(B)}
+        @test chol.U ≈ cholesky(Matrix(B)).U
+        @test cc == B
+    end
 end
 
 end # module