bugfix for ldiv!(D::Diagonal, B::StridedVecOrMat) and tests (#32104)

(cherry picked from commit 85603e1)

Author: dkarrasch

stdlib/LinearAlgebra/src/diagonal.jl

@@ -481,7 +481,7 @@ function ldiv!(D::Diagonal, B::StridedVecOrMat)
             if di == 0
                 throw(SingularException(i))
             end
-            B[i,j] /= di
+            B[i,j] = di \ B[i,j]
         end
     end
     return B

stdlib/LinearAlgebra/test/diagonal.jl

@@ -441,6 +441,15 @@ end
     @test det(D) == 4
 end
 
+@testset "linear solve for block diagonal matrices" begin
+    D = Diagonal([rand(2,2) for _ in 1:5])
+    b = [rand(2,2) for _ in 1:5]
+    B = [rand(2,2) for _ in 1:5, _ in 1:5]
+    @test ldiv!(D, copy(b)) ≈ Diagonal(inv.(D.diag)) * b
+    @test ldiv!(D, copy(B)) ≈ Diagonal(inv.(D.diag)) * B
+    @test rdiv!(copy(B), D) ≈ B * Diagonal(inv.(D.diag))
+end
+
 @testset "multiplication with Symmetric/Hermitian" begin
     for T in (Float64, ComplexF64)
         D = Diagonal(randn(T, n))