Currently, `eigvals(D::Diagonal{<:Number})` returns `D.diag`, which means that subsequent changes to `D` will change previously computed eigenvalues.

```
julia> D = Diagonal([1,2,3])
3×3 Diagonal{Int64, Vector{Int64}}:
 1  ⋅  ⋅
 ⋅  2  ⋅
 ⋅  ⋅  3

julia> λ = eigvals(D)
3-element Vector{Int64}:
 1
 2
 3

julia> D[3,3] = 4
4

julia> λ
3-element Vector{Int64}:
 1
 2
 4
```

Creating a copy should fix this