Create a copy while evaluating eigvals(::Diagonal)

[jishnub]

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.

[jishnub]

@dkarrasch would like your view on this

[dkarrasch]

Yes, this is the right way to do it. This should be backported to all versions >= v1.6. This aliasing issue was also found in #43856, but the "fix" (in the context of eigen) there included tests that weren't compatible with v1.6.