eigvals(::SymTridiagonal) errors if off-diagonal has wrong length

[dlfivefifty]

The following errors. I can prepare a fix but I don't understand why SymTridiagonal(randn(5), randn(5)) works: I would expect it to require a length 4 off-diagonal vector. The docs don't clarify anything...

julia> A = SymTridiagonal(randn(5), randn(5))
5×5 SymTridiagonal{Float64, Vector{Float64}}:
 1.54728  1.29122     ⋅          ⋅          ⋅ 
 1.29122  1.7491     0.820245    ⋅          ⋅ 
  ⋅       0.820245  -0.933958   0.816364    ⋅ 
  ⋅        ⋅         0.816364  -0.2896    -0.552447
  ⋅        ⋅          ⋅        -0.552447   0.669905

julia> eigvals(A)
ERROR: DimensionMismatch("ev has length 5 but needs one less than dv's length, 5)")
Stacktrace:
 [1] stev!(job::Char, dv::Vector{Float64}, ev::Vector{Float64})
   @ LinearAlgebra.LAPACK ~/Projects/julia-1.6/usr/share/julia/stdlib/v1.6/LinearAlgebra/src/lapack.jl:3774
 [2] eigvals!
   @ ~/Projects/julia-1.6/usr/share/julia/stdlib/v1.6/LinearAlgebra/src/tridiag.jl:293 [inlined]
 [3] eigvals(A::SymTridiagonal{Float64, Vector{Float64}})
   @ LinearAlgebra ~/Projects/julia-1.6/usr/share/julia/stdlib/v1.6/LinearAlgebra/src/tridiag.jl:294
 [4] top-level scope
   @ REPL[3]:1

[fredrikekre]

6b954c9 8647373 for why SymTridiagonal(randn(5), randn(5)) works (cc @andreasnoack).

[dlfivefifty]

Ha the issue was predicted: 6b954c9#commitcomment-8013969

[andreasnoack]

I don't see why we should error in the call to eigvals. We should just ignore that last element.

[dlfivefifty]

Do you mean allow ev to have one extra entry in the stev! call, changing

julia/stdlib/LinearAlgebra/src/lapack.jl

Line 3774 in cc639ec

throw(DimensionMismatch("ev has length $(length(ev)) but needs one less than dv's length, $n)"))

That's an easy fix

[andreasnoack]

Yes exactly.