Possibly unreachable line in `lapack.jl`

[ajinkya-k]

Is the following line in lapack.jl even reachable?

LinearAlgebra.jl/src/lapack.jl

Line 4141 in 2a1696a

error("failed to converge eigenvectors:\n$(findall(!iszero, ifail))")

The call to chklapackerror(info[]) on line 4138

LinearAlgebra.jl/src/lapack.jl

Line 4138 in 2a1696a

chklapackerror(info[])

should throw an error if any of the eigenvectors failed to converge and therefore its unlikely the if block would ever be reached.

can someone suggest a test for this? I tried a bunch of matrices from the MatrixDepot.jl packages but none of them threw any errors when running eigen.

The relevant code for context (comments with <-- mine)

# .... # 
# lapack.jl lines 4138 - 4142
            chklapackerror(info[]) 
            if any(ifail .!= 0) # <-- should be unreachable because of the line above
                # TODO: better error message / type
                error("failed to converge eigenvectors:\n$(findall(!iszero, ifail))") # <-- this line here
            end

# ... # 

[aravindh-krishnamoorthy]

This will always throw an error. If you look at the documentation in LAPACK's dstein function, you'll note that INFO > 0 is set as a warning for nonconvergence, and the corresponding nonconvergent eigenvalue numbers are given in IFAIL. In

LinearAlgebra.jl/src/lapack.jl

Line 34 in 2a1696a

function chklapackerror(ret::BlasInt, f...)

positive INFO throws a LAPACKException. Life can perhaps be improved by turning the INFO > 0 case for dstein into a warning, or, if we're being strict, by including IFAIL values in the thrown LAPACKException.