Stop cg_lanczos! resolving to the shifted workspace - #1251
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`get_KrylovJL_solver` tested `KrylovAlg === Krylov.cg_lanczos!` twice. The
first branch returned `CgLanczosShiftWorkspace`, so it won, and the second
branch returning `CgLanczosWorkspace` was unreachable.
`cg_lanczos!` is listed in the `KrylovJL` docstring as a supported
`KrylovAlg`, so this is reachable from documented usage:
solve(LinearProblem(A, b), KrylovJL(KrylovAlg = Krylov.cg_lanczos!))
MethodError: no method matching CgLanczosShiftWorkspace(::Matrix{Float64}, ::Vector{Float64})
Dropping the first branch lets it fall through to the right workspace. The
solve then succeeds, to a relative error of 5.9e-9 on a symmetric positive
definite system.
`cg_lanczos_shift!` and `cgls_lanczos_shift!` are deliberately left
unmapped. Both exist in Krylov 0.10, but they take a `shifts` argument that
this wrapper never passes, and neither appears in the docstring's list, so
they keep hitting "Invalid Krylov method detected" rather than being
advertised and broken.
Spotted in SciML#554, which is otherwise superseded: block GMRES and MINRES are
already interfaced through `BlockGmresWorkspace` and `BlockMinresWorkspace`
for a matrix right-hand side.
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Additional context
get_KrylovJL_solvermatchedcg_lanczos!twice. The first branch returned the shifted workspace and won, so the correct branch below it was dead code. Sincecg_lanczos!is listed in theKrylovJLdocstring,KrylovJL(KrylovAlg = Krylov.cg_lanczos!)threw aMethodErrorfrom the workspace constructor. Dropping the stray branch fixes it.The shifted variants stay unmapped on purpose: they take a
shiftsargument this wrapper never passes, and they are not in the documented list.Spotted in #554, which is otherwise superseded now that block GMRES and MINRES are reached by passing a matrix
btoKrylovJL_GMRES/KrylovJL_MINRES.AI Disclosure: Used Opus 5