Fix default Krylov preconditioner dispatch

Dispatch on the selected inner Krylov workspace so DefaultLinearSolver forwards supplied left and right preconditioners.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://chatgpt.com/codex/tasks/01a03a17-ad6f-7131-82fc-d0fd57ea6512

Author: ChrisRackauckas

src/iterative_wrappers.jl

@@ -680,41 +680,41 @@ function SciMLBase.solve!(cache::LinearCache, alg::KrylovJL; kwargs...)
         ldiv = true, history = true, filtered_kwargs...,
     )
 
-    if cache.cacheval isa Krylov.CgWorkspace
+    if cacheval isa Krylov.CgWorkspace
         N !== I &&
             @SciMLMessage(
             "$(alg.KrylovAlg) doesn't support right preconditioning.",
             verbose, :no_right_preconditioning
         )
         Krylov.krylov_solve!(args...; M, kwargs...)
-    elseif cache.cacheval isa Krylov.GmresWorkspace
+    elseif cacheval isa Krylov.GmresWorkspace
         Krylov.krylov_solve!(args...; M, N, restart = alg.gmres_restart > 0, kwargs...)
-    elseif cache.cacheval isa Krylov.FgmresWorkspace
+    elseif cacheval isa Krylov.FgmresWorkspace
         Krylov.krylov_solve!(args...; M, N, kwargs...)
-    elseif cache.cacheval isa Krylov.BicgstabWorkspace
+    elseif cacheval isa Krylov.BicgstabWorkspace
         Krylov.krylov_solve!(args...; M, N, kwargs...)
-    elseif cache.cacheval isa Krylov.MinresWorkspace
+    elseif cacheval isa Krylov.MinresWorkspace
         N !== I &&
             @SciMLMessage(
             "$(alg.KrylovAlg) doesn't support right preconditioning.",
             verbose, :no_right_preconditioning
         )
         Krylov.krylov_solve!(args...; M, kwargs...)
-    elseif cache.cacheval isa Krylov.BlockGmresWorkspace
+    elseif cacheval isa Krylov.BlockGmresWorkspace
         Krylov.krylov_solve!(args...; M, N, restart = alg.gmres_restart > 0, kwargs...)
-    elseif cache.cacheval isa Krylov.BlockMinresWorkspace
+    elseif cacheval isa Krylov.BlockMinresWorkspace
         N !== I &&
             @SciMLMessage(
             "$(alg.KrylovAlg) doesn't support right preconditioning.",
             verbose, :no_right_preconditioning
         )
         Krylov.krylov_solve!(args...; M, kwargs...)
-    elseif cache.cacheval isa Krylov.LsmrWorkspace ||
-            cache.cacheval isa Krylov.LsqrWorkspace ||
-            cache.cacheval isa Krylov.LslqWorkspace
+    elseif cacheval isa Krylov.LsmrWorkspace ||
+            cacheval isa Krylov.LsqrWorkspace ||
+            cacheval isa Krylov.LslqWorkspace
         Krylov.krylov_solve!(args...; M, N, kwargs...)
-    elseif cache.cacheval isa Krylov.CglsWorkspace ||
-            cache.cacheval isa Krylov.CrlsWorkspace
+    elseif cacheval isa Krylov.CglsWorkspace ||
+            cacheval isa Krylov.CrlsWorkspace
         N !== I &&
             @SciMLMessage(
             "$(alg.KrylovAlg) doesn't support right preconditioning.",

test/Core/default_algs.jl

@@ -1,6 +1,15 @@
 using LinearSolve, RecursiveFactorization, LinearAlgebra, SparseArrays, Test
 using SciMLOperators: FunctionOperator, MatrixOperator, WOperator, has_concretization
 
+struct CountingIdentityPreconditioner
+    calls::Base.RefValue{Int}
+end
+
+function LinearAlgebra.ldiv!(y, P::CountingIdentityPreconditioner, x)
+    P.calls[] += 1
+    return copyto!(y, x)
+end
+
 @test LinearSolve.defaultalg(nothing, zeros(3)).alg === LinearSolve.DefaultAlgorithmChoice.GenericLUFactorization
 prob = LinearProblem(rand(3, 3), rand(3))
 solve(prob)
@@ -710,3 +719,20 @@ end
         LinearSolve.DefaultAlgorithmChoice.LHLFactorization
     @test solve(LinearProblem(Wdense, bw)).u ≈ ref rtol = 1.0e-8
 end
+
+@testset "default matrix-free solver uses supplied preconditioners" begin
+    n = 4
+    Jm = rand(n, n) + n * I
+    b = rand(n)
+    fj(v, u, p, t) = Jm * v
+    fj(w, v, u, p, t) = mul!(w, Jm, v)
+    Jfree = FunctionOperator(fj, zeros(n), zeros(n); islinear = true)
+    Wfree = WOperator{true}(I, 0.1, Jfree, zeros(n))
+    Pl = CountingIdentityPreconditioner(Ref(0))
+    Pr = CountingIdentityPreconditioner(Ref(0))
+
+    solve(LinearProblem(Wfree, b), nothing; Pl, Pr)
+
+    @test Pl.calls[] > 0
+    @test Pr.calls[] > 0
+end