Keep RFLUFactorization off the direct dual AD path (fixes #1052)

Project.toml

@@ -1,6 +1,6 @@
 name = "LinearSolve"
 uuid = "7ed4a6bd-45f5-4d41-b270-4a48e9bafcae"
-version = "3.86.0"
+version = "3.86.1"
 authors = ["SciML"]
 
 [deps]

ext/LinearSolveForwardDiffExt.jl

@@ -393,11 +393,15 @@ end
 # Check if the algorithm should use the direct dual solve path
 # (algorithms that can work directly with Dual numbers without the primal/partials separation)
 function _use_direct_dual_solve(alg)
+    # NOTE: RFLUFactorization is intentionally *not* on the direct path. Even when
+    # A carries duals, its fast Float64 factorization is BLAS/SIMD-grade, and routing
+    # the Dual problem through it falls back to generic scalar dual arithmetic, losing
+    # that speedup entirely (~40x slower, see issue #1052). The split path keeps the
+    # fast primal factorization and reuses it across the partial back-solves.
     return alg isa GenericLUFactorization ||
         alg isa LinearSolve.SpecializedLUFactorization ||
         alg isa LinearSolve.SpecializedQRFactorization ||
-        alg isa LinearSolve.PureKLUFactorization ||
-        alg isa LinearSolve.RFLUFactorization
+        alg isa LinearSolve.PureKLUFactorization
 end
 
 function _use_direct_dual_solve(alg::DefaultLinearSolver)

test/Core/forwarddiff_overloads.jl

@@ -67,6 +67,19 @@ prob = LinearProblem(plain_A, b)
 @test ≈(solve(prob, GenericLUFactorization()), plain_A \ b, rtol = 1.0e-9)
 @test ≈(solve(prob, RFLUFactorization()), plain_A \ b, rtol = 1.0e-9)
 
+# Regression test for #1052: RFLUFactorization must stay on the split
+# primal/partials path and NOT take the direct dual solve. Its fast Float64
+# factorization is BLAS/SIMD-grade; routing the Dual problem through it falls
+# back to generic scalar dual arithmetic (~40x slower). Guard the routing
+# decision directly so RFLU is never re-added to the direct path.
+@testset "RFLU stays off the direct dual path (#1052)" begin
+    ext = Base.get_extension(LinearSolve, :LinearSolveForwardDiffExt)
+    @test !ext._use_direct_dual_solve(RFLUFactorization())
+    # Sanity: the genuinely-cheap-in-dual algorithms are still on the direct path.
+    @test ext._use_direct_dual_solve(GenericLUFactorization())
+    @test ext._use_direct_dual_solve(SpecializedLUFactorization())
+end
+
 # Overload Dense
 
 A, b = h([ForwardDiff.Dual(5.0, 1.0, 0.0), ForwardDiff.Dual(5.0, 0.0, 1.0)])