From 8e1d439515d2737301e585ff3d2b0d6a01e9cbda Mon Sep 17 00:00:00 2001
From: Kai Bao <kai.bao@sintef.no>
Date: Thu, 30 Apr 2026 14:45:59 +0200
Subject: [PATCH] fall back to use ilu0 preconditioner for polymer injectivity
 run

instead of throw, so that those cases can be run with default
parameters.
---
 opm/simulators/linalg/ISTLSolver.hpp | 16 +++++++++++++---
 regressionTests.cmake                |  2 +-
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/opm/simulators/linalg/ISTLSolver.hpp b/opm/simulators/linalg/ISTLSolver.hpp
index 8ecb0e4be1..aeafcb202b 100644
--- a/opm/simulators/linalg/ISTLSolver.hpp
+++ b/opm/simulators/linalg/ISTLSolver.hpp
@@ -52,6 +52,8 @@
 #include <opm/simulators/linalg/AbstractISTLSolver.hpp>
 #include <opm/simulators/linalg/printlinearsolverparameter.hpp>
 
+#include <fmt/format.h>
+
 #include <any>
 #include <cstddef>
 #include <functional>
@@ -223,10 +225,18 @@ std::unique_ptr<Matrix> blockJacobiAdjacency(const Grid& grid,
 
             if (isIncompatibleWithCprw) {
                 // Polymer injectivity is incompatible with the CPRW linear solver.
+                // Instead of aborting the run, emit a warning and fall back to ILU0
+                // so that polymer injectivity cases work with default parameters.
                 if (parameters_[0].linsolver_ == "cprw" || parameters_[0].linsolver_ == "hybrid") {
-                    OPM_THROW(std::runtime_error,
-                              "The polymer injectivity model is incompatible with the CPRW linear solver.\n"
-                              "Choose a different option, for example --linear-solver=ilu0");
+                    const bool on_io_rank = (simulator_.gridView().comm().rank() == 0);
+                    const std::string msg =
+                        fmt::format("The polymer injectivity model is incompatible with the '{}' "
+                                    "linear solver. Falling back to --linear-solver=ilu0.",
+                                    parameters_[0].linsolver_);
+                    if (on_io_rank) {
+                        OpmLog::warning(msg);
+                    }
+                    parameters_[0].linsolver_ = "ilu0";
                 }
             }
 
diff --git a/regressionTests.cmake b/regressionTests.cmake
index 19df609ca3..5586df0ac3 100644
--- a/regressionTests.cmake
+++ b/regressionTests.cmake
@@ -269,7 +269,7 @@ add_test_compareECLFiles(CASENAME polymer_injectivity
                          SIMULATOR flow
                          ABS_TOL ${abs_tol}
                          REL_TOL ${rel_tol}
-                         TEST_ARGS --tolerance-wells=1.e-6 --linear-solver=ilu0)
+                         TEST_ARGS --tolerance-wells=1.e-6)
 
 add_test_compareECLFiles(CASENAME polymer_simple2D
                          FILENAME 2D_THREEPHASE_POLY_HETER