[solver][fix] Make sure to only call communication code if we are able to communicate

dumux/linear/istlsolvers.hh

@@ -143,16 +143,24 @@ public:
     {
         initializeParameters_(paramGroup);
 #if HAVE_MPI
-        solverCategory_ = Detail::solverCategory<LinearSolverTraits>(gridView);
-        if (solverCategory_ != Dune::SolverCategory::sequential)
+        if constexpr (LinearSolverTraits::canCommunicate)
         {
-            parallelHelper_ = std::make_unique<ParallelISTLHelper<LinearSolverTraits>>(gridView, dofMapper);
-            communication_ = std::make_shared<Comm>(gridView.comm(), solverCategory_);
-            scalarProduct_ = Dune::createScalarProduct<XVector>(*communication_, solverCategory_);
-            parallelHelper_->createParallelIndexSet(*communication_);
+            solverCategory_ = Detail::solverCategory<LinearSolverTraits>(gridView);
+            if (solverCategory_ != Dune::SolverCategory::sequential)
+            {
+                parallelHelper_ = std::make_unique<ParallelISTLHelper<LinearSolverTraits>>(gridView, dofMapper);
+                communication_ = std::make_shared<Comm>(gridView.comm(), solverCategory_);
+                scalarProduct_ = Dune::createScalarProduct<XVector>(*communication_, solverCategory_);
+                parallelHelper_->createParallelIndexSet(*communication_);
+            }
+            else
+                scalarProduct_ = std::make_shared<ScalarProduct>();
         }
         else
+        {
+            solverCategory_ = Dune::SolverCategory::sequential;
             scalarProduct_ = std::make_shared<ScalarProduct>();
+        }
 #else
         solverCategory_ = Dune::SolverCategory::sequential;
         scalarProduct_ = std::make_shared<ScalarProduct>();
@@ -174,10 +182,13 @@ public:
         solverCategory_ = Detail::solverCategory(gridView);
         scalarProduct_ = scalarProduct;
         communication_ = communication;
-        if (solverCategory_ != Dune::SolverCategory::sequential)
+        if constexpr (LinearSolverTraits::canCommunicate)
         {
-            parallelHelper_ = std::make_unique<ParallelISTLHelper<LinearSolverTraits>>(gridView, dofMapper);
-            parallelHelper_->createParallelIndexSet(communication);
+            if (solverCategory_ != Dune::SolverCategory::sequential)
+            {
+                parallelHelper_ = std::make_unique<ParallelISTLHelper<LinearSolverTraits>>(gridView, dofMapper);
+                parallelHelper_->createParallelIndexSet(communication);
+            }
         }
     }
 #endif