Add functionality needed to have empty vars for MC system

src/DataStructures/Variables.hpp

@@ -594,6 +594,11 @@ class Variables<tmpl::list<>> {
   template <typename T>
   Variables(const T* /*pointer*/, const size_t /*size*/) {}
   static constexpr size_t size() { return 0; }
+  static constexpr size_t number_of_grid_points() { return 0; }
+  void assign_subset(const Variables<tmpl::list<>>& /*unused*/) {}
+  void assign_subset(const tuples::TaggedTuple<>& /*unused*/) {}
+  // Initialization for empty variables should ignore any input.
+  void initialize(size_t /*number_of_grid_points*/) {}
 };
 
 // gcc8 screams when the empty Variables has pup as a member function, so we

src/Evolution/DgSubcell/Actions/Initialize.hpp

@@ -103,7 +103,8 @@ struct SetSubcellGrid {
       subcell::Tags::ReconstructionOrder<Dim>,
       evolution::dg::subcell::Tags::InterpolatorsFromFdToNeighborFd<Dim>,
       evolution::dg::subcell::Tags::InterpolatorsFromDgToNeighborFd<Dim>,
-      evolution::dg::subcell::Tags::InterpolatorsFromNeighborDgToFd<Dim>>;
+      evolution::dg::subcell::Tags::InterpolatorsFromNeighborDgToFd<Dim>,
+      typename System::variables_tag>;
   using compute_tags =
       tmpl::list<Tags::MeshCompute<Dim>, Tags::LogicalCoordinatesCompute<Dim>,
                  ::domain::Tags::MappedCoordinates<

src/Evolution/DgSubcell/BackgroundGrVars.hpp

@@ -142,10 +142,13 @@ struct BackgroundGrVars : tt::ConformsTo<db::protocols::Mutator> {
             cell_centered_impl(active_gr_vars, time, subcell_inertial_coords,
                                solution_or_data);
           }
-
-          face_centered_impl(subcell_face_gr_vars, time, functions_of_time,
-                             logical_to_grid_map, grid_to_inertial_map,
-                             subcell_mesh, solution_or_data);
+          if constexpr (not std::is_same_v<
+                        typename SubcellFaceGrVars::value_type::tags_list,
+                        tmpl::list<>>){
+            face_centered_impl(subcell_face_gr_vars, time, functions_of_time,
+                               logical_to_grid_map, grid_to_inertial_map,
+                               subcell_mesh, solution_or_data);
+          }
         }
       }
 
@@ -158,19 +161,23 @@ struct BackgroundGrVars : tt::ConformsTo<db::protocols::Mutator> {
              "The subcell mesh must have isotropic basis, quadrature. and "
              "extents but got "
                  << subcell_mesh);
-      const size_t num_face_centered_mesh_grid_pts =
-          (subcell_mesh.extents(0) + 1) * subcell_mesh.extents(1) *
-          subcell_mesh.extents(2);
-      for (size_t d = 0; d < volume_dim; ++d) {
-        gsl::at(*subcell_face_gr_vars, d)
-            .initialize(num_face_centered_mesh_grid_pts);
+      if constexpr (not std::is_same_v<
+                    typename SubcellFaceGrVars::value_type::tags_list,
+                    tmpl::list<>>){
+        const size_t num_face_centered_mesh_grid_pts =
+            (subcell_mesh.extents(0) + 1) * subcell_mesh.extents(1) *
+            subcell_mesh.extents(2);
+        for (size_t d = 0; d < volume_dim; ++d) {
+          gsl::at(*subcell_face_gr_vars, d)
+              .initialize(num_face_centered_mesh_grid_pts);
+        }
+        face_centered_impl(subcell_face_gr_vars, time, functions_of_time,
+                           logical_to_grid_map, grid_to_inertial_map,
+                           subcell_mesh, solution_or_data);
       }
 
       cell_centered_impl(inactive_gr_vars, time, subcell_inertial_coords,
                          solution_or_data);
-      face_centered_impl(subcell_face_gr_vars, time, functions_of_time,
-                         logical_to_grid_map, grid_to_inertial_map,
-                         subcell_mesh, solution_or_data);
     }
   }
 

src/ParallelAlgorithms/Events/ObserveTimeStep.hpp

@@ -175,7 +175,13 @@ class ObserveTimeStep : public Event {
                   const ArrayIndex& array_index,
                   const ParallelComponent* const /*meta*/,
                   const ObservationValue& observation_value) const {
-    const size_t number_of_grid_points = variables.number_of_grid_points();
+    // For empty vars, we use 1 grid point to avoid divisions by zero
+    // after reduction.
+    size_t number_of_grid_points = 1;
+    if constexpr (not std::is_same_v<typename System::variables_tag::tags_list,
+                                       tmpl::list<>>) {
+      number_of_grid_points = variables.number_of_grid_points();
+    }
     const double slab_size = time_step.slab().duration().value();
     const double step_size = abs(time_step.value());
     const double wall_time = sys::wall_time();

tests/Unit/DataStructures/Test_Variables.cpp

@@ -78,6 +78,12 @@ void test_empty_variables() {
   CHECK(get_output(tuple_with_empty_vars) == "(3,hello,{})");
 
   CHECK(not contains_allocations(empty_vars));
+  // Functions do nothing, but need to compile
+  empty_vars.initialize(0);
+  const Variables<tmpl::list<>> input_vars;
+  empty_vars.assign_subset(input_vars);
+  CHECK(empty_vars.number_of_grid_points() == 0);
+  CHECK(empty_vars.size() == 0);
 }
 
 template <typename VectorType>