Error when extract face dofs of face boardering FE_Nothing

[simonsticko]

I thought the following would work, but it results in an exception. Is this a bug or not?

Set up a triangulation with 2 cells:

|cell 0|cell 1|

and a DoFHandler with elements over the triangulation:

|FE_Q|FE_Nothing|

Try to extract the face dofs of cell 0 at the shared face.

#include <deal.II/dofs/dof_handler.h>

#include <deal.II/fe/fe_nothing.h>
#include <deal.II/fe/fe_q.h>

#include <deal.II/grid/grid_generator.h>

#include <vector>

using namespace dealii;

template <int dim>
void
make_two_element_mesh(Triangulation<dim> &triangulation)
{
  const Point<dim> lower_left;
  Point<dim>       upper_right;

  const unsigned int        n_cells = 2;
  std::vector<unsigned int> repetitions;
  upper_right[0] = n_cells;
  repetitions.push_back(n_cells);
  for (unsigned int d = 1; d < dim; ++d)
    {
      upper_right[d] = 1;
      repetitions.push_back(1);
    }

  GridGenerator::subdivided_hyper_rectangle(triangulation,
                                            repetitions,
                                            lower_left,
                                            upper_right);
}



int
main()
{
  const int dim = 1;

  Triangulation<dim> triangulation;
  make_two_element_mesh(triangulation);

  hp::FECollection<dim> fe_collection;
  fe_collection.push_back(FE_Nothing<dim>());
  fe_collection.push_back(FE_Q<dim>(1));

  DoFHandler<dim> dof_handler(triangulation);
  dof_handler.begin_active()->set_active_fe_index(1);
  dof_handler.distribute_dofs(fe_collection);

  const auto cell = dof_handler.begin_active();

  std::vector<types::global_dof_index> dof_indices(
    fe_collection[1].dofs_per_cell);

  // Get the dofs at the shared face.
  cell->face(1)->get_dof_indices(dof_indices);
}

--------------------------------------------------------
An error occurred in line <296> of file </home/simon/bin/dealii/include/deal.II/dofs/dof_accessor.templates.h> in function
    static void dealii::internal::DoFAccessorImplementation::Implementation::process_dof_index(const dealii::DoFHandler<dim, spacedim>&, unsigned int, unsigned int, unsigned int, unsigned int, const std::integral_constant<int, structdim>&, GlobalIndexType&, const DoFPProcessor&) [with int dim = 1; int spacedim = 1; int structdim = 0; GlobalIndexType = unsigned int; DoFPProcessor = dealii::internal::DoFAccessorImplementation::Implementation::get_dof_index<1, 1, 0>::<lambda(const auto:8&, auto:9&)>]
The violated condition was: 
    ptr != dof_handler.hp_object_fe_indices[structdim].begin() + dof_handler.hp_object_fe_ptr[structdim][obj_index + 1]
Additional information: 
    You are requesting an active FE index that is not assigned to any of
    the cells connected to this entity.

Stacktrace:
-----------
#0  ./example: void dealii::internal::DoFAccessorImplementation::Implementation::process_dof_index<1, 1, 0, unsigned int, dealii::internal::DoFAccessorImplementation::Implementation::get_dof_index<1, 1, 0>(dealii::DoFHandler<1, 1> const&, unsigned int, unsigned int, unsigned int, unsigned int, std::integral_constant<int, 0> const&)::{lambda(auto:1 const&, auto:2&)#1}>(dealii::DoFHandler<1, 1> const&, unsigned int, unsigned int, unsigned int, unsigned int, std::integral_constant<int, 0> const&, unsigned int&, dealii::internal::DoFAccessorImplementation::Implementation::get_dof_index<1, 1, 0>(dealii::DoFHandler<1, 1> const&, unsigned int, unsigned int, unsigned int, unsigned int, std::integral_constant<int, 0> const&)::{lambda(auto:1 const&, auto:2&)#1} const&)
#1  ./example: unsigned int dealii::internal::DoFAccessorImplementation::Implementation::get_dof_index<1, 1, 0>(dealii::DoFHandler<1, 1> const&, unsigned int, unsigned int, unsigned int, unsigned int, std::integral_constant<int, 0> const&)
#2  ./example: dealii::DoFAccessor<0, 1, 1, false>::get_dof_indices(std::vector<unsigned int, std::allocator<unsigned int> >&, unsigned int) const
#3  ./example: main

---

[peterrum]

I think this is rather an issue of the size of dof_indices and that get_dof_indices() does not get the right index. The following code is working:

  {
  std::vector<types::global_dof_index> dof_indices(
    fe_collection[0].dofs_per_face);

    cell->face(1)->get_dof_indices(dof_indices, 0);

    for(const auto i : dof_indices)
      std::cout << i << " ";
    std::cout << std::endl;
  }

  {
  std::vector<types::global_dof_index> dof_indices(
    fe_collection[1].dofs_per_face);

    cell->face(1)->get_dof_indices(dof_indices, 1);

    for(const auto i : dof_indices)
      std::cout << i << " ";
    std::cout << std::endl;
  }

[kronbichler]

Is there a way to print better assertions that inform the user that the wrong size has been handed in?