[deal.II] VectorTools::interpolate_boundary_values doesn't work for DG

[Abbas]

Hello, 

The VectorTools::interpolate_boundary_values function works for 
interpolating fe_Q<dim> functions but not fe_DGQ<dim> . 
Was this meant to be the case? 
I attached a minimal code that illustrates this.
   

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

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

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

#include <deal.II/numerics/data_out.h>
#include <deal.II/base/function.h>
#include <deal.II/numerics/vector_tools.h>

#include <fstream>
using namespace dealii;
int main()

{
    // create an feq obkect and interpolate a value of 1 at boundary 0
    {
        Triangulation<2> tria;
        GridGenerator::hyper_cube(tria, 0, 1, true);
        tria.refine_global(4);
        FE_Q<2> fe(1);
        DoFHandler<2> dof(tria);

        Vector<double> solution;

        dof.distribute_dofs(fe);
        solution.reinit(dof.n_dofs());

        std::map<types::global_dof_index, double> boundary_values;
        VectorTools::interpolate_boundary_values(dof,
                                                 0,
                                                 Functions::ConstantFunction<2>(1),
                                                 boundary_values);
        for (auto &boundary_value : boundary_values)
            solution(boundary_value.first) = boundary_value.second;

        DataOut<2> data_out;

        data_out.attach_dof_handler(dof);
        data_out.add_data_vector(solution, "solution");
        data_out.build_patches();

        const std::string filename = "solution-feq.vtu";
        std::ofstream output(filename);
        data_out.write_vtu(output);
    }

    // Same as above but
    // we create an feGQq obkect and interpolate
    {
        Triangulation<2> tria;
        GridGenerator::hyper_cube(tria, 0, 1, true);
        tria.refine_global(4);
        FE_DGQ<2> fe(1);                 // only difference between this and the above code 
        DoFHandler<2> dof(tria);

        Vector<double> solution;

        dof.distribute_dofs(fe);
        solution.reinit(dof.n_dofs());

        std::map<types::global_dof_index, double> boundary_values;
        VectorTools::interpolate_boundary_values(dof,
                                                 0,
                                                 Functions::ConstantFunction<2>(1),
                                                 boundary_values);
        for (auto &boundary_value : boundary_values)
            solution(boundary_value.first) = boundary_value.second;

        DataOut<2> data_out;

        data_out.attach_dof_handler(dof);
        data_out.add_data_vector(solution, "solution");
        data_out.build_patches();

        const std::string filename = "solution-fedgq.vtu";
        std::ofstream output(filename);
        data_out.write_vtu(output);
    }
}