Behavior of FEEvaluation::reinit()

[peterrum]

The function FEEvaluation::reinit() can be called either with a cell-batch index or with an array of indices of cells that should make up a new macro cell. Calling the first function and after that the second one works. But it does not work in the opposite way around, due to the assert:

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An error occurred in line <6765> of file </home/munch/sw-index/dealii2/include/deal.II/matrix_free/fe_evaluation.h> in function
    void dealii::FEEvaluation<dim, fe_degree, n_q_points_1d, n_components_, Number, VectorizedArrayType>::reinit(unsigned int) [with int dim = 2; int fe_degree = -1; int n_q_points_1d = 0; int n_components_ = 1; Number = double; VectorizedArrayType = dealii::VectorizedArray<double, 8>]
The violated condition was: 
    this->mapped_geometry == nullptr
Additional information: 
    FEEvaluation was initialized without a matrix-free object. Integer
    indexing is not possible

Stacktrace:
-----------
#0  ./main: dealii::FEEvaluation<2, -1, 0, 1, double, dealii::VectorizedArray<double, 8ul> >::reinit(unsigned int)
#1  ./main: main

What do we want? Should we allow switching between modi? Or should we add an additional assert in the second function?

Minimal test:

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

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

#include <deal.II/lac/affine_constraints.h>

#include <deal.II/matrix_free/fe_evaluation.h>
#include <deal.II/matrix_free/matrix_free.h>

using namespace dealii;

int
main()
{
  const unsigned int dim    = 2;
  const unsigned int degree = 2;

  FE_Q<dim> fe(degree);

  std::cout << fe.n_dofs_per_cell() << std::endl;

  Triangulation<dim> tria;
  GridGenerator::hyper_cube(tria);

  DoFHandler<dim> dof_handler(tria);
  dof_handler.distribute_dofs(fe);

  AffineConstraints<double> constraints;

  MatrixFree<dim, double> mf;
  mf.reinit(dof_handler, constraints, QGauss<dim>(degree + 1));

  FEEvaluation<dim, -1, 0, 1, double> phi(mf);

  phi.reinit(0);
  phi.reinit(std::array<unsigned int, VectorizedArray<double>::size()>{{0}});
  phi.reinit(0);
}