Fix some warnings.

[marcfehling]

According to https://cdash.dealii.43-1.org/viewBuildError.php?type=1&buildid=992

[source/dofs/dof_handler_policy.cc:3664](https://github.com/dealii/dealii/blob/master/source/dofs/dof_handler_policy.cc#L3664):27: warning: loop variable ‘index’ creates a copy from type ‘const std::pair<const unsigned int, unsigned int>’ [-Wrange-loop-construct]
[source/dofs/dof_handler_policy.cc:3664](https://github.com/dealii/dealii/blob/master/source/dofs/dof_handler_policy.cc#L3664):27: note: use reference type to prevent copying

[source/cgal/intersections.cc:387](https://github.com/dealii/dealii/blob/master/source/cgal/intersections.cc#L387):37: warning: loop variable ‘f’ of type ‘const dealii::CGALWrappers::Face_handle&’ {aka ‘const CGAL::internal::CC_iterator<CGAL::Compact_container<CGAL::Delaunay_mesh_face_base_2<CGAL::Simple_cartesian<CORE::Expr>, CGAL::Constrained_triangulation_face_base_2<CGAL::Simple_cartesian<CORE::Expr>, CGAL::Triangulation_face_base_with_info_2<dealii::CGALWrappers::FaceInfo2, CGAL::Simple_cartesian<CORE::Expr>, CGAL::Triangulation_face_base_2<CGAL::Simple_cartesian<CORE::Expr>, CGAL::Triangulation_ds_face_base_2<CGAL::Triangulation_data_structure_2<CGAL::Triangulation_vertex_base_2<CGAL::Simple_cartesian<CORE::Expr>, CGAL::Triangulation_ds_vertex_base_2<void> >, CGAL::Delaunay_mesh_face_base_2<CGAL::Simple_cartesian<CORE::Expr>, CGAL::Constrained_triangulation_face_base_2<CGAL::Simple_cartesian<CORE::Expr>, CGAL::Triangulation_face_base_with_info_2<dealii::CGALWrappers::FaceInfo2, CGAL::Simple_cartesian<CORE::Expr> > > > > > > > > >, CGAL::Default, CGAL::Default, CGAL::Default>, false>&’} binds to a temporary constructed from type ‘CGAL::Triangulation_2<CGAL::Simple_cartesian<CORE::Expr>, CGAL::Triangulation_data_structure_2<CGAL::Triangulation_vertex_base_2<CGAL::Simple_cartesian<CORE::Expr>, CGAL::Triangulation_ds_vertex_base_2<void> >, CGAL::Delaunay_mesh_face_base_2<CGAL::Simple_cartesian<CORE::Expr>, CGAL::Constrained_triangulation_face_base_2<CGAL::Simple_cartesian<CORE::Expr>, CGAL::Triangulation_face_base_with_info_2<dealii::CGALWrappers::FaceInfo2, CGAL::Simple_cartesian<CORE::Expr> > > > > >::Finite_faces_iterator’ [-Wrange-loop-construct]
[source/cgal/intersections.cc:387](https://github.com/dealii/dealii/blob/master/source/cgal/intersections.cc#L387):37: note: use non-reference type ‘const dealii::CGALWrappers::Face_handle’ {aka ‘const CGAL::internal::CC_iterator<CGAL::Compact_container<CGAL::Delaunay_mesh_face_base_2<CGAL::Simple_cartesian<CORE::Expr>, CGAL::Constrained_triangulation_face_base_2<CGAL::Simple_cartesian<CORE::Expr>, CGAL::Triangulation_face_base_with_info_2<dealii::CGALWrappers::FaceInfo2, CGAL::Simple_cartesian<CORE::Expr>, CGAL::Triangulation_face_base_2<CGAL::Simple_cartesian<CORE::Expr>, CGAL::Triangulation_ds_face_base_2<CGAL::Triangulation_data_structure_2<CGAL::Triangulation_vertex_base_2<CGAL::Simple_cartesian<CORE::Expr>, CGAL::Triangulation_ds_vertex_base_2<void> >, CGAL::Delaunay_mesh_face_base_2<CGAL::Simple_cartesian<CORE::Expr>, CGAL::Constrained_triangulation_face_base_2<CGAL::Simple_cartesian<CORE::Expr>, CGAL::Triangulation_face_base_with_info_2<dealii::CGALWrappers::FaceInfo2, CGAL::Simple_cartesian<CORE::Expr> > > > > > > > > >, CGAL::Default, CGAL::Default, CGAL::Default>, false>’} to make the copy explicit or ‘const CGAL::Triangulation_2<CGAL::Simple_cartesian<CORE::Expr>, CGAL::Triangulation_data_structure_2<CGAL::Triangulation_vertex_base_2<CGAL::Simple_cartesian<CORE::Expr>, CGAL::Triangulation_ds_vertex_base_2<void> >, CGAL::Delaunay_mesh_face_base_2<CGAL::Simple_cartesian<CORE::Expr>, CGAL::Constrained_triangulation_face_base_2<CGAL::Simple_cartesian<CORE::Expr>, CGAL::Triangulation_face_base_with_info_2<dealii::CGALWrappers::FaceInfo2, CGAL::Simple_cartesian<CORE::Expr> > > > > >::Finite_faces_iterator&’ to prevent copying

[drwells]

Do we really need the explicit type change here, or would using const auto & work?

[marcfehling]

or would using const auto & work?

Potentially yes. But then I would be in favor of using const auto & in all loops of that kind in this file, i.e.

dealii/source/cgal/intersections.cc

Line 151 in 2166009

for (CDT::Face_handle f : cdt.all_face_handles())

dealii/source/cgal/intersections.cc

Line 440 in 2166009

for (Face_handle f : cdt.finite_face_handles())

[tjhei]

I would say this is unrelated to the warnings fixed here.