Provide some overloads and instantiations

include/deal.II/grid/tria_accessor.h

@@ -2384,6 +2384,14 @@ class TriaAccessor<0, 1, spacedim>
   void
   copy_from(const TriaAccessor &);
 
+  /**
+   * Copy operator. We need this function to support generic
+   * programming, but it just throws an exception because it cannot do
+   * the required operations.
+   */
+  void
+  copy_from(const TriaAccessorBase<0, 1, spacedim> &);
+
   /**
    * Return the state of the iterator. Since an iterator to points can not be
    * incremented or decremented, its state remains constant, and in particular

include/deal.II/grid/tria_accessor.templates.h

@@ -2769,6 +2769,19 @@ TriaAccessor<0, 1, spacedim>::copy_from(const TriaAccessor &t)
 
 
 
+template <int spacedim>
+inline void
+TriaAccessor<0, 1, spacedim>::copy_from(
+  const TriaAccessorBase<0, 1, spacedim> &)
+{
+  // We cannot convert from TriaAccessorBase to
+  // TriaAccessor<0,1,spacedim> because the latter is not derived from
+  // the former. We should never get here.
+  Assert(false, ExcInternalError());
+}
+
+
+
 template <int spacedim>
 inline bool
 TriaAccessor<0, 1, spacedim>::operator<(

source/grid/tria_accessor.inst.in

@@ -51,3 +51,9 @@ for (deal_II_dimension : DIMENSIONS; deal_II_space_dimension : DIMENSIONS)
       CellAccessor<deal_II_dimension, deal_II_space_dimension>>;
 #endif
   }
+
+for (deal_II_space_dimension : DIMENSIONS)
+  {
+    template class TriaAccessor<0, 1, deal_II_space_dimension>;
+    template class TriaIterator<TriaAccessor<0, 1, deal_II_space_dimension>>;
+  }