improve assert message for non matching reference cell types

include/deal.II/dofs/dof_accessor.templates.h

@@ -2636,9 +2636,8 @@ DoFCellAccessor<dimension_, space_dimension_, level_dof_access>::get_fe() const
   const auto &fe = this->dof_handler->get_fe(active_fe_index());
 
   Assert(this->reference_cell() == fe.reference_cell(),
-         ExcMessage(
-           "The reference-cell type of the cell does not match the one of the "
-           "finite element!"));
+         internal::ExcNonMatchingReferenceCellTypes(this->reference_cell(),
+                                                    fe.reference_cell()));
 
   return fe;
 }

include/deal.II/grid/reference_cell.h

@@ -2687,6 +2687,46 @@ namespace internal
      */
     const ArrayView<const T> vertices_1;
   };
+
+  class ExcNonMatchingReferenceCellTypes : public dealii::ExceptionBase
+  {
+  public:
+    /**
+     * Constructor.
+     */
+    ExcNonMatchingReferenceCellTypes(const ReferenceCell &entity_type_0,
+                                     const ReferenceCell &entity_type_1)
+      : entity_type_0(entity_type_0)
+      , entity_type_1(entity_type_1)
+    {}
+
+    /**
+     * Destructor.
+     */
+    virtual ~ExcNonMatchingReferenceCellTypes() noexcept override = default;
+
+    /**
+     * Print error message to @p out.
+     */
+    virtual void
+    print_info(std::ostream &out) const override
+    {
+      out << "The reference-cell type of " << entity_type_0.to_string()
+          << " does not match with " << entity_type_1.to_string()
+          << ". This could happen if one object (e.g. Triangulation) holds "
+          << "reference cells from a hexahedral mesh and the other one "
+          << "(FiniteElement) from a simplex mesh." << std::endl;
+    }
+
+    /**
+     * First entity type.
+     */
+    const ReferenceCell &entity_type_0;
+    /**
+     * Second entity type.
+     */
+    const ReferenceCell &entity_type_1;
+  };
 } // namespace internal