Allow for blocked elements with block size 1

cpp/dolfinx/fem/FiniteElement.cpp

@@ -124,7 +124,7 @@ template <std::floating_point T>
 FiniteElement<T>::FiniteElement(const ufcx_finite_element& e)
     : _signature(e.signature), _space_dim(e.space_dimension),
       _value_shape(e.value_shape, e.value_shape + e.value_rank),
-      _bs(e.block_size)
+      _bs(e.block_size), _is_mixed(e.element_type == ufcx_mixed_element)
 {
   const ufcx_shape _shape = e.cell_shape;
   switch (_shape)
@@ -293,7 +293,7 @@ FiniteElement<T>::FiniteElement(const ufcx_finite_element& e)
 template <std::floating_point T>
 FiniteElement<T>::FiniteElement(const basix::FiniteElement<T>& element,
                                 const std::vector<std::size_t>& value_shape)
-    : _value_shape(element.value_shape())
+    : _value_shape(element.value_shape()), _is_mixed(false)
 {
   if (!_value_shape.empty() and !value_shape.empty())
   {
@@ -440,7 +440,7 @@ int FiniteElement<T>::num_sub_elements() const noexcept
 template <std::floating_point T>
 bool FiniteElement<T>::is_mixed() const noexcept
 {
-  return !_sub_elements.empty() and _bs == 1;
+  return _is_mixed;
 }
 //-----------------------------------------------------------------------------
 template <std::floating_point T>

cpp/dolfinx/fem/FiniteElement.h

@@ -139,17 +139,20 @@ class FiniteElement
   tabulate(std::span<const geometry_type> X, std::array<std::size_t, 2> shape,
            int order) const;
 
-  /// @brief Number of sub elements (for a mixed or blocked element)
+  /// @brief Number of sub elements (for a mixed or blocked element).
   /// @return The number of sub elements
   int num_sub_elements() const noexcept;
 
-  /// Check if element is a mixed element, i.e. composed of two or more
-  /// elements of different types. A block element, e.g. a Lagrange
-  /// element with block size > 1 is not considered mixed.
-  /// @return True is element is mixed.
+  /// @brief Check if element is a mixed element.
+  ///
+  /// A mixed element i composed of two or more elements of different
+  /// types (a block element, e.g. a Lagrange element with block size >=
+  /// 1 is not considered mixed).
+  ///
+  /// @return True if element is mixed.
   bool is_mixed() const noexcept;
 
-  /// Subelements (if any)
+  /// Get subelements (if any)
   const std::vector<std::shared_ptr<const FiniteElement<geometry_type>>>&
   sub_elements() const noexcept;
 
@@ -686,6 +689,9 @@ class FiniteElement
   // number of DOFs co-located at each dof 'point'.
   int _bs;
 
+  // Indicate whether this is a mixed element
+  bool _is_mixed;
+
   // Indicate whether the element needs permutations or transformations
   bool _needs_dof_permutations;
   bool _needs_dof_transformations;