SymmetricTensor access returns const reference

include/deal.II/base/symmetric_tensor.h

@@ -421,7 +421,7 @@ namespace internal
     {
       using tensor_type = const ::dealii::SymmetricTensor<rank, dim, Number>;
 
-      using reference = Number;
+      using reference = const Number &;
     };
 
     /**

tests/tensors/symmetric_tensor_46.cc

@@ -0,0 +1,43 @@
+// ---------------------------------------------------------------------
+//
+// Copyright (C) 2023 by the deal.II authors
+//
+// This file is part of the deal.II library.
+//
+// The deal.II library is free software; you can use it, redistribute
+// it, and/or modify it under the terms of the GNU Lesser General
+// Public License as published by the Free Software Foundation; either
+// version 2.1 of the License, or (at your option) any later version.
+// The full text of the license can be found in the file LICENSE.md at
+// the top level directory of deal.II.
+//
+// ---------------------------------------------------------------------
+
+
+// Test that a const SymmetricTensor is not assignable, i.e., the accessors
+// return a const reference
+
+#include <deal.II/base/symmetric_tensor.h>
+#include <deal.II/base/tensor.h>
+#include <deal.II/base/vectorization.h>
+
+#include "../tests.h"
+
+int
+main()
+{
+  initlog();
+
+  {
+    const SymmetricTensor<2, 3, double> st{};
+    static_assert(std::is_same<decltype(st[0][0]), const double &>::value);
+  }
+
+  {
+    const SymmetricTensor<2, 3, VectorizedArray<double>> stv{};
+    static_assert(std::is_same<decltype(stv[0][0]),
+                               const VectorizedArray<double> &>::value);
+  }
+
+  deallog << "OK" << std::endl;
+}

tests/tensors/symmetric_tensor_46.output

@@ -0,0 +1,2 @@
+
+DEAL::OK