From e5d8ddf6be0945cf108e2e36360fe7bffadef10a Mon Sep 17 00:00:00 2001
From: Sebastian Proell <sebastian.proell@tum.de>
Date: Fri, 3 Mar 2023 15:42:06 +0100
Subject: [PATCH] SymmetricTensor access returns const reference

---
 include/deal.II/base/symmetric_tensor.h  |  2 +-
 tests/tensors/symmetric_tensor_46.cc     | 43 ++++++++++++++++++++++++
 tests/tensors/symmetric_tensor_46.output |  2 ++
 3 files changed, 46 insertions(+), 1 deletion(-)
 create mode 100644 tests/tensors/symmetric_tensor_46.cc
 create mode 100644 tests/tensors/symmetric_tensor_46.output

diff --git a/include/deal.II/base/symmetric_tensor.h b/include/deal.II/base/symmetric_tensor.h
index 4a8397a6d0..9ba9418450 100644
--- a/include/deal.II/base/symmetric_tensor.h
+++ b/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 &;
     };
 
     /**
diff --git a/tests/tensors/symmetric_tensor_46.cc b/tests/tensors/symmetric_tensor_46.cc
new file mode 100644
index 0000000000..a361211ae9
--- /dev/null
+++ b/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;
+}
diff --git a/tests/tensors/symmetric_tensor_46.output b/tests/tensors/symmetric_tensor_46.output
new file mode 100644
index 0000000000..0fd8fc12f0
--- /dev/null
+++ b/tests/tensors/symmetric_tensor_46.output
@@ -0,0 +1,2 @@
+
+DEAL::OK
-- 
2.39.5