Simon Praetorius · a505c20e
--- a/dune/common/fvector.hh
+++ b/dune/common/fvector.hh
    //===== forward methods to container
    static constexpr size_type size () noexcept { return 1; }

-    constexpr reference operator[] ([[maybe_unused]] size_type i)
+    reference operator[] ([[maybe_unused]] size_type i)
    {
      DUNE_ASSERT_BOUNDS(i == 0);
--- a/dune/common/concepts/scalar.hh 0 → 100644
+++ b/dune/common/concepts/scalar.hh 0 → 100644
+// SPDX-FileCopyrightText: Copyright © DUNE Project contributors, see file LICENSE.md in module root
+// SPDX-License-Identifier: LicenseRef-GPL-2.0-only-with-DUNE-exception
+// -*- tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+// vi: set et ts=4 sw=2 sts=2:
+#ifndef DUNE_COMMON_CONCEPTS_SCALAR_HH
+#define DUNE_COMMON_CONCEPTS_SCALAR_HH
+
+// check whether c++20 concept can be used
+#if __has_include(<version>) && __has_include(<concepts>)
--- a/dune/common/concepts/scalar.hh 0 → 100644
+++ b/dune/common/concepts/scalar.hh 0 → 100644
+    #define DUNE_ENABLE_CONCEPTS 1
+    #endif
+  #endif
+#endif
+
+#if DUNE_ENABLE_CONCEPTS
+
+#include <complex>
+#include <dune/common/typetraits.hh>
+
+namespace Dune::Concept {
+
+template <class N>
+concept Scalar = Dune::IsNumber<N>::value;
+
+static_assert(Scalar<short>);
--- a/dune/common/densematrix.hh
+++ b/dune/common/densematrix.hh
      }
    };

+    template <class A, class B>
+    using MatrixElementsAssignable = decltype(
+        *std::begin(*std::declval<typename A::iterator>()) = *std::begin(*std::declval<typename B::const_iterator>()));
+
    template< class DenseMatrix, class RHS >
-    class DenseMatrixAssigner< DenseMatrix, RHS, std::enable_if_t< !std::is_same< typename RHS::const_iterator, void >::value
-        && std::is_convertible< typename RHS::const_iterator::value_type, typename DenseMatrix::iterator::value_type >::value > >
+    class DenseMatrixAssigner< DenseMatrix, RHS,
+      std::enable_if_t<Dune::Std::is_detected_v<MatrixElementsAssignable,DenseMatrix,RHS>>>