make DerivativeTraits compatible to Dune-functions DefaultDerivativeTraits

dune/localfunctions/common/derivative.hh

@@ -40,7 +40,16 @@ namespace Dune
                             std::void_t<std::common_type_t<S, T>>>
     {
       using ST = std::common_type_t<S, T>;
-      using type = FieldVector<ST, n>;
+      // using type = FieldVector<ST, n>;
+      using type = FieldMatrix<ST, 1, n>;
+    };
+
+    template <typename S, typename T, int n, int m>
+    struct DerivativeTraits<FieldMatrix<S, 1, n>(FieldVector<T, m>),
+                            std::void_t<std::common_type_t<S, T>>>
+    {
+      using ST = std::common_type_t<S, T>;
+      using type = FieldMatrix<ST, n, m>;
     };
 
     // call `f.derivative(Domain)`
@@ -105,6 +114,15 @@ namespace Dune
           J[j][i] = df_i[j];
       }
 
+      template <typename S, int m, typename T, int n,
+                std::enable_if_t<std::is_convertible_v<S, T>, int> = 0>
+      void assignPartialDerivative(const FieldMatrix<S, 1, m> &df_i, FieldMatrix<T, m, n> &J,
+                                   std::size_t i)
+      {
+        for (int j = 0; j < m; ++j)
+          J[j][i] = df_i[0][j];
+      }
+
       template <typename S, typename T, int n,
                 std::enable_if_t<std::is_convertible_v<S, T>, int> = 0>
       void assignPartialDerivative(const S &df_i, FieldMatrix<T, 1, n> &J, std::size_t i)