Provide a C++17 implementation.

include/deal.II/base/tensor.h

@@ -1320,6 +1320,7 @@ Tensor<rank_, dim, Number>::Tensor(const ArrayLike &initializer,
 }
 
 
+#  ifdef DEAL_II_HAVE_CXX20
 
 template <int rank_, int dim, typename Number>
 constexpr DEAL_II_HOST_DEVICE_ALWAYS_INLINE
@@ -1375,6 +1376,55 @@ Tensor<rank_, dim, Number>::Tensor()
       }(std::make_index_sequence<dim>()))
 {}
 
+#  else
+
+// The C++17 case works in essence the same, except that we can't use a
+// lambda function with explicit template parameters, i.e., we can't do
+//   []<std::size_t... I>(const std::index_sequence<I...> &)
+// as above because that's a C++20 feature. Lambda functions in C++17 can
+// have template packs as arguments, but we need the ability to *name*
+// that template pack (the 'I' above) and that's not possible in C++17.
+//
+// We work around this by moving the lambda function to a global function
+// and using the traditional template syntax on it.
+namespace internal
+{
+  namespace TensorInitialization
+  {
+    template <int rank, int dim, typename Number, std::size_t... I>
+    constexpr std::array<typename Tensor<rank, dim, Number>::value_type, dim>
+    make_zero_array(const std::index_sequence<I...> &)
+    {
+      static_assert(sizeof...(I) == dim, "This is bad.");
+
+      if constexpr (rank == 1)
+        {
+          auto get_zero_and_ignore_argument = [](int) {
+            return internal::NumberType<Number>::value(0.0);
+          };
+          return {{(get_zero_and_ignore_argument(I))...}};
+        }
+      else
+        {
+          auto get_zero_and_ignore_argument = [](int) {
+            return Tensor<rank - 1, dim, Number>();
+          };
+          return {{(get_zero_and_ignore_argument(I))...}};
+        }
+    };
+  } // namespace TensorInitialization
+} // namespace internal
+
+
+template <int rank_, int dim, typename Number>
+constexpr DEAL_II_HOST_DEVICE_ALWAYS_INLINE
+Tensor<rank_, dim, Number>::Tensor()
+  : values(internal::TensorInitialization::make_zero_array<rank_, dim, Number>(
+      std::make_index_sequence<dim>()))
+{}
+
+
+#  endif
 
 
 template <int rank_, int dim, typename Number>