From 12f2084c6ce0670a7b470aa1157bab02de31c294 Mon Sep 17 00:00:00 2001
From: Denis Davydov <davydden@gmail.com>
Date: Sat, 23 Feb 2019 18:09:57 +0100
Subject: [PATCH] add has_begin detector

---
 include/deal.II/matrix_free/fe_evaluation.h   | 21 +++++++++++++++++++
 .../matrix_free/fe_evaluation_type_traits.cc  |  9 ++++++++
 ...tion_type_traits.with_trilinos=true.output |  3 +++
 3 files changed, 33 insertions(+)

diff --git a/include/deal.II/matrix_free/fe_evaluation.h b/include/deal.II/matrix_free/fe_evaluation.h
index 8da043d70b..4b088f977d 100644
--- a/include/deal.II/matrix_free/fe_evaluation.h
+++ b/include/deal.II/matrix_free/fe_evaluation.h
@@ -3450,6 +3450,27 @@ namespace internal
   };
 
 
+
+  // same as above to check
+  // T::const_iterator T::begin() const
+  template <typename T>
+  struct has_begin
+  {
+  private:
+    static void
+    detect(...);
+
+    template <typename U>
+    static decltype(std::declval<U const>().begin())
+    detect(const U &);
+
+  public:
+    static constexpr bool value =
+      std::is_same<typename T::const_iterator,
+                   decltype(detect(std::declval<T>()))>::value;
+  };
+
+
   // access to generic vectors that have operator ().
   // FIXME: this is wrong for Trilinos/Petsc MPI vectors
   template <typename VectorType,
diff --git a/tests/matrix_free/fe_evaluation_type_traits.cc b/tests/matrix_free/fe_evaluation_type_traits.cc
index 12ab08476b..d3363649aa 100644
--- a/tests/matrix_free/fe_evaluation_type_traits.cc
+++ b/tests/matrix_free/fe_evaluation_type_traits.cc
@@ -129,5 +129,14 @@ main()
                TrilinosWrappers::MPI::Vector>::value
           << std::endl;
 
+  // check has_begin():
+  deallog
+    << "has_begin:" << std::endl
+    << "LinearAlgebra::distributed::Vector = "
+    << internal::has_begin<LinearAlgebra::distributed::Vector<double>>::value
+    << std::endl
+    << "TrilinosWrappers::MPI::Vector = "
+    << internal::has_begin<TrilinosWrappers::MPI::Vector>::value << std::endl;
+
   deallog << "OK" << std::endl;
 }
diff --git a/tests/matrix_free/fe_evaluation_type_traits.with_trilinos=true.output b/tests/matrix_free/fe_evaluation_type_traits.with_trilinos=true.output
index 4e82d52202..71bac87207 100644
--- a/tests/matrix_free/fe_evaluation_type_traits.with_trilinos=true.output
+++ b/tests/matrix_free/fe_evaluation_type_traits.with_trilinos=true.output
@@ -10,4 +10,7 @@ DEAL::Dummy2::operator()
 DEAL::has_partitioners_are_compatible:
 DEAL::LinearAlgebra::distributed::Vector = 1
 DEAL::TrilinosWrappers::MPI::Vector = 0
+DEAL::has_begin:
+DEAL::LinearAlgebra::distributed::Vector = 1
+DEAL::TrilinosWrappers::MPI::Vector = 1
 DEAL::OK
-- 
2.39.5