From c0ca8a9fa3e03ad7ecb270adfe760a1bff7c0829 Mon Sep 17 00:00:00 2001
From: Eugene Zhulenev <ezhulenev@google.com>
Date: Tue, 9 Oct 2018 15:28:23 -0700
Subject: [PATCH] Compile time detection for unimplemented stl-style iterators

---
 Eigen/src/Core/DenseBase.h                | 14 +++++-
 Eigen/src/Core/StlIterators.h             | 13 ++++++
 Eigen/src/Core/util/ForwardDeclarations.h |  1 +
 test/stl_iterators.cpp                    | 56 +++++++++++++++++++++++
 4 files changed, 83 insertions(+), 1 deletion(-)

diff --git a/Eigen/src/Core/DenseBase.h b/Eigen/src/Core/DenseBase.h
index 0c34bce40..eedeacc94 100644
--- a/Eigen/src/Core/DenseBase.h
+++ b/Eigen/src/Core/DenseBase.h
@@ -583,11 +583,23 @@ template<typename Derived> class DenseBase
     typedef typename internal::conditional< (Flags&DirectAccessBit)==DirectAccessBit,
                                             internal::pointer_based_stl_iterator<Derived>,
                                             internal::generic_randaccess_stl_iterator<Derived>
-                                          >::type iterator;
+                                          >::type iterator_type;
 
     typedef typename internal::conditional< (Flags&DirectAccessBit)==DirectAccessBit,
                                             internal::pointer_based_stl_iterator<const Derived>,
                                             internal::generic_randaccess_stl_iterator<const Derived>
+                                          >::type const_iterator_type;
+
+    // Stl-style iterators are supported only for vectors.
+
+    typedef typename internal::conditional< IsVectorAtCompileTime,
+                                            iterator_type,
+                                            internal::not_an_iterator<const Derived>
+                                          >::type iterator;
+
+    typedef typename internal::conditional< IsVectorAtCompileTime,
+                                            const_iterator_type,
+                                            internal::not_an_iterator<const Derived>
                                           >::type const_iterator;
     #endif
 
diff --git a/Eigen/src/Core/StlIterators.h b/Eigen/src/Core/StlIterators.h
index 24eef1269..5f2edc4e6 100644
--- a/Eigen/src/Core/StlIterators.h
+++ b/Eigen/src/Core/StlIterators.h
@@ -11,6 +11,19 @@ namespace Eigen {
 
 namespace internal {
 
+// Iterator type for XprType that do not support stl-style iterators. Allows to
+// detect that expression does not support stl iterators at compile time.
+template<typename XprType>
+class not_an_iterator
+{
+  not_an_iterator() : mp_xpr(0), m_index(0) {}
+  not_an_iterator(XprType& xpr, Index index) : mp_xpr(&xpr), m_index(index) {}
+
+ protected:
+  XprType *mp_xpr;
+  Index m_index;
+};
+
 template<typename XprType,typename Derived>
 class indexed_based_stl_iterator_base
 {
diff --git a/Eigen/src/Core/util/ForwardDeclarations.h b/Eigen/src/Core/util/ForwardDeclarations.h
index 3ab3a5f50..49c4e0c31 100644
--- a/Eigen/src/Core/util/ForwardDeclarations.h
+++ b/Eigen/src/Core/util/ForwardDeclarations.h
@@ -135,6 +135,7 @@ template<typename Derived> class SolverBase;
 template<typename XprType> class InnerIterator;
 
 namespace internal {
+template<typename XprType> class not_an_iterator;
 template<typename XprType> class generic_randaccess_stl_iterator;
 template<typename XprType> class pointer_based_stl_iterator;
 template<typename XprType, DirectionType Direction> class subvector_stl_iterator;
diff --git a/test/stl_iterators.cpp b/test/stl_iterators.cpp
index a9fe3afc2..75e23b2fb 100644
--- a/test/stl_iterators.cpp
+++ b/test/stl_iterators.cpp
@@ -358,6 +358,59 @@ void test_stl_iterators(int rows=Rows, int cols=Cols)
 #endif
 }
 
+
+#if EIGEN_HAS_CXX11
+// When the compiler sees expression IsContainerTest<C>(0), if C is an
+// STL-style container class, the first overload of IsContainerTest
+// will be viable (since both C::iterator* and C::const_iterator* are
+// valid types and NULL can be implicitly converted to them).  It will
+// be picked over the second overload as 'int' is a perfect match for
+// the type of argument 0.  If C::iterator or C::const_iterator is not
+// a valid type, the first overload is not viable, and the second
+// overload will be picked.
+template <class C,
+    class Iterator = decltype(::std::declval<const C&>().begin()),
+    class = decltype(::std::declval<const C&>().end()),
+    class = decltype(++::std::declval<Iterator&>()),
+    class = decltype(*::std::declval<Iterator>()),
+    class = typename C::const_iterator>
+bool IsContainerType(int /* dummy */) { return true; }
+
+template <class C>
+bool IsContainerType(long /* dummy */) { return false; }
+#endif // EIGEN_HAS_CXX11
+
+template <typename Scalar, int Rows, int Cols>
+void test_stl_container_detection(int rows=Rows, int cols=Cols)
+{
+#if EIGEN_HAS_CXX11
+  typedef Matrix<Scalar,Rows,1> VectorType;
+  typedef Matrix<Scalar,Rows,Cols,ColMajor> ColMatrixType;
+  typedef Matrix<Scalar,Rows,Cols,RowMajor> RowMatrixType;
+
+  ColMatrixType A = ColMatrixType::Random(rows, cols);
+  RowMatrixType B = RowMatrixType::Random(rows, cols);
+
+  Index i;
+
+  using ColMatrixColType = decltype(A.col(i));
+  using ColMatrixRowType = decltype(A.row(i));
+  using RowMatrixColType = decltype(B.col(i));
+  using RowMatrixRowType = decltype(B.row(i));
+
+  // Vector and matrix col/row are valid Stl-style container.
+  VERIFY_IS_EQUAL(IsContainerType<VectorType>(0), true);
+  VERIFY_IS_EQUAL(IsContainerType<ColMatrixColType>(0), true);
+  VERIFY_IS_EQUAL(IsContainerType<ColMatrixRowType>(0), true);
+  VERIFY_IS_EQUAL(IsContainerType<RowMatrixColType>(0), true);
+  VERIFY_IS_EQUAL(IsContainerType<RowMatrixRowType>(0), true);
+
+  // But the matrix itself is not a valid Stl-style container.
+  VERIFY_IS_EQUAL(IsContainerType<ColMatrixType>(0), rows == 1 || cols == 1);
+  VERIFY_IS_EQUAL(IsContainerType<RowMatrixType>(0), rows == 1 || cols == 1);
+#endif
+}
+
 EIGEN_DECLARE_TEST(stl_iterators)
 {
   for(int i = 0; i < g_repeat; i++) {
@@ -366,4 +419,7 @@ EIGEN_DECLARE_TEST(stl_iterators)
     CALL_SUBTEST_1(( test_stl_iterators<int,Dynamic,Dynamic>(internal::random<int>(5,10), internal::random<int>(5,10)) ));
     CALL_SUBTEST_1(( test_stl_iterators<int,Dynamic,Dynamic>(internal::random<int>(10,200), internal::random<int>(10,200)) ));
   }
+
+  CALL_SUBTEST_1(( test_stl_container_detection<float,1,1>() ));
+  CALL_SUBTEST_1(( test_stl_container_detection<float,5,5>() ));
 }