From d58678069c6c90ac6fe6bdeac5e95fd9ea1a2235 Mon Sep 17 00:00:00 2001
From: Christoph Hertzberg <chtz@informatik.uni-bremen.de>
Date: Fri, 9 Apr 2021 17:03:28 +0000
Subject: Make iterators default constructible and assignable, by making...

---
 Eigen/src/Core/StlIterators.h   | 19 ++++++++++++-------
 Eigen/src/Core/util/XprHelper.h |  5 ++---
 test/stl_iterators.cpp          | 30 +++++++++++++++++++++++++++++-
 3 files changed, 43 insertions(+), 11 deletions(-)

diff --git a/Eigen/src/Core/StlIterators.h b/Eigen/src/Core/StlIterators.h
index a2bc0dece..09041db1d 100644
--- a/Eigen/src/Core/StlIterators.h
+++ b/Eigen/src/Core/StlIterators.h
@@ -7,6 +7,9 @@
 // Public License v. 2.0. If a copy of the MPL was not distributed
 // with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
+#ifndef EIGEN_STLITERATORS_H
+#define EIGEN_STLITERATORS_H
+
 namespace Eigen {
 
 namespace internal {
@@ -30,10 +33,10 @@ public:
   typedef Index difference_type;
   typedef std::random_access_iterator_tag iterator_category;
 
-  indexed_based_stl_iterator_base() : mp_xpr(0), m_index(0) {}
-  indexed_based_stl_iterator_base(XprType& xpr, Index index) : mp_xpr(&xpr), m_index(index) {}
+  indexed_based_stl_iterator_base() EIGEN_NO_THROW : mp_xpr(0), m_index(0) {}
+  indexed_based_stl_iterator_base(XprType& xpr, Index index) EIGEN_NO_THROW : mp_xpr(&xpr), m_index(index) {}
 
-  indexed_based_stl_iterator_base(const non_const_iterator& other)
+  indexed_based_stl_iterator_base(const non_const_iterator& other) EIGEN_NO_THROW
     : mp_xpr(other.mp_xpr), m_index(other.m_index)
   {}
 
@@ -190,17 +193,17 @@ public:
   typedef typename internal::conditional<bool(is_lvalue), value_type&, const value_type&>::type reference;
 
 
-  pointer_based_stl_iterator() : m_ptr(0) {}
-  pointer_based_stl_iterator(XprType& xpr, Index index) : m_incr(xpr.innerStride())
+  pointer_based_stl_iterator() EIGEN_NO_THROW : m_ptr(0) {}
+  pointer_based_stl_iterator(XprType& xpr, Index index) EIGEN_NO_THROW : m_incr(xpr.innerStride())
   {
     m_ptr = xpr.data() + index * m_incr.value();
   }
 
-  pointer_based_stl_iterator(const non_const_iterator& other)
+  pointer_based_stl_iterator(const non_const_iterator& other) EIGEN_NO_THROW
     : m_ptr(other.m_ptr), m_incr(other.m_incr)
   {}
 
-  pointer_based_stl_iterator& operator=(const non_const_iterator& other)
+  pointer_based_stl_iterator& operator=(const non_const_iterator& other) EIGEN_NO_THROW
   {
     m_ptr = other.m_ptr;
     m_incr.setValue(other.m_incr);
@@ -456,3 +459,5 @@ inline typename DenseBase<Derived>::const_iterator DenseBase<Derived>::cend() co
 }
 
 } // namespace Eigen
+
+#endif // EIGEN_STLITERATORS_H
diff --git a/Eigen/src/Core/util/XprHelper.h b/Eigen/src/Core/util/XprHelper.h
index 9c7771797..2c63a9524 100644
--- a/Eigen/src/Core/util/XprHelper.h
+++ b/Eigen/src/Core/util/XprHelper.h
@@ -136,15 +136,14 @@ template<typename T, int Value> class variable_if_dynamic
     EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE EIGEN_CONSTEXPR
     operator T() const { return T(Value); }
     EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE
-    void setValue(T) const {}
+    void setValue(T v) const { EIGEN_ONLY_USED_FOR_DEBUG(v); eigen_assert(v == T(Value)); }
 };
 
 template<typename T> class variable_if_dynamic<T, Dynamic>
 {
     T m_value;
-    EIGEN_DEVICE_FUNC variable_if_dynamic() { eigen_assert(false); }
   public:
-    EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE explicit variable_if_dynamic(T value) : m_value(value) {}
+    EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE explicit variable_if_dynamic(T value = 0) EIGEN_NO_THROW : m_value(value) {}
     EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE T value() const { return m_value; }
     EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE operator T() const { return m_value; }
     EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE void setValue(T value) { m_value = value; }
diff --git a/test/stl_iterators.cpp b/test/stl_iterators.cpp
index 997f8016c..72bbf8250 100644
--- a/test/stl_iterators.cpp
+++ b/test/stl_iterators.cpp
@@ -7,9 +7,9 @@
 // Public License v. 2.0. If a copy of the MPL was not distributed
 // with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
+#include "main.h"
 #include <iterator>
 #include <numeric>
-#include "main.h"
 
 template< class Iterator >
 std::reverse_iterator<Iterator>
@@ -47,6 +47,18 @@ bool is_pointer_based_stl_iterator(const internal::pointer_based_stl_iterator<Xp
 template<typename XprType>
 bool is_generic_randaccess_stl_iterator(const internal::generic_randaccess_stl_iterator<XprType> &) { return true; }
 
+template<typename Iter>
+bool is_default_constructible_and_assignable(const Iter& it)
+{
+#if EIGEN_HAS_CXX11
+  VERIFY(std::is_default_constructible<Iter>::value);
+  VERIFY(std::is_nothrow_default_constructible<Iter>::value);
+#endif
+  Iter it2;
+  it2 = it;
+  return (it==it2);
+}
+
 template<typename Xpr>
 void check_begin_end_for_loop(Xpr xpr)
 {
@@ -124,6 +136,22 @@ void test_stl_iterators(int rows=Rows, int cols=Cols)
   
   Index i, j;
 
+  // Verify that iterators are default constructible (See bug #1900)
+  {
+    VERIFY( is_default_constructible_and_assignable(v.begin()));
+    VERIFY( is_default_constructible_and_assignable(v.end()));
+    VERIFY( is_default_constructible_and_assignable(cv.begin()));
+    VERIFY( is_default_constructible_and_assignable(cv.end()));
+
+    VERIFY( is_default_constructible_and_assignable(A.row(0).begin()));
+    VERIFY( is_default_constructible_and_assignable(A.row(0).end()));
+    VERIFY( is_default_constructible_and_assignable(cA.row(0).begin()));
+    VERIFY( is_default_constructible_and_assignable(cA.row(0).end()));
+
+    VERIFY( is_default_constructible_and_assignable(B.row(0).begin()));
+    VERIFY( is_default_constructible_and_assignable(B.row(0).end()));
+  }
+
   // Check we got a fast pointer-based iterator when expected
   {
     VERIFY( is_pointer_based_stl_iterator(v.begin()) );
-- 
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