From e7d8ba747c7b161eff59959d4d2a5acf788ee00e Mon Sep 17 00:00:00 2001
From: Gael Guennebaud <g.gael@free.fr>
Date: Thu, 10 Oct 2019 17:41:47 +0200
Subject: [PATCH] bug #1752: make is_convertible equivalent to the std c++11
 equivalent and fallback to std::is_convertible when c++11 is enabled.

---
 Eigen/src/Core/util/Meta.h | 13 +++++++++++++
 test/meta.cpp              |  8 ++++++--
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/Eigen/src/Core/util/Meta.h b/Eigen/src/Core/util/Meta.h
index db948cea19..fe9550d71f 100755
--- a/Eigen/src/Core/util/Meta.h
+++ b/Eigen/src/Core/util/Meta.h
@@ -174,6 +174,11 @@ template<typename T> struct add_const_on_value_type<T*>        { typedef T const
 template<typename T> struct add_const_on_value_type<T* const>  { typedef T const* const type; };
 template<typename T> struct add_const_on_value_type<T const* const>  { typedef T const* const type; };
 
+#if EIGEN_HAS_CXX11
+
+using std::is_convertible;
+
+#else
 
 template<typename From, typename To>
 struct is_convertible_impl
@@ -211,6 +216,14 @@ struct is_convertible
   enum { value = is_convertible_impl<From,To>::value };
 };
 
+template<typename T>
+struct is_convertible<T,T&> { enum { value = false }; };
+
+template<typename T>
+struct is_convertible<const T,const T&> { enum { value = true }; };
+
+#endif
+
 /** \internal Allows to enable/disable an overload
   * according to a compile time condition.
   */
diff --git a/test/meta.cpp b/test/meta.cpp
index 51395acd06..b432d93161 100644
--- a/test/meta.cpp
+++ b/test/meta.cpp
@@ -87,8 +87,12 @@ EIGEN_DECLARE_TEST(meta)
   STATIC_CHECK(( internal::is_convertible<const Matrix3f&,const Matrix3f&>::value ));
   STATIC_CHECK((!internal::is_convertible<const Matrix3f&,Matrix3f&>::value ));
   STATIC_CHECK((!internal::is_convertible<const Matrix3f,Matrix3f&>::value ));
-  STATIC_CHECK(( internal::is_convertible<Matrix3f,Matrix3f&>::value )); // std::is_convertible returns false here though Matrix3f from; Matrix3f& to = from; is valid.
-  //STATIC_CHECK((!internal::is_convertible<Matrix3f,Matrix3d>::value )); //does not work because the conversion is prevented by a static assertion
+  STATIC_CHECK(!( internal::is_convertible<Matrix3f,Matrix3f&>::value ));
+
+  STATIC_CHECK(!( internal::is_convertible<int,int&>::value ));
+  STATIC_CHECK(( internal::is_convertible<const int,const int& >::value ));
+
+  //STATIC_CHECK((!internal::is_convertible<Matrix3f,Matrix3d>::value )); //does not even compile because the conversion is prevented by a static assertion
   STATIC_CHECK((!internal::is_convertible<Array33f,int>::value ));
   STATIC_CHECK((!internal::is_convertible<MatrixXf,float>::value ));
   {