Add a DynamicIndex constant for signed quantities and use it to fix the conflict

between Diagonal<S,-1> (the first sub diagonal) and a runtime super/sub diagonal which is now:
Diagonal<S,DynamicIndex>

Eigen/src/Core/Diagonal.h

@@ -56,12 +56,12 @@ struct traits<Diagonal<MatrixType,DiagIndex> >
   typedef typename remove_reference<MatrixTypeNested>::type _MatrixTypeNested;
   typedef typename MatrixType::StorageKind StorageKind;
   enum {
-    RowsAtCompileTime = (int(DiagIndex) == Dynamic || int(MatrixType::SizeAtCompileTime) == Dynamic) ? Dynamic
+    RowsAtCompileTime = (int(DiagIndex) == DynamicIndex || int(MatrixType::SizeAtCompileTime) == Dynamic) ? Dynamic
-    : (EIGEN_PLAIN_ENUM_MIN(MatrixType::RowsAtCompileTime - EIGEN_PLAIN_ENUM_MAX(-DiagIndex, 0),
+                      : (EIGEN_PLAIN_ENUM_MIN(MatrixType::RowsAtCompileTime - EIGEN_PLAIN_ENUM_MAX(-DiagIndex, 0),
-                            MatrixType::ColsAtCompileTime - EIGEN_PLAIN_ENUM_MAX( DiagIndex, 0))),
+                                              MatrixType::ColsAtCompileTime - EIGEN_PLAIN_ENUM_MAX( DiagIndex, 0))),
     ColsAtCompileTime = 1,
     MaxRowsAtCompileTime = int(MatrixType::MaxSizeAtCompileTime) == Dynamic ? Dynamic
-                         : DiagIndex == Dynamic ? EIGEN_SIZE_MIN_PREFER_FIXED(MatrixType::MaxRowsAtCompileTime,
+                         : DiagIndex == DynamicIndex ? EIGEN_SIZE_MIN_PREFER_FIXED(MatrixType::MaxRowsAtCompileTime,
                                                                               MatrixType::MaxColsAtCompileTime)
                          : (EIGEN_PLAIN_ENUM_MIN(MatrixType::MaxRowsAtCompileTime - EIGEN_PLAIN_ENUM_MAX(-DiagIndex, 0),
                                                  MatrixType::MaxColsAtCompileTime - EIGEN_PLAIN_ENUM_MAX( DiagIndex, 0))),
@@ -158,7 +158,7 @@ template<typename MatrixType, int _DiagIndex> class Diagonal
 
   protected:
     typename MatrixType::Nested m_matrix;
-    const internal::variable_if_dynamic<Index, DiagIndex> m_index;
+    const internal::variable_if_dynamicindex<Index, DiagIndex> m_index;
 
   private:
     // some compilers may fail to optimize std::max etc in case of compile-time constants...
@@ -205,18 +205,18 @@ MatrixBase<Derived>::diagonal() const
   *
   * \sa MatrixBase::diagonal(), class Diagonal */
 template<typename Derived>
-inline typename MatrixBase<Derived>::template DiagonalIndexReturnType<Dynamic>::Type
+inline typename MatrixBase<Derived>::template DiagonalIndexReturnType<DynamicIndex>::Type
 MatrixBase<Derived>::diagonal(Index index)
 {
-  return typename DiagonalIndexReturnType<Dynamic>::Type(derived(), index);
+  return typename DiagonalIndexReturnType<DynamicIndex>::Type(derived(), index);
 }
 
 /** This is the const version of diagonal(Index). */
 template<typename Derived>
-inline typename MatrixBase<Derived>::template ConstDiagonalIndexReturnType<Dynamic>::Type
+inline typename MatrixBase<Derived>::template ConstDiagonalIndexReturnType<DynamicIndex>::Type
 MatrixBase<Derived>::diagonal(Index index) const
 {
-  return typename ConstDiagonalIndexReturnType<Dynamic>::Type(derived(), index);
+  return typename ConstDiagonalIndexReturnType<DynamicIndex>::Type(derived(), index);
 }
 
 /** \returns an expression of the \a DiagIndex-th sub or super diagonal of the matrix \c *this

Eigen/src/Core/MatrixBase.h

@@ -239,11 +239,11 @@ template<typename Derived> class MatrixBase
     // Note: The "MatrixBase::" prefixes are added to help MSVC9 to match these declarations with the later implementations.
     // On the other hand they confuse MSVC8...
     #if (defined _MSC_VER) && (_MSC_VER >= 1500) // 2008 or later
-    typename MatrixBase::template DiagonalIndexReturnType<Dynamic>::Type diagonal(Index index);
+    typename MatrixBase::template DiagonalIndexReturnType<DynamicIndex>::Type diagonal(Index index);
-    typename MatrixBase::template ConstDiagonalIndexReturnType<Dynamic>::Type diagonal(Index index) const;
+    typename MatrixBase::template ConstDiagonalIndexReturnType<DynamicIndex>::Type diagonal(Index index) const;
     #else
-    typename DiagonalIndexReturnType<Dynamic>::Type diagonal(Index index);
+    typename DiagonalIndexReturnType<DynamicIndex>::Type diagonal(Index index);
-    typename ConstDiagonalIndexReturnType<Dynamic>::Type diagonal(Index index) const;
+    typename ConstDiagonalIndexReturnType<DynamicIndex>::Type diagonal(Index index) const;
     #endif
 
     #ifdef EIGEN2_SUPPORT

Eigen/src/Core/util/Constants.h

@@ -28,13 +28,18 @@
 
 namespace Eigen {
 
-/** This value means that a quantity is not known at compile-time, and that instead the value is
+/** This value means that a positive quantity (e.g., a size) is not known at compile-time, and that instead the value is
   * stored in some runtime variable.
   *
   * Changing the value of Dynamic breaks the ABI, as Dynamic is often used as a template parameter for Matrix.
   */
 const int Dynamic = -1;
 
+/** This value means that a signed quantity (e.g., a signed index) is not known at compile-time, and that instead its value
+  * has to be specified at runtime.
+  */
+const int DynamicIndex = 0xffffff;
+
 /** This value means +Infinity; it is currently used only as the p parameter to MatrixBase::lpNorm<int>().
   * The value Infinity there means the L-infinity norm.
   */

Eigen/src/Core/util/XprHelper.h

@@ -80,6 +80,27 @@ template<typename T> class variable_if_dynamic<T, Dynamic>
     void setValue(T value) { m_value = value; }
 };
 
+/** \internal like variable_if_dynamic but for DynamicIndex
+  */
+template<typename T, int Value> class variable_if_dynamicindex
+{
+  public:
+    EIGEN_EMPTY_STRUCT_CTOR(variable_if_dynamicindex)
+    explicit variable_if_dynamicindex(T v) { EIGEN_ONLY_USED_FOR_DEBUG(v); assert(v == T(Value)); }
+    static T value() { return T(Value); }
+    void setValue(T) {}
+};
+
+template<typename T> class variable_if_dynamicindex<T, DynamicIndex>
+{
+    T m_value;
+    variable_if_dynamicindex() { assert(false); }
+  public:
+    explicit variable_if_dynamicindex(T value) : m_value(value) {}
+    T value() const { return m_value; }
+    void setValue(T value) { m_value = value; }
+};
+
 template<typename T> struct functor_traits
 {
   enum

test/diagonal.cpp

@@ -46,17 +46,14 @@ template<typename MatrixType> void diagonal(const MatrixType& m)
   if (rows>2)
   {
     enum {
-      N1 = MatrixType::RowsAtCompileTime>1 ?  1 : 0,
+      N1 = MatrixType::RowsAtCompileTime>2 ?  2 : 0,
-      N2 = MatrixType::RowsAtCompileTime>2 ? -2 : 0
+      N2 = MatrixType::RowsAtCompileTime>1 ? -1 : 0
     };
 
     // check sub/super diagonal
-    if(m1.template diagonal<N1>().RowsAtCompileTime!=Dynamic)
+    if(MatrixType::SizeAtCompileTime!=Dynamic)
     {
       VERIFY(m1.template diagonal<N1>().RowsAtCompileTime == m1.diagonal(N1).size());
-    }
-    if(m1.template diagonal<N2>().RowsAtCompileTime!=Dynamic)
-    {
       VERIFY(m1.template diagonal<N2>().RowsAtCompileTime == m1.diagonal(N2).size());
     }