Allow for negative strides.

Note that using a stride of -1 is still not possible because it would
clash with the definition of Eigen::Dynamic.

This fixes #747.

Eigen/src/Core/Stride.h

@@ -26,7 +26,7 @@ namespace Eigen {
   *
   * The outer stride is the pointer increment between two consecutive rows of a row-major matrix or
   * between two consecutive columns of a column-major matrix.
-  *
+  * 
   * These two values can be passed either at compile-time as template parameters, or at runtime as
   * arguments to the constructor.
   *
@@ -38,6 +38,10 @@ namespace Eigen {
   * \include Map_general_stride.cpp
   * Output: \verbinclude Map_general_stride.out
   *
+  * Both strides can be negative, however, a negative stride of -1 cannot be specified at compiletime
+  * because of the ambiguity with Dynamic which is defined to -1 (historically, negative strides were
+  * not allowed).
+  * 
   * \sa class InnerStride, class OuterStride, \ref TopicStorageOrders
   */
 template<int _OuterStrideAtCompileTime, int _InnerStrideAtCompileTime>
@@ -55,6 +59,8 @@ class Stride
     Stride()
       : m_outer(OuterStrideAtCompileTime), m_inner(InnerStrideAtCompileTime)
     {
+      // FIXME: for Eigen 4 we should use DynamicIndex instead of Dynamic.
+      // FIXME: for Eigen 4 we should also unify this API with fix<>
       eigen_assert(InnerStrideAtCompileTime != Dynamic && OuterStrideAtCompileTime != Dynamic);
     }
 
@@ -63,7 +69,6 @@ class Stride
     Stride(Index outerStride, Index innerStride)
       : m_outer(outerStride), m_inner(innerStride)
     {
-      eigen_assert(innerStride>=0 && outerStride>=0);
     }
 
     /** Copy constructor */