From 0d7a23d34e9e898f538220816704e5add4b03f17 Mon Sep 17 00:00:00 2001
From: Benoit Steiner <benoit.steiner.goog@gmail.com>
Date: Thu, 29 Oct 2015 17:29:49 -0700
Subject: [PATCH] Extended the reduction code so that reducing an empty set
 returns the neural element for the operation

---
 .../Eigen/CXX11/src/Core/util/CXX11Meta.h       | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/unsupported/Eigen/CXX11/src/Core/util/CXX11Meta.h b/unsupported/Eigen/CXX11/src/Core/util/CXX11Meta.h
index 7cd04a99e..7e4929ff8 100644
--- a/unsupported/Eigen/CXX11/src/Core/util/CXX11Meta.h
+++ b/unsupported/Eigen/CXX11/src/Core/util/CXX11Meta.h
@@ -252,6 +252,13 @@ template<
   typename... Ts
 > struct reduce;
 
+template<
+  typename Reducer
+> struct reduce<Reducer>
+{
+  constexpr static inline int run() { return Reducer::Identity; }
+};
+
 template<
   typename Reducer,
   typename A,
@@ -275,8 +282,14 @@ template<
 
 /* generic binary operations */
 
-struct sum_op           { template<typename A, typename B> constexpr static inline auto run(A a, B b) -> decltype(a + b)   { return a + b;   } };
-struct product_op       { template<typename A, typename B> constexpr static inline auto run(A a, B b) -> decltype(a * b)   { return a * b;   } };
+struct sum_op           {
+  template<typename A, typename B> constexpr static inline auto run(A a, B b) -> decltype(a + b)   { return a + b;   }
+  static constexpr int Identity = 0;
+};
+struct product_op       {
+  template<typename A, typename B> constexpr static inline auto run(A a, B b) -> decltype(a * b)   { return a * b;   }
+  static constexpr int Identity = 1;
+};
 
 struct logical_and_op   { template<typename A, typename B> constexpr static inline auto run(A a, B b) -> decltype(a && b)  { return a && b;  } };
 struct logical_or_op    { template<typename A, typename B> constexpr static inline auto run(A a, B b) -> decltype(a || b)  { return a || b;  } };