From 6ae7d842a31e1e1937f88a1acf513c7bf51aee9c Mon Sep 17 00:00:00 2001
From: Benoit Jacob <jacob.benoit.1@gmail.com>
Date: Sun, 24 Jan 2010 21:44:18 -0500
Subject: [PATCH] generate a compilation error when using
 ReturnByValue::coeff() or coeffRef(), instead of doing an infinite recursion

---
 Eigen/src/Core/ReturnByValue.h | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/Eigen/src/Core/ReturnByValue.h b/Eigen/src/Core/ReturnByValue.h
index 0199d3740..920269365 100644
--- a/Eigen/src/Core/ReturnByValue.h
+++ b/Eigen/src/Core/ReturnByValue.h
@@ -70,6 +70,18 @@ template<typename Derived> class ReturnByValue
     { static_cast<const Derived* const>(this)->evalTo(dst); }
     inline int rows() const { return static_cast<const Derived* const>(this)->rows(); }
     inline int cols() const { return static_cast<const Derived* const>(this)->cols(); }
+
+#ifndef EIGEN_PARSED_BY_DOXYGEN
+#define Unusable YOU_ARE_TRYING_TO_ACCESS_A_SINGLE_COEFFICIENT_IN_A_SPECIAL_EXPRESSION_WHERE_THAT_IS_NOT_ALLOWED_BECAUSE_THAT_WOULD_BE_INEFFICIENT
+    class Unusable{
+      Unusable(const Unusable&) {}
+      Unusable& operator=(const Unusable&) {return *this;}
+    };
+    const Unusable& coeff(int) const { return *reinterpret_cast<const Unusable*>(this); }
+    const Unusable& coeff(int,int) const { return *reinterpret_cast<const Unusable*>(this); }
+    Unusable& coeffRef(int) { return *reinterpret_cast<Unusable*>(this); }
+    Unusable& coeffRef(int,int) { return *reinterpret_cast<Unusable*>(this); }
+#endif
 };
 
 template<typename Derived>