From 511810797e35471568091c44e947329f0060698e Mon Sep 17 00:00:00 2001
From: Simon Praetorius <simon.praetorius@tu-dresden.de>
Date: Fri, 24 Mar 2017 17:45:56 +0100
Subject: [PATCH] Issue with mpreal and std::numeric_limits, i.e. digits is not
 a constant. Added a digits() traits in NumTraits with fallback to static
 constant. Specialization for mpreal added in MPRealSupport.

---
 Eigen/src/Core/NumTraits.h      | 34 +++++++++++++++++++++++++++++++++
 Eigen/src/Core/StableNorm.h     |  2 +-
 unsupported/Eigen/MPRealSupport |  3 +++
 3 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/Eigen/src/Core/NumTraits.h b/Eigen/src/Core/NumTraits.h
index aebc0c259..4d896a098 100644
--- a/Eigen/src/Core/NumTraits.h
+++ b/Eigen/src/Core/NumTraits.h
@@ -41,6 +41,34 @@ struct default_digits10_impl<T,false,true> // Integer
   static int run() { return 0; }
 };
 
+
+// default implementation of digits(), based on numeric_limits if specialized,
+// 0 for integer types, and log2(epsilon()) otherwise.
+template< typename T,
+          bool use_numeric_limits = std::numeric_limits<T>::is_specialized,
+          bool is_integer = NumTraits<T>::IsInteger>
+struct default_digits_impl
+{
+  static int run() { return std::numeric_limits<T>::digits; }
+};
+
+template<typename T>
+struct default_digits_impl<T,false,false> // Floating point
+{
+  static int run() {
+    using std::log;
+    using std::ceil;
+    typedef typename NumTraits<T>::Real Real;
+    return int(ceil(-log(NumTraits<Real>::epsilon())/log(static_cast<Real>(2))));
+  }
+};
+
+template<typename T>
+struct default_digits_impl<T,false,true> // Integer
+{
+  static int run() { return 0; }
+};
+
 } // end namespace internal
 
 /** \class NumTraits
@@ -118,6 +146,12 @@ template<typename T> struct GenericNumTraits
     return internal::default_digits10_impl<T>::run();
   }
 
+  EIGEN_DEVICE_FUNC
+  static inline int digits()
+  {
+    return internal::default_digits_impl<T>::run();
+  }
+
   EIGEN_DEVICE_FUNC
   static inline Real dummy_precision()
   {
diff --git a/Eigen/src/Core/StableNorm.h b/Eigen/src/Core/StableNorm.h
index be04ed44d..4ea598ba8 100644
--- a/Eigen/src/Core/StableNorm.h
+++ b/Eigen/src/Core/StableNorm.h
@@ -74,7 +74,7 @@ blueNorm_impl(const EigenBase<Derived>& _vec)
     // are used. For any specific computer, each of the assignment
     // statements can be replaced
     ibeta = std::numeric_limits<RealScalar>::radix;                 // base for floating-point numbers
-    it    = std::numeric_limits<RealScalar>::digits;                // number of base-beta digits in mantissa
+    it    = NumTraits<RealScalar>::digits();                        // number of base-beta digits in mantissa
     iemin = std::numeric_limits<RealScalar>::min_exponent;          // minimum exponent
     iemax = std::numeric_limits<RealScalar>::max_exponent;          // maximum exponent
     rbig  = (std::numeric_limits<RealScalar>::max)();               // largest floating-point number
diff --git a/unsupported/Eigen/MPRealSupport b/unsupported/Eigen/MPRealSupport
index 7f0b70c63..b770abc4a 100644
--- a/unsupported/Eigen/MPRealSupport
+++ b/unsupported/Eigen/MPRealSupport
@@ -90,6 +90,9 @@ int main()
 #ifdef MPREAL_HAVE_DYNAMIC_STD_NUMERIC_LIMITS
     static inline int digits10 (long Precision = mpfr::mpreal::get_default_prec())  { return std::numeric_limits<Real>::digits10(Precision); }
     static inline int digits10 (const Real& x)                                      { return std::numeric_limits<Real>::digits10(x); }
+    
+    static inline int digits ()               { return std::numeric_limits<Real>::digits(); }
+    static inline int digits (const Real& x)  { return std::numeric_limits<Real>::digits(x); }
 #endif
 
     static inline Real dummy_precision()