Simplify the use of custom scalar types, the rule is to never directly call a standard math function using std:: but rather put a using std::foo before and simply call foo:

using std::max;
max(a,b);

Eigen/src/Core/MathFunctions.h

@@ -87,7 +87,8 @@ struct real_impl<std::complex<RealScalar> >
 {
   static inline RealScalar run(const std::complex<RealScalar>& x)
   {
-    return std::real(x);
+    using std::real;
+    return real(x);
   }
 };
 
@@ -122,7 +123,8 @@ struct imag_impl<std::complex<RealScalar> >
 {
   static inline RealScalar run(const std::complex<RealScalar>& x)
   {
-    return std::imag(x);
+    using std::imag;
+    return imag(x);
   }
 };
 
@@ -244,7 +246,8 @@ struct conj_impl<std::complex<RealScalar> >
 {
   static inline std::complex<RealScalar> run(const std::complex<RealScalar>& x)
   {
-    return std::conj(x);
+    using std::conj;
+    return conj(x);
   }
 };
 
@@ -270,7 +273,8 @@ struct abs_impl
   typedef typename NumTraits<Scalar>::Real RealScalar;
   static inline RealScalar run(const Scalar& x)
   {
-    return std::abs(x);
+    using std::abs;
+    return abs(x);
   }
 };
 
@@ -305,7 +309,8 @@ struct abs2_impl<std::complex<RealScalar> >
 {
   static inline RealScalar run(const std::complex<RealScalar>& x)
   {
-    return std::norm(x);
+    using std::norm;
+    return norm(x);
   }
 };
 
@@ -369,10 +374,12 @@ struct hypot_impl
   typedef typename NumTraits<Scalar>::Real RealScalar;
   static inline RealScalar run(const Scalar& x, const Scalar& y)
   {
+    using std::max;
+    using std::min;
     RealScalar _x = abs(x);
     RealScalar _y = abs(y);
-    RealScalar p = std::max(_x, _y);
-    RealScalar q = std::min(_x, _y);
+    RealScalar p = max(_x, _y);
+    RealScalar q = min(_x, _y);
     RealScalar qp = q/p;
     return p * sqrt(RealScalar(1) + qp*qp);
   }
@@ -420,7 +427,8 @@ struct sqrt_default_impl
 {
   static inline Scalar run(const Scalar& x)
   {
-    return std::sqrt(x);
+    using std::sqrt;
+    return sqrt(x);
   }
 };
 
@@ -460,7 +468,7 @@ inline EIGEN_MATHFUNC_RETVAL(sqrt, Scalar) sqrt(const Scalar& x)
 // This macro instanciate all the necessary template mechanism which is common to all unary real functions.
 #define EIGEN_MATHFUNC_STANDARD_REAL_UNARY(NAME) \
   template<typename Scalar, bool IsInteger> struct NAME##_default_impl {            \
-    static inline Scalar run(const Scalar& x) { return std::NAME(x); }              \
+    static inline Scalar run(const Scalar& x) { using std::NAME; return NAME(x); }  \
   };                                                                                \
   template<typename Scalar> struct NAME##_default_impl<Scalar, true> {              \
     static inline Scalar run(const Scalar&) {                                       \
@@ -495,7 +503,8 @@ struct atan2_default_impl
   typedef Scalar retval;
   static inline Scalar run(const Scalar& x, const Scalar& y)
   {
-    return std::atan2(x, y);
+    using std::atan2;
+    return atan2(x, y);
   }
 };
 
@@ -534,7 +543,8 @@ struct pow_default_impl
   typedef Scalar retval;
   static inline Scalar run(const Scalar& x, const Scalar& y)
   {
-    return std::pow(x, y);
+    using std::pow;
+    return pow(x, y);
   }
 };
 
@@ -726,7 +736,8 @@ struct scalar_fuzzy_default_impl<Scalar, false, false>
   }
   static inline bool isApprox(const Scalar& x, const Scalar& y, const RealScalar& prec)
   {
-    return abs(x - y) <= std::min(abs(x), abs(y)) * prec;
+    using std::min;
+    return abs(x - y) <= min(abs(x), abs(y)) * prec;
   }
   static inline bool isApproxOrLessThan(const Scalar& x, const Scalar& y, const RealScalar& prec)
   {
@@ -764,7 +775,8 @@ struct scalar_fuzzy_default_impl<Scalar, true, false>
   }
   static inline bool isApprox(const Scalar& x, const Scalar& y, const RealScalar& prec)
   {
-    return abs2(x - y) <= std::min(abs2(x), abs2(y)) * prec * prec;
+    using std::min;
+    return abs2(x - y) <= min(abs2(x), abs2(y)) * prec * prec;
   }
 };