Simplify ScalarBinaryOpTraits by removing the Defined enum, and extend its documentation.

Eigen/src/Core/util/Meta.h

@@ -358,6 +358,19 @@ struct result_of<Func(ArgType0,ArgType1,ArgType2)> {
 };
 #endif
 
+// Check whether T::ReturnType does exist
+template <typename T>
+struct has_ReturnType
+{
+  typedef char yes[1];
+  typedef char no[2];
+
+  template <typename C> static yes& testFunctor(C const *, typename C::ReturnType const * = 0);
+  static no& testFunctor(...);
+
+  static const bool value = sizeof(testFunctor(static_cast<T*>(0))) == sizeof(yes);
+};
+
 /** \internal In short, it computes int(sqrt(\a Y)) with \a Y an integer.
   * Usage example: \code meta_sqrt<1023>::ret \endcode
   */