Improve readability of type trait used to distinguish scalar/vector t…

…ypes

sophus/ceres_local_parameterization.hpp

@@ -1,30 +1,31 @@
 #ifndef SOPHUS_CERES_LOCAL_PARAMETERIZATION_HPP
 #define SOPHUS_CERES_LOCAL_PARAMETERIZATION_HPP
 
+#include <ceres/local_parameterization.h>
+
 namespace Sophus {
 
+template <class T, std::size_t = sizeof(T)>
+constexpr std::true_type complete(T*);
+constexpr std::false_type complete(...);
+
+template <class T>
+using IsSpecialized = decltype(complete(std::declval<T*>()));
+
 /// Type trait used to distinguish mappable vector types from scalars
+///
 /// We use this class to distinguish Sophus::Vector<Scalar, N> from Scalar types
 /// in LieGroup<T>::Tangent
 ///
-/// Fortunately, ceres::Jet is not mappable
-template <typename T>
-struct is_mappable_type_t {
-  template <typename U>
-  using EigenTraits = Eigen::internal::traits<U>;
-  // Eigen::Map<T> requires Eigen::internal::traits<T> type to be complete
-  template <typename U>
-  static auto map_test(U*)
-      -> std::integral_constant<bool, sizeof(EigenTraits<U>) ==
-                                          sizeof(EigenTraits<U>)>;
-  static auto map_test(...) -> std::false_type;
-
-  using type = decltype(map_test((T*)nullptr));
-  static constexpr bool value = type::value;
-};
+/// Having Eigen::internal::traits<T> specialized for T is a prerequisite for
+/// constructing Eigen::Map<T> (we will call such T "mappable" type)
+///
+/// Fortunately, ceres::Jet is not a mappable type
+template <class T>
+using HasEigenTraits = IsSpecialized<Eigen::internal::traits<std::decay_t<T>>>;
 
-template <typename T>
-constexpr bool is_mappable_type_v = is_mappable_type_t<T>::value;
+template <class T>
+constexpr bool HasEigenTraitsV = HasEigenTraits<T>::value;
 
 /// Helper for mapping tangent vectors (scalars) over pointers to data
 template <typename T, typename E = void>
@@ -38,7 +39,7 @@ struct Mapper {
 };
 
 template <typename T>
-struct Mapper<T, typename std::enable_if<is_mappable_type_v<T>>::type> {
+struct Mapper<T, typename std::enable_if<HasEigenTraitsV<T>>::type> {
   using Scalar = typename T::Scalar;
   using Map = Eigen::Map<T>;
   using ConstMap = Eigen::Map<const T>;