Simplify base class detection for Eigen types

include/pybind11/common.h

@@ -390,6 +390,21 @@ template <template<class> class Predicate, class Default, class... T> using firs
 template <typename T, typename... /*Us*/> struct deferred_type { using type = T; };
 template <typename T, typename... Us> using deferred_t = typename deferred_type<T, Us...>::type;
 
+template <template<typename...> class Base>
+struct is_template_base_of_impl {
+    template <typename... Us> static std::true_type check(Base<Us...> *);
+    static std::false_type check(...);
+};
+
+/// Check if a template is the base of a type. For example:
+/// `is_template_base_of<Base, T>` is true if `struct T : Base<U> {}` where U can be anything
+template <template<typename...> class Base, typename T>
+#if !defined(_MSC_VER)
+using is_template_base_of = decltype(is_template_base_of_impl<Base>::check((T*)nullptr));
+#else // MSVC2015 has trouble with decltype in template aliases
+struct is_template_base_of : decltype(is_template_base_of_impl<Base>::check((T*)nullptr)) { };
+#endif
+
 /// Ignore that a variable is unused in compiler warnings
 inline void ignore_unused(const int *) { }
 

include/pybind11/eigen.h

@@ -34,47 +34,18 @@
 NAMESPACE_BEGIN(pybind11)
 NAMESPACE_BEGIN(detail)
 
-template <typename T> class is_eigen_dense {
-private:
-    template<typename Derived> static std::true_type test(const Eigen::DenseBase<Derived> &);
-    static std::false_type test(...);
-public:
-    static constexpr bool value = decltype(test(std::declval<T>()))::value;
-};
-
-// Eigen::Ref<Derived> satisfies is_eigen_dense, but isn't constructible, so it needs a special
-// type_caster to handle argument copying/forwarding.
-template <typename T> class is_eigen_ref {
-private:
-    template<typename Derived> static enable_if_t<
-        std::is_same<typename std::remove_const<T>::type, Eigen::Ref<Derived>>::value,
-        Derived> test(const Eigen::Ref<Derived> &);
-    static void test(...);
-public:
-    typedef decltype(test(std::declval<T>())) Derived;
-    static constexpr bool value = !std::is_void<Derived>::value;
-};
-
-template <typename T> class is_eigen_sparse {
-private:
-    template<typename Derived> static std::true_type test(const Eigen::SparseMatrixBase<Derived> &);
-    static std::false_type test(...);
-public:
-    static constexpr bool value = decltype(test(std::declval<T>()))::value;
-};
+template <typename T> using is_eigen_dense = is_template_base_of<Eigen::DenseBase, T>;
+template <typename T> using is_eigen_sparse = is_template_base_of<Eigen::SparseMatrixBase, T>;
+template <typename T> using is_eigen_ref = is_template_base_of<Eigen::RefBase, T>;
 
 // Test for objects inheriting from EigenBase<Derived> that aren't captured by the above.  This
 // basically covers anything that can be assigned to a dense matrix but that don't have a typical
 // matrix data layout that can be copied from their .data().  For example, DiagonalMatrix and
 // SelfAdjointView fall into this category.
-template <typename T> class is_eigen_base {
-private:
-    template<typename Derived> static std::true_type test(const Eigen::EigenBase<Derived> &);
-    static std::false_type test(...);
-public:
-    static constexpr bool value = !is_eigen_dense<T>::value && !is_eigen_sparse<T>::value &&
-        decltype(test(std::declval<T>()))::value;
-};
+template <typename T> using is_eigen_base = bool_constant<
+    is_template_base_of<Eigen::EigenBase, T>::value
+    && !is_eigen_dense<T>::value && !is_eigen_sparse<T>::value
+>;
 
 template<typename Type>
 struct type_caster<Type, enable_if_t<is_eigen_dense<Type>::value && !is_eigen_ref<Type>::value>> {
@@ -159,10 +130,13 @@ protected:
     static PYBIND11_DESCR cols() { return _<T::ColsAtCompileTime>(); }
 };
 
-template<typename Type>
-struct type_caster<Type, enable_if_t<is_eigen_dense<Type>::value && is_eigen_ref<Type>::value>> {
+// Eigen::Ref<Derived> satisfies is_eigen_dense, but isn't constructable, so it needs a special
+// type_caster to handle argument copying/forwarding.
+template <typename CVDerived, int Options, typename StrideType>
+struct type_caster<Eigen::Ref<CVDerived, Options, StrideType>> {
 protected:
-    using Derived = typename std::remove_const<typename is_eigen_ref<Type>::Derived>::type;
+    using Type = Eigen::Ref<CVDerived, Options, StrideType>;
+    using Derived = typename std::remove_const<CVDerived>::type;
     using DerivedCaster = type_caster<Derived>;
     DerivedCaster derived_caster;
     std::unique_ptr<Type> value;