Allows users to specialize polymorphic_type_hook with std::enable_if.

Currently user specializations of the form

template <typename itype> struct polymorphic_type_hook<itype, std::enable_if_t<...>> { ... };

will fail if itype is also polymorphic, because the existing specialization will also
be enabled, which leads to 2 equally viable candidates. With this change, user provided
specializations have higher priority than the built in specialization for polymorphic types.

include/pybind11/cast.h

@@ -835,19 +835,25 @@ NAMESPACE_END(detail)
 // You may specialize polymorphic_type_hook yourself for types that want to appear
 // polymorphic to Python but do not use C++ RTTI. (This is a not uncommon pattern
 // in performance-sensitive applications, used most notably in LLVM.)
+//
+// polymorphic_type_hook_base allows users to specialize polymorphic_type_hook with
+// std::enable_if. User provided specializations will always have higher priority than
+// the default implementation and specialization provided in polymorphic_type_hook_base.
 template <typename itype, typename SFINAE = void>
-struct polymorphic_type_hook
+struct polymorphic_type_hook_base
 {
     static const void *get(const itype *src, const std::type_info*&) { return src; }
 };
 template <typename itype>
-struct polymorphic_type_hook<itype, detail::enable_if_t<std::is_polymorphic<itype>::value>>
+struct polymorphic_type_hook_base<itype, detail::enable_if_t<std::is_polymorphic<itype>::value>>
 {
     static const void *get(const itype *src, const std::type_info*& type) {
         type = src ? &typeid(*src) : nullptr;
         return dynamic_cast<const void*>(src);
     }
 };
+template <typename itype, typename SFINAE = void>
+struct polymorphic_type_hook : public polymorphic_type_hook_base<itype> {};
 
 NAMESPACE_BEGIN(detail)