Working around MSVC 2015 bug (to be proven via GitHub CI).

include/pybind11/cast.h

@@ -1385,6 +1385,10 @@ struct smart_holder_type_caster : smart_holder_type_caster_load<T>,
 
 
     // clang-format off
     // clang-format off
 
 
+#if defined(_MSC_VER) && _MSC_VER < 1910
+    // Working around MSVC 2015 bug.
+    template <typename T_> using cast_op_type = detail::cast_op_type<T_>;
+#else
     template <typename T_>
     template <typename T_>
     using cast_op_type = conditional_t<
     using cast_op_type = conditional_t<
         std::is_same<remove_reference_t<T_>, T const *>::value,
         std::is_same<remove_reference_t<T_>, T const *>::value,
@@ -1394,6 +1398,7 @@ struct smart_holder_type_caster : smart_holder_type_caster_load<T>,
                       conditional_t<std::is_same<T_, T const &>::value,
                       conditional_t<std::is_same<T_, T const &>::value,
                                     T const &,
                                     T const &,
                                     T &>>>;
                                     T &>>>;
+#endif
 
 
     operator T const&() { return this->loaded_as_lvalue_ref(); }
     operator T const&() { return this->loaded_as_lvalue_ref(); }
     operator T&()       { return this->loaded_as_lvalue_ref(); }
     operator T&()       { return this->loaded_as_lvalue_ref(); }