`iterator_category_to_system` returns `host_system_tag` for `input_device_iterator_tag` instead of `device_system_tag`

Hello,

I was trying to design my own iterators to make them compatible with thrust. I stumbled across some problems and dug inside the code. Ultimately, it seems that the following statement fails to compile:

static_assert(std::is_same::type, thrust::device_system_tag>::value, "test");

While the following passes:

static_assert(std::is_same::type, thrust::host_system_tag>::value, "test");

That is not really surprising given the code of iterator_category_to_system and the fact that input_device_iterator_tag actually inherits from std::input_iterator_tag, which is just another name for input_host_iterator_tag (see iterator_categories.h).

Am I missing something or there is a bug there?

Note that all classes inheriting from thrust::iterator_adapter seem to be fine. Still trying to figure out why.