Sender introspection determines tag differently from data and children

[maartenarnst]

Currently, the sender introspection determines the tag based on the description, but the data and children directly from the tuple.

stdexec/include/stdexec/__detail/__sender_introspection.hpp

Lines 65 to 69 in 1687898

	template <class _Sender>
	using tag_of_t = decltype(__detail::__tag_of_v<_Sender>);
	template <class _Sender>
	using __data_of = __tuple_element_t<1, _Sender>;

Hence, when we write a custom sender like

struct MySender : __tuple<Tag, Data, Child> {
    ...
};

we still need to explicitly provide the description for stdexec to be able to determine the tag.

I'm not sure to understand why the approach for tag is different. Perhaps there could be a fallback so that tag_of_t is determined from __tuple_element_t<0, _Sender>; if the __desc_t path is not available.