some using declarations points to weird places

[correaa]

In this case, it points to the first line where the class is defined, instead of a local using or the implementation in the base class.

https://correaa.gitlab.io/boost-multi/multi/reference/boost/multi/const_subarray-0ba/empty.html

It points to
https://gitlab.com/correaa/boost-multi/-/blob/develop/include/boost/multi/array_ref.hpp#L1317
(there is nothing special in this line, except that this is the first line of the class definition)

instead of
https://gitlab.com/correaa/boost-multi/-/blob/develop/include/boost/multi/array_ref.hpp#L304