The result of `value_at` with `std::tuple` does not match result of `value_at` with `boost::fusion::vector`

[denzor200]

https://godbolt.org/z/nWY9xvehE
Looks like a issue in std::tuple integration.

[Kojoley]

Has to be because of this

fusion/include/boost/fusion/adapted/std_tuple/std_tuple_iterator.hpp

Line 36 in 37ed70f

typename add_const<Tuple>::type, Index>

[denzor200]

Has to be because of this

fusion/include/boost/fusion/adapted/std_tuple/std_tuple_iterator.hpp

Line 36 in 37ed70f

typename add_const<Tuple>::type, Index>

Should we fix it?