Warning predefinition std::tuple_size

[tdegeus]

It appears that std::tuple_size is predefined here:

https://github.com/xtensor-stack/xtl/blob/86d8071ff347450e7394eb958fd9bb9f2ede92b4/include/xtl/xspan_impl.hpp#L763

However, some compilers (e.g. linux/clang6.0) seem to define it as struct not a class:

In file included from /home/travis/miniconda/include/xtl/xspan.hpp:13:
864/home/travis/miniconda/include/xtl/xspan_impl.hpp:763:1: warning: 'tuple_size'
865      defined as a class template here but previously declared as a struct
866      template [-Wmismatched-tags]
867class tuple_size<tcb::span<E, S>> : public integral_constant<size_t, sta...
868^
869/usr/bin/../lib/gcc/x86_64-linux-gnu/5.5.0/../../../../include/c++/5.5.0/tuple:801:5: note: 
870      did you mean class here?
871    struct tuple_size;
872    ^