Minimal example of bug: https://godbolt.org/z/91Tz6sbcx

In the example above, auto is being deduced as std::tuple<> instead of std:tuple<long int> by the compiler. Since we do not provide loop over in the example above, it should use this specialisation, however in computing

class = internal::enable_if_callable<Fn, typename Shape::index_type>

the compiler tries to deduce the return type of the function and produces an actual error (see this for more detail) which causes the other specialisation of for_each_index to be used, and since loop order is not provided, index_of_rank<sizeof...(LoopOrder)> evaluates to std::tuple<>.

There is a workaround for the bug by explicitly defining the return type of the lambda: https://godbolt.org/z/9bob8TYdd (added a -> void) however perhaps the sizeof...(LoopOrder) conditions should be the first enable ifs in the template parameters and specialisations should be done based on that?

I just found the bug but credit to @plietar for figuring this out