Range Algorithms do not compile with `std::span`

[gcerretani]

Algorithms like find_if do not compile when used on instances of std::span. They work with boost::span.

You can try this code on https://godbolt.org/z/Mq4GhqMYn:

#include <ranges>
#include <span>

#include <boost/core/span.hpp>
#include <boost/range/algorithm/find.hpp>

constexpr int a[] { 0, 1, 2, 3, 4, 5, 6, 7, 8 };
boost::span boost_span{a};
std::span std_span{a};

auto foo0() {
    return boost::range::find(boost_span, 2); // ok
}

auto foo1() {
    return boost::range::find(std_span, 2); // fails
}

auto foo2() {
    return std::ranges::find(boost_span, 2); // ok
}

auto foo3() {
    return std::ranges::find(std_span, 2); // ok
}

The error message is something like

error: no type named 'type' in 'struct boost::range_iterator<const std::span<const int, 9>, void>'

[gcerretani]

The problem depends by std::span that does not provice a std::span<T>::const_iterator, that is usually provided by the other standard containers.

Could be related to https://stackoverflow.com/questions/62757700/why-does-stdspan-lack-cbegin-and-cend-methods