An rvalue boost::span doesn't work for algorithms that expect mutable ranges

[jsg72]

An rvalue span does not work for algorithms that modify elements. In the following example, using an lvalue span works, but using an rvalue span fails to compile.

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

template <typename T>
boost::span<T> to_span( std::vector<T>& v ) {
        return v;
}

int main() {

   std::vector<unsigned char> vec{ 1, 2, 3, 4, 5 };
   boost::span<unsigned char> sp{ vec };

   boost::fill( sp, 0 ); // works
   boost::fill( to_span(vec), 0); // fails

   return 0;
}

It looks to me that the issue is that boost::begin and boost::end have overloads for lvalue ranges and const lvalue ranges but not rvalue ranges, so if you pass an rvalue range to an algorithm, it gets treated as a const lvalue range and has const iterators. That seems to be the right thing to do for containers, but not so much for proxy types like span.