`iterator_range` does not accept iterators having "reference to function type" as `reference` nested type

[Michel Morin]

This is a very minor thing.

This code works:

#include <iostream>
#include <boost/iterator/transform_iterator.hpp>

void zero() { std::cout << "zero" << std::endl; }
void non_zero() { std::cout << "non-zero" << std::endl; }

typedef void function_type();
function_type& f(int i)
{
    if (i == 0) {
        return zero;
    }
    else {
        return non_zero;
    }
}

int main(int argc, char* argv[])
{
    int ar[2] = {0, 1};
    
    typedef boost::transform_iterator<function_type&(*)(int), int*> iterator;
    
    iterator it_0 = boost::make_transform_iterator(ar + 0, f);
    iterator it_1 = boost::make_transform_iterator(ar + 1, f);
    
    (*it_0)();
    (*it_1)();
    
    return 0;
}

But this code does not compile:

#include <iostream>
#include <boost/iterator/transform_iterator.hpp>
#include <boost/range/iterator_range.hpp>

void zero() { std::cout << "zero" << std::endl; }
void non_zero() { std::cout << "non-zero" << std::endl; }

typedef void function_type();
function_type& f(int i)
{
    if (i == 0) {
        return zero;
    }
    else {
        return non_zero;
    }
}

int main(int argc, char* argv[])
{
    int ar[2] = {0, 1};
    
    typedef boost::transform_iterator<function_type&(*)(int), int*> iterator;
    typedef boost::iterator_range<iterator> range;
    
    range rng(
        boost::make_transform_iterator(ar, f)
      , boost::make_transform_iterator(ar + 2, f)
    );
    
    rng[0]();
    rng[1]();
    
    return 0;
}

This is because iterator_range does not accept iterators having "reference to function type" as reference nested type.

Patch attached. Tested on gcc-4.7 and clang trunk.