boost::apply needs to change from lvalue reference to forward reference

[jhcarl0814]

boost::apply does not compile when injecting rvalue at function/arguments positions using nested boost::bind:

#include<iostream>
#include<functional>
#include<boost/bind/bind.hpp>
#include<boost/bind/apply.hpp>

class f_t
{
public:
    void operator()(int&)&
    {
        std::cout << 1 << std::endl;
    }
    void operator()(int&&)&
    {
        std::cout << 2 << std::endl;
    }
    void operator()(int&)&&
    {
        std::cout << 3 << std::endl;
    }
    void operator()(int&&)&&
    {
        std::cout << 4 << std::endl;
    }
}f;

int& get_lvalue_arg()
{
    static int a;
    return a;
}

int get_prvalue_arg()
{
    return 1;
}

auto& get_lvalue_f()
{
    return f;
}

auto get_prvalue_f() // function returning [(prvalue) function object]; the same goes for function returning [(prvalue) function pointer]
{
    return f;
}

int main()
{
    boost::bind(boost::apply<void>(), boost::bind(get_lvalue_f), boost::bind(get_lvalue_arg))();
    boost::bind(boost::apply<void>(), boost::bind(get_lvalue_f), boost::bind(get_prvalue_arg))(); //compile error
    boost::bind(boost::apply<void>(), boost::bind(get_prvalue_f), boost::bind(get_lvalue_arg))(); //compile error
    boost::bind(boost::apply<void>(), boost::bind(get_prvalue_f), boost::bind(get_prvalue_arg))(); //compile error

    boost::bind(boost::apply<void>(), boost::bind(boost::apply<f_t&>(), boost::placeholders::_1), boost::bind(boost::apply<int&>(), boost::placeholders::_2))(get_lvalue_f, get_lvalue_arg);
    boost::bind(boost::apply<void>(), boost::bind(boost::apply<f_t&>(), boost::placeholders::_1), boost::bind(boost::apply<int>(), boost::placeholders::_2))(get_lvalue_f, get_prvalue_arg); //compile error
    boost::bind(boost::apply<void>(), boost::bind(boost::apply<f_t>(), boost::placeholders::_1), boost::bind(boost::apply<int&>(), boost::placeholders::_2))(get_prvalue_f, get_lvalue_arg); //compile error
    boost::bind(boost::apply<void>(), boost::bind(boost::apply<f_t>(), boost::placeholders::_1), boost::bind(boost::apply<int>(), boost::placeholders::_2))(get_prvalue_f, get_prvalue_arg); //compile error

    std::bind(f, std::bind(get_lvalue_arg))();
    std::bind(f, std::bind(get_prvalue_arg))();
}

expected output:

1
2
3
4
1
2
3
4
1
2

workaround:
in <boost/bind/apply.hpp>, change:

template<class F, class A1> result_type operator()(F & f, A1 & a1) const
{
    return f(a1);
}

to:

template<class F, class A1> result_type operator()(F&& f, A1&& a1) const
{
    return ((F&&)f)((A1&&)a1);
}

[pdimov]

Should be fixed on develop. Thank you for the report.