good to know! but I still get an error, on this new version.

error: no matching function for call to ‘test(<unresolved overloaded function type>)’
/tmp/tt.cpp:25:6: note: candidate is: void test(F) [with F = char (csquare_impl::*)(char)const]

but "run" is not overloaded !, and I don't know how to cast to resolve the overload.

#include <typeinfo>
#include <iostream>
#include <boost/type_traits/function_traits.hpp>
#include <boost/type_traits/remove_pointer.hpp>
#include <boost/function_types/parameter_types.hpp>
#include <boost/mpl/vector.hpp>
#include <boost/mpl/int.hpp>
#include <boost/mpl/at.hpp>

using namespace std;

double dsquare(double l){ return l*l; }
int isquare(int l){ return l*l; }

struct csquare_impl{
    char run(char c) const{return 'c';}
};
csquare_impl csquare;

template<class F>
void test(F f){
    // works
    // typename boost::function_traits<typename boost::remove_pointer<F>::type>::arg1_type test_var;
    // works too, more elegant(?)
    typename boost::mpl::at_c<typename boost::function_types::parameter_types<F>::type, 0 >::type test_var;
    cout << typeid(test_var).name() << endl;
}

int main(){
    test(dsquare); // ok, prints "d"(double)
    test(isquare); // ok, prints "i"(int)
    test(csquare.csquare_impl::run);  // not ok, want to print "c"(char)
    return 0;
}


BTW, this type traits is a mess, everything it is all over the place, Boost.FunctionTypes, Boost.TypeTraits.FunctionTraits, Boost.Functional.