Cannot deduce template argument that is a function

Why cannot F be deduced for proxy()?

It should be possible because I am restricting it - only for functions that return an int.

#include <utility>
#include <iostream>
#include <type_traits>
using namespace std;

int foo(int bar) {
    cout << "int" << endl;
    return 2;
}

float foo(float bar) {
    cout << "float" << endl;
    return 1;
}

template <typename F, typename... Args>
typename enable_if<
    is_same<
        typename result_of<F(Args...)>::type,
        int
        >::value,
    typename result_of<F(Args...)>::type
    >::type
proxy(F func, Args&&... args) {
    return func(forward<Args>(args)...);
}

int main() {
    proxy(foo, 5);
}

Here is the error:

b.cpp:29:17: error: no matching function for call to 'proxy(<unresolved overloaded function type>, int)'
b.cpp:24:1: note:   template argument deduction/substitution failed:
b.cpp:29:17: note:   couldn't deduce template parameter 'F'