template and decltype for void f(int)

I am learning write my own traits using C++11 borrowing only std::true_type and std::false_type

Why is_farg1<void(*)(int)>::value is true while is_farg1<decltype(f)>::value returned false?

If my trait is incorrectly written then how can I check if the given function has one and only one argument so I could use also decltype(f)?

#include <iostream>
template <typename F>

template <typename F>
struct is_farg1: std::false_type {};

template <typename R, typename A>
struct is_farg1<R (*)(A)> : std::true_type {};

void f(int) {}

int main() {
    std::cout << "is_farg1<decltype(f)>  : " << is_farg1<decltype(f)>::value << std::endl;
    std::cout << "is_farg1<void(*)(int)> : " << is_farg1<void(*)(int)>::value << std::endl;

    return 0;
}

Output:

is_farg1<decltype(f)>  : 0
is_farg1<void(*)(int)> : 1