std::result_of simple function

#include <iostream>
#include <type_traits>

double f(int i)
{
        return i+0.1;
}

struct F
{
        public:
        double operator ()(int i) { return i+0.1; }
};

int
main(int, char**)
{
        std::result_of<F(int)>::type x;     // ok
        // std::result_of<f(int)>::type x; // error: template argument 1 is invalid
        x = 0.1;
        std::cerr << x << std::endl;
}

Please explain why std::result_of<f(int)>::type x; is invalid...

cppreference says "(std::result_of) Deduces the return type of a function call expression at compile type.".

what's the problem?