Why is the constructor not called when assigning a functor to a std::function?

Question

I don't understand what happens with the following code using functors and std::function.

My explanations are interlaced with the code (the concatenated code is compilable).

#include <chrono>
#include <functional>

class O
{
public:
    void func();
};

Class O has a func-method to do something.

class FunctorClass
{
    O &o_;
    FunctorClass(O &o)
        : o_(o) {}

    std::chrono::milliseconds operator()()
    {
        o_.func();
        return std::chrono::seconds(1);
    }
};

Class FunctorClass takes a reference of an object of O and declares a operator() which returns an interval and calls func of member o_.

class TaskLoop
{
    std::function<std::chrono::milliseconds(void)> task_;

public:
    TaskLoop(std::function<std::chrono::milliseconds(void)> t)
        : task_(t)
        {}
};

TaskLoop is starting a thread and calls the task_ and sleep for the time-interval returned by this call. (Thread handling is left out here for clarity if any)

int main(void)
{
    O o;
    TaskLoop loop(FunctorClass(o));

    getchar(); // just to have the process waiting
    return 0;
}

In the main function I create an object of O. When creating the loop-object I thought I created a FunctorClass-object which itself it taken o as argument.

But this is not happening. The constructor of FunctorClass is private.

This code compiles with two different versions of GCC (4.8 and 6.x) without a related warning.

What happens? The compiler is optimizing out things, but why?

Sorry for the lengthy code, this is already a reduced test-case.

PS: When doing it with a new/shared_ptr I get the necessary compiler-warnings and errors:

std::shared_ptr<TaskLoop> t = std::make_shared<>(FunctorClass(o));