Lambda closure cannot be converted to std::function

I was playing around a bit with lambdas and binds and cannot understand why at the second call of f there is error with lambda closure cannot be converted into std::function<int(int)>. I kindly ask for an explanation. Thanks in advance.

#include <vector>
#include <functional>

void f(std::function<int(int)>f1, int x)
{
    f1(x);
}
int g(int x, int y)
{
    std::cout<<x+y;
    return x;
}

int main()
{
 f([](int x, int y = 10){std::cout<<x+y;return x;},20);
 f([](int x, int y = 10){g(x,y);},20);
    return 0;
}