unique ptr and move semantics

I have the following question about unique pointers and the ownership transfer Assume the following

class Base
{
    Base()
    {
    }
    void foo(unique_ptr<Base> p)
    {
    }
};
void main()
{
    Base b;
    b.foo(std::unique_ptr<Base> p(new Base);
}

The above section of code creates a temporal unique pointer and I expected a compile error. But the code compiles. I expected the only valid function prototype to be the

 void foo(unique_ptr<Base> && p)

I am surprize that and the two signatures of the functions are valid. Could you please explain why the void foo(unique_ptr p) is valid when we pass an rvalue reference? Thank you