What does “int& foo()” mean in C++?

Question

While reading this explanation on lvalues and rvalues, these lines of code stuck out to me:

int& foo();
foo() = 42; // OK, foo() is an lvalue

Answer 1

int& foo(); is a function returning a reference to int. Your provided function returns int without reference.

You may do

int& foo()
{
    static int i = 42;
    return i;
}

int main()
{
    int& foo();
    foo() = 42;
}