Why do I get `warning: control reaches end of non-void function` with switch-case?

Consider code like this:

enum class Foo {
    A,
    B
};

int deliverPizza(Foo foo) 
{
    switch (foo) {
        case Foo::A:
            return 0;
        case Foo::B:
            return 1;
    }
}

int main()
{
    return deliverPizza(Foo::A);
}

Compiled with $ g++ -o main main.cpp -Wreturn-type:

main.cpp: In function ‘int deliverPizza(Foo)’:
main.cpp:14:1: warning: control reaches end of non-void function [-Wreturn-type]

Why is this? As far as I can see all the cases are handled within the switch. Is it that GCC doesn't understand that the cases return?