What is the namespace of a friend-defined function

If I have a function defined as friend inside a class. What is the namespace of that function?

namespace A{namespace B{
    struct S{
        friend void f(S const&){};
    };
}}

int main(){
    A::B::S s{};
    f(s); // ok
    ::f(s); // not ok, no f in global namespace
    A::B::f(s); // no f in A::B
    A::B::S::f(s); // not ok, no f in A::B
}

Does it even have a namespace? Does it make sense for it to have a namespace?

What if I want to disambiguate a call in the commonly used idiom?

using std::swap;
swap(a, b);

Am I forced to define it outside the class and declare it a friend?