Three compilers disagree

#include <iostream>

template <class A, class B> struct Foo {
    template <class Z>
    void operator-(Z z) const { std::cout << "Member\n"; }
};

template <class A, class Z> void operator- (const Foo<A, int>& foo, Z z)  {
    std::cout << "Standalone\n";
}

int main () {
    Foo<double, int> x;
    x-2;
}