c++ - Ambiguous operator overload on clang

When I try to compile this test program:

struct comma_guard
{
    template
    const comma_guard& operator,(T&&) const
    {
        return *this;
    }
};
struct foo {};
template T operator,(T x, foo)
{
    return x;
}
int main()
{
    (comma_guard(), foo());
}

I get a compile error on clang:

comma_guard.cpp:20:19: error: use of overloaded operator ',' is ambiguous (with operand types 'comma_guard' and 'foo')
    (comma_guard(), foo());
     ~~~~~~~~~~~~~^ ~~~~~
comma_guard.cpp:6:24: note: candidate function [with T = foo]
    const comma_guard& operator,(T&&) const
                       ^
comma_guard.cpp:13:21: note: candidate function [with T = comma_guard]
template T operator,(T x, foo)
                    ^

This compiles fine on gcc. From my understanding of ADL lookup, the member function in comma_guard should be preferred and so therefore shouldn't be ambiguous. Is this correct? Is this a bug in clang? Also, is there a workaround so that the operator in comma_guard will always be preferred?

Update: So it seems clang doesn't consider it a class member when its templated. So if I define the comma_guard like this, it will work:

struct comma_guard
{
    struct any
    {
        template
        any(T&&);
    };
    const comma_guard& operator,(any) const;
};

Which is correct according to C++?