How should RHS operators be handled?

[RyanJK5]

Since interfaces are defined as structs, there is not an obvious way for how binary operators should be defined where the interface is the right-hand side operand, e.g.:

struct S {
    friend std::ostream& operator<<(std::ostream& out, const S& self);
};

struct Printable {
    // how should this be specified?
};

Is this something we want to support? If so, how should it be written?

One possibility is to write the friend declaration in Printable, but I do not know if there's a way to discover that via reflection. Another is to use the approach from duck, which uses annotations:

struct Printable {
    [[=rjk::right_side]]
    std::ostream& operator<<(std::ostream& out) const;
};

struct AddableWithInt {
    [[=rjk::both_sides]]
    int operator+(int) const;
};

However this could be considered too complex / not matching STL style. Any thoughts?