https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102609
--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> --- I don't see anything forbidding that declaration, but I think it can only be called if S& has an implicit conversion to S* because the object parameter is an lvalue of type S, and so it can only match S* via an implicit conversion.
