http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57610
--- Comment #7 from Ole Kniemeyer <o_kniemeyer at maxon dot net> --- OK, thanks for your comments. I wasn't aware of the ongoing discussion about this issue (I was referring to the the draft version N3376 of the C++11 standard). A B-copy would be OK, but as I said copying B into A is not a good idea if those classes have virtual functions. The N3376 wording would be much better for such cases. In my case A is an interface (with dummy implementations of the virtual functions), B an implementation of A and X an unrelated class with conversion operator to B. Now if a function has a const A& parameter (so it works with the interface), N3376 allows me to pass an X object to this function by implicitly creating a temporary B object which makes X look like an A. With the variant discussed in the links you posted this wouldn't work (copying B into A makes no sense).
