The text "Note that using a reference member is almost always wrong" appears in the exception text for http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#define-copy-move-and-destroy-consistently in appendix C. As far as I can see there is no other treatment of this issue anywhere in the core guidelines. To me this seems like quite a strong (and in my opinion wrong) statement with no justification offered. If it is indeed the opinion put forth by the core guidelines that this is the case then I think it deserves its own rule. If not then I think that comment should be removed or revised.