I like that idea!

One C++03 usage that has bothered me for a long time is a base class which stores one or more std::string data members, with virtual methods that return const std::string&. In an override method on one existing subclass, I decide to compute a std::string return value. But I can't return a temporary, and I can't return a stack variable! I have two bad choices: I can add a new private std::string data member to the subclass specifically to store the return value from this method; or I can refactor every declaration of this method in the entire class hierarchy to return by value.

(We will loftily ignore the even worse choice of storing a module-static std::string for the method return value.)

I have several times decided to refactor the method signature throughout the class hierarchy. But by the time I'm doing that much work, I typically also change the signature of every virtual method returning const std::string& to return by value -- to spare myself, or someone else, the pain of having to do it later.

I've pretty much concluded that a virtual method returning const& is an anti-pattern. I like the idea of a compiler warning about it.