In cases where Foo and Bar are stricly incompatible, auto is fine here. In cases where Foo and Bar are similar and might cause trouble, auto shouldn't be used here.
Yeah, I was omitting a bias I have: I tend not to write class hierarchies, so my types are generally strictly incompatible. Things would be different if the code involved some big class hierarchy (as is sometimes the case, for instance in widget toolkits).