[Bug c++/108165] -Wdangling-reference false positive

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108165

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Romain Geissler from comment #3)
> In my real life case B was std::string and used a "string literal" at call
> site, and I guess using the implicit conversion from const char[] to
> std::string is something that might happen in many call sites in big code
> bases.

And in general that is not safe.

const std::string& f(const std::string& s) { return s; }
const std::string& s = f(""); // BUG

Warning here would be entirely correct. A temporary string is created from the
string literal, then a reference to that temporary is returned, and bound to
another reference.
This is a dangling reference, and a serious bug, and exactly what the new
warning is designed to diagnose.

> Is it expected that -Wdangling-reference doesn't take into account the
> definition of f ?

Yes.

> The problem of dangling reference in general needs
> function definitions to be effective, otherwise I fear there might be quite
> some false positives.

Yes, but not in a case like f(const std::string&) above. The warning is correct
in most real cases.

The situation for the code in comment 0 is different though, there is no A
temporary. The temporary is the second argument of type B, and that isn't
returned. This seems like a bug in the implementation of the warning's
heuristics, not a problem with the design of the warning.