[Bug c++/108165] -Wdangling-reference false positive
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108165 Sam Varshavchik changed: What|Removed |Added CC||mr...@courier-mta.com --- Comment #16 from Sam Varshavchik --- *** Bug 109538 has been marked as a duplicate of this bug. ***
[Bug c++/108165] -Wdangling-reference false positive
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108165 Marek Polacek changed: What|Removed |Added Status|NEW |RESOLVED Resolution|--- |WONTFIX --- Comment #15 from Marek Polacek --- I'm sorry but I think I'm going to close this PR without attempting to deal with the false positive in the compiler. Please add a #pragma around the function that provokes this warning. Sorry again.
[Bug c++/108165] -Wdangling-reference false positive
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108165 --- Comment #14 from Martin Liška --- (In reply to Marek Polacek from comment #13) > I would really not like to do that, the false positives rate is pretty low. You right! I noticed the warning for about 3 packages of 3300 we have in a testing CI.
[Bug c++/108165] -Wdangling-reference false positive
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108165 --- Comment #13 from Marek Polacek --- I would really not like to do that, the false positives rate is pretty low.
[Bug c++/108165] -Wdangling-reference false positive
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108165 --- Comment #12 from Martin Liška --- (In reply to Marek Polacek from comment #11) > No, because as Comment 9 says, there's no good way to suppress the warning. > I'm currently leaning towards closing the BZ and suggesting adding a #pragma > to disable the warning. Another solution would be the removal of the warning from -Wall?
[Bug c++/108165] -Wdangling-reference false positive
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108165 --- Comment #11 from Marek Polacek --- No, because as Comment 9 says, there's no good way to suppress the warning. I'm currently leaning towards closing the BZ and suggesting adding a #pragma to disable the warning.
[Bug c++/108165] -Wdangling-reference false positive
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108165 --- Comment #10 from Martin Liška --- @Marek, is there any progress on this?
[Bug c++/108165] -Wdangling-reference false positive
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108165 --- Comment #9 from Marek Polacek --- But even that won't work for Wdangling-reference6.C where the argtype is int but the rettype is std::pair. Really, all I could do is to warn only when all the arguments to the function returning a reference are temporaries, e.g. don't warn on f(a, a); f(a, 42); f(42, a); only on f(42, 42);
[Bug c++/108165] -Wdangling-reference false positive
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108165 --- Comment #8 from Marek Polacek --- (For std::any et al I guess we also have to look for void*.)
[Bug c++/108165] -Wdangling-reference false positive
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108165 --- Comment #7 from Marek Polacek --- Sure, I could (lookup_member?). It's still just guessing but maybe it would be worth it. Let me try...
[Bug c++/108165] -Wdangling-reference false positive
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108165 Jakub Jelinek changed: What|Removed |Added CC||jakub at gcc dot gnu.org --- Comment #6 from Jakub Jelinek --- (In reply to Marek Polacek from comment #5) > Sorry, I'm not sure if the false positive in comment 0 can be fixed. I > can't simply compare the type of the temporary argument and the return type, > because we may be returning a subobject of the temporary argument, which is > still dangerous. Well, you could e.g. walk recursively all the TYPE_FIELDS of the type of the temporary and compare to the type the return type references. If the temporary has some subobject of that type or if the temporary itself has that type, warn, otherwise not warn. There would still be false positives and false negatives, but those are without actually looking at the definition unavoidable.
[Bug c++/108165] -Wdangling-reference false positive
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108165 --- Comment #5 from Marek Polacek --- Sorry, I'm not sure if the false positive in comment 0 can be fixed. I can't simply compare the type of the temporary argument and the return type, because we may be returning a subobject of the temporary argument, which is still dangerous.
[Bug c++/108165] -Wdangling-reference false positive
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108165
--- Comment #4 from Jonathan Wakely ---
(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.
[Bug c++/108165] -Wdangling-reference false positive
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108165 Martin Liška changed: What|Removed |Added CC||marxin at gcc dot gnu.org, ||mpolacek at gcc dot gnu.org Status|UNCONFIRMED |NEW Last reconfirmed||2022-12-19 Ever confirmed|0 |1
[Bug c++/108165] -Wdangling-reference false positive
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108165 --- Comment #3 from Romain Geissler --- 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. Is it expected that -Wdangling-reference doesn't take into account the definition of f ? The problem of dangling reference in general needs function definitions to be effective, otherwise I fear there might be quite some false positives.
[Bug c++/108165] -Wdangling-reference false positive
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108165 --- Comment #2 from Andrew Pinski --- or is it because of the different types? GCC does not look into that either and does not look into if they are castable either ...
[Bug c++/108165] -Wdangling-reference false positive
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108165 --- Comment #1 from Andrew Pinski --- So at the warning is not flow sensitive at all and does not take into account the definition of f; only the call location of f is taken into account. In this case, the call site of f has a temporary and the warning is saying possibly because a temporary is made in the call arguments of f.
